Comparison of development of step-kinematics of assisted 60 m sprints with different pulling forces between experienced male and female sprinters

Roland van den Tillaar

doi:10.1371/journal.pone.0255302

. 2021 Jul 27;16(7):e0255302. doi: 10.1371/journal.pone.0255302

Comparison of development of step-kinematics of assisted 60 m sprints with different pulling forces between experienced male and female sprinters

Roland van den Tillaar ^1,^*

Editor: Dragan Mirkov²

PMCID: PMC8315524 PMID: 34314453

Abstract

The purpose of this study was to compare step-by-step kinematics of normal and assisted 60 m sprints with different loads in experienced sprinters. Step-by-step kinematics were measured using inertial measuring units (IMU) integrated with a 3-axis gyroscope and a laser gun in 24 national level male and female sprinters during a normal 60 m sprint and sprints with a 3, 4, and 5 kg pulling force. The main findings were that using increasing assisted loads resulted in faster 60 m times, as a result of higher step velocity mainly caused by longer step lengths in both genders and by shorter contact times in women. Men had longer step lengths, longer contact times, and shorter flight times than women. However, the assisted loads had a greater effect on women than on men, as shown by their larger decrease in sprint times. These time differences in gender were the result of more and longer duration increases in maximal step velocity with increasing assisted loads for women (70–80% of distance) than men (65–70% of distance). This was mainly caused by shorter contact times, and by more increased step lengths in women compared to men. In terms of practical application, it is notable that employing this approach, when using assisted loads can help athletes to reach higher step velocities and hold this for longer, which may be a training impulse to move the speed barrier upwards.

Introduction

In many sports like track and field, rugby, American football and soccer, sprint ability is highly important to outrun their opponents to win, get the ball or to score points [1–3]. There are different views on the type of training needed to enhance sprint performance. These views are derived from the force-velocity curve of movement, which in turn is based on the force-velocity curve of muscle contraction described by Hill [4]. Firstly, there is training based upon the principle of an overload of force, e.g. general resistance training and resisted sprints [5–8]. Secondly, there is training based upon the principle of an overload of velocity, e.g. assisted sprints [9–11]. Different ways are used to create this assisted condition like running behind a car, downhill running [12], or by using a pulley system [13, 14]. Some pulley systems, such as elastic bands or being pulled by a partner [9] cannot easy control the amount of assistance exactly over the whole sprint distance. Thereby, these systems are not well suited for research purposes in which assisted load must be controlled to know what exact loads are necessary and for the possibility of replicating the study.

Only a few studies have investigated the kinematics during assisted sprints [9, 10, 13, 15–17]. In assisted runs higher step velocities were found, which were caused by a longer step length and shorter contact times [10, 15–17]. Furthermore, a higher stride rate was found in national level sprinters [9, 17] when performing assisted sprints compared with traditional sprints. However, van den Tillaar and colleagues [10, 15] have only investigated the acute effect of assisted sprints upon kinematics for the first 20–30 m, while Mero and Komi [9] investigated kinematics over a 10 m distance (between 35 and 45 m) and Clark et al. [16] investigated only one stride at 35 m. None of these studies investigated the kinematics for the entire sprint from start until reaching maximal velocity and the acute effect of assisted runs with different loads. Only Clark et al. [16] compared different assisted sprint loads with the normal sprint. However, due to the use of elastic towing it was difficult to control the exact pulling force.

In addition, studies of van den Tillaar and colleagues [10, 15] only investigated running kinematics of female soccer and female team handball players, while Mero and Komi [9], Mann and Murphy [1], and Slawinski et al. [18] only investigated sprint kinematics at different parts of the sprint in experienced male and female sprinters. Slawinski et al. [18] and Brechue [19] found that women achieve peak velocity during a sprint earlier than men and that the peak velocity was lower than in men. This earlier occurrence and lower peak velocity was mainly due to their shorter step length [1, 9], reduced limb/joint stiffness, and their horizontal velocity and step length plateauing sooner than men [19]. When comparing the effect of assisted sprints upon step kinematics between men and women only Mero and Komi [9] investigated this, when using a tightened rubber rope with a runner pulling on average 30–45 N. The use of another partner and a rubber rope causes inaccuracies in pulling force as shown by the variation in pulling force of 30–45 N [16]. Due to this variability in pulling force it is difficult to replicate this accurately in training and for research purposes.

Nowadays, there are also pulley systems, such as the 1080 Sprint^TM and DynaSpeed^TM, which can give a constant active resistance during the whole sprint by using a motor to employ a constant pulling force [20, 21]. These pulley systems can help perform controlled assisted sprints with different loads. As not much is known about the acute effect of assisted sprints with different loads on step kinematics for men and women, more detailed information is needed. The acute effect of several different loads upon sprinting step kinematics for men and women could help in prescribing training programs involving assisted sprints that are specific for gender.

Therefore, the aim of the study was to investigate the effect of different assisted sprint loads on step-by-step kinematics during a 60 m sprint in experienced male and female sprinters. It was hypothesized that step velocity would increase with increased assisted sprint loads and that this increased step velocity would cause a higher maximal velocity. Since a higher maximal velocity is based upon changes in step kinematics it was hypothesised that the assisted sprint loads resulted in longer step length, shorter contact times, and higher step rate as found in earlier studies [9, 10, 16, 17]. In addition, it was expected that 60 m times in women would decrease relatively more than in men, with equal pulling force as women experience more pulling force relative to their body mass.

Methods

To investigate the effect of different assisted sprint loads in men and women upon step-by-step kinematics during a 60 m sprint, a cross-sectional study design was used in which 24 experienced male and female sprinters performed a normal 60 m and assisted 60 m with 3, 4, or 5 kg of pulling force provided by a motorised pulley system. Kinematic-dependent variables were step velocity, contact and flight time, step length and frequency over the whole 60 m for both genders and all conditions.

Subjects

Twelve experienced male sprinters (age: 22 ± 6 years, body mass: 74.5 ± 6.8 kg, body height 1.82 ± 0.07 m, personal best on 100 m times of 11.16 (10.27–11.97) s), and twelve experienced female sprinters (age: 22 ± 3 years, body mass: 60.7 ± 5.1 kg, body height: 1.68 ± 0.06 m, with personal 100 m best times of 12.58 (11.74–13.07) s, participated in the present study. They were instructed to avoid any maximal sprint training for at least 48 hours prior to testing. Each subject was informed of the testing procedures and possible risks, and written consent was obtained prior to the study from them and parents or legal guardians when they were under 18 years old. The study complied with current ethical regulations for research, was approved by the Norwegian Center for Research Data and performed according to the Declaration of Helsinki.

Procedures

Testing was conducted during the indoor season in preparation for the national championships. The subjects were instructed to avoid undertaking any maximal sprint training in the 48 hours prior to testing. After an individualized warm-up of around a 30–45 minutes, each subject performed one 60 m sprint, followed by a 3, 4, and 5 kg assisted 60 m sprints. The warm-up was individualized, since the subjects were experienced sprinters who knew best what type of warm-up works for them to attain their greatest sprinting performance and to avoid possible injuries. The assistance of 3, 4, and 5 kg during the 60 m assisted sprints was provided by DynaSpeed (Ergotest Technology AS, Langesund, Norway), which is a portable motorised pulling system to provide assistance/resistance while sprinting. The Dynaspeed was placed at 85m from the subject with the cable attached to the subject on a belt firmly tightened around the waist (Fig 1). The assisted load (3, 4 or 5 kg) was determined and controlled by MuscleLab v10.202.93.5131 software (Ergotest Technology AS, Langesund, Norway). Rest between each trial was 10–12 minutes to avoid fatigue. All subjects had some experience with assisted sprints, but not with this device.

Sprint times were measured with two pairs of wireless photocells placed at height of 1 m (Brower Timing Systems, Draper, UT, USA). Subjects initiated each sprint from a standing start in a split stance, with the lead foot behind a line taped on the floor 0.3 m from the first set of photocells. Distance measurements were recorded continuously during each attempt using a CMP3 Distance Sensor laser gun (Noptel Oy, Oulu, Finland), sampling at 2.56 KHz. Step velocity was automatically calculated by MuscleLab v10.57 (Ergotest Technology AS, Langesund, Norway). Contact and flight times throughout the run were derived from an identified running pattern (Fig 2) using wireless 9 degrees of freedom inertial measurement units (IMU) integrated with a 3-axis gyroscope attached on top of the shoelaces of the spikes of each foot. Sampling rate of the gyroscope was 500 Hz with maximal measuring range of 2000°·s^-1 ± 3% (Ergotest Technology AS, Langesund, Norway). This running pattern was determined in an unpublished pilot study that compared contact and flight time data measured with an infrared contact mat over 30 m (Ergotest Technology AS, Langesund, Norway) using the patterns of the angular velocity of plantar flexion/extension (ICC = 0.94). This made it possible to measure contact and flight time directly with the IMU per step and to determine each step and count the number of steps over the whole 60 m. Step frequency and step length were calculated for each step by the formulas: Step frequency = 1 / (contact time + flight time), Step length = velocity * (contact time + flight time). The velocity for each step was derived from the laser gun, which was the average velocity during one flight and contact time (step). All recordings were synchronized with the MuscleLab v10.57 (Ergotest Technology AS, Langesund, Norway). To account for the difference in number of steps between the conditions and between genders, kinematic data was averaged for each 5% of the total distance. Thus, the average kinematics were calculated in 20 points (every 3 m) for the whole 60 m for each condition.

Statistical analyses

Normality was tested using the Shapiro-Wilks test of normality. To compare the sprint times between genders and for different assisted sprints, a 2 (gender) x 4 (normal, 3, 4, and 5 kg assisted sprint) analysis of variance (ANOVA) with repeated measures was used. To assess the effect of different assisted sprints and gender on kinematics, a mixed model 2 (gender) x 4 (normal, 3, 4, and 5 kg assisted sprint) x 20 (percentage of total 60 m sprint distance) ANOVA for each kinematic variable was used. When significant differences were found a two-way ANOVA (Gender and percentage of 60 m sprint) per assisted sprint load was also performed. Post hoc comparisons with least mean differences were performed for pairwise comparisons between each following percentage (5%) of the 60 m sprints. The level of significance was set at p < 0.05, and all data are expressed as mean ± standard deviation (SD). Analysis was performed with SPSS Statistics for Windows, version 25.0 (IBM Corp., Armonk, NY, USA). Effect size was evaluated with partial eta squared (η²) where 0.01 < η² < 0.06 constituted a small effect, 0.06 < η² < 0.14 a medium effect, and η² ≥ 0.14 a large effect [22].

Results

The 60 m times decreased from the normal sprint condition significantly with each assisted pulling load (F = 108, p < 0.001, η² = 0.83,) for both genders. Relative and absolute decrease in sprint times (Table 1) was more in women compared with men (F = 19.2, p < 0.001, η² = 0.30). The number of steps also decreased significantly with increasing assisted loads for both women and men (F = 31.1, p < 0.001, η² = 0.62) with no significant difference in decrease between men and women (F = 1.1, p = 0.34, η² = 0.06). The men had significantly fewer steps than the women in all conditions (Table 2). Post hoc comparison revealed that the number of steps did decrease significantly from normal to 3 kg assisted sprint and again from 4 kg assisted to 5 kg assisted sprints (Table 2).

Table 1. Mean (±SD) sprint times and absolute and relative change in sprint times per gender.

Gender	Normal	3 kg assisted	4 kg assisted	5 kg assisted
Total sprint times 60 m
Women (s)	8.17±0.30	7.89±0.27	7.76±0.28	7.62±0.30
Men (s)	7.23±0.30	7.12±0.34	7.03±0.34	6.93±0.34
Absolute change with normal 60 m
Women (s)		0.28±0.16	0.41±0.17	0.55±0.18
Men (s)		0.12±0.10	0.20±0.07	0.30±0.10
Relative change with normal 60 m
Women (%)		3.39±1.88	5.02±1.98	6.72±2.15
Men (%)		1.62±1.42	2.80±1.06	4.21±1.36

Open in a new tab

Significant different between men and women in every condition and between conditions on a p<0.05 level.

Table 2. Mean (±SD) number of steps during the 60 m per gender under each condition.

Gender	Normal	3 kg assisted	4 kg assisted	5 kg assisted
Women (n)^†	31.6±1.1^*	30.7±0.8	30.5±0.9	30.2±0.9^*
Men (n)	36.1±1.5^*	34.9±1.6	34.7±1.7	34.0±1.7^*

Open in a new tab

* indicates a significant difference with all other conditions on a p<0.05 level.

† indicates a significant difference in number of steps with men in each condition on a p<0.01 level.

Kinematics

All kinematics were affected by running distance in both men and women (F ≥ 21, p < 0.001, η² ≥ 0.64), while the assisted conditions influenced velocity and step length (F ≥ 9.8, p < 0.001, η² ≥ 0.39) in both genders. On step frequency and contact times only a significant effect in women (F ≥ 3.5, p ≤ 0.031, η² ≥ 0.34) was found, but no significant effect on flight time (F = 0.45, p = 0.72, η² = 0.04, Table 3). In addition, a gender effect was found for all kinematics in all conditions (F ≥ 5.9, p ≤ 0.026, η² ≥ 0.25), except for step frequency (F = 0.3, p = 0.57, η² = 0.03). Furthermore, a significant interaction effect was found for running distance and gender for step velocity and flight time for all running conditions (F ≥ 5.1, p ≤ 0.001, η² ≥ 0.30), and for the effect of condition*running distance on step velocity and step length (F ≥ 1.6, p ≤ 0.004, η² ≥ 0.13), and there was also a significant interaction effect of condition*distance*gender on step velocity (F = 1.5, p = 0.015, η² = 0.10).

Table 3. Average (±SD) step velocity, step length and frequency and contact and flight time over 60 m per condition with relative change per gender.

Gender	Normal	3 kg assisted	4 kg assisted	5 kg assisted
Step velocity ^†
Women (m/s)	7.68±0.40	7.98±0.29^* (+3.9%)	8.17±0.29^* (+6.5%)	8.35 ±0.30^* (+8.8%)
Men (m/s)	8.81±0.30	9.00±0.35^* (+2.1%)	9.10±0.37^* (+3.3%)	9.29±0.36^* (5.4%)
Step frequency
Women (Hz)	4.30±0.20	4.26±0.19 (-0.9%)	4.29±0.24 (-0.2%)	4.28 ±0.24 (-0.4%)
Men (Hz)	4.38±0.23	4.38±0.24 (0.0%)	4.36±0.24 (-0.6%)	4.40±0.23 (+0.5%)
Step length ^†
Women (m)	1.80±0.08	1.87±0.09^* (+3.6%)	1.90±0.08^* (+5.6%)	1.94 ±0.10^* (+7.9%)
Men (m)	2.02±0.08	2.06±0.07^* (+2.1%)	2.08±0.10^* (+3.3%)	2.11±0.07^* (+4.8%)
Contact time ^†
Women (s)	0.102±0.010	0.100±0.082 (-1.2%)	0.098±0.065 (-3.6%)	0.098±0.082^* (-3.8%)
Men (s)	0.113±0.009	0.111±0.011 (-1.3%)	0.111±0.010 (-1.3%)	0.110±0.011^* (-2.3%)
Flight time ^†
Women (s)	0.133±0.067	0.135±0.080 (+1.4%)	0.136±0.084 (+1.9%)	0.134±0.073 (+0.9%)
Men (s)	0.119±0.013	0.118±0.012 (-0.5%)	0.118±0.014 (-0.3%)	0.118±0.012 (-0.7%)

Open in a new tab

† indicates a significant difference for this parameter between men and women in every condition on a p<0.05 level.

* indicates a significant difference with the normal 60m condition on a p<0.05 level.

Post hoc comparisons revealed that men reached a higher step velocity and that they obtained this earlier (at 65–70% of the 60m) than women (75–80% of the 60m) for the different assisted conditions (Fig 3, Table 4). Men had longer step lengths, longer contact times, and shorter flight times than women (Figs 3 and 4). The assisted loads resulted in a higher step velocity with each increased assisted load in both men (+2.1–5.4%) and women (+3.9–8.8%) Fig 3). Step length increased with every assisted load for women (+3–7.9%) and for men between normal and 3 kg (+2.1%) and between 3 and 5 kg assisted loads (+4.8%, Fig 4). Contact times decreased significantly between 3 and 5 kg assisted sprints in women (-3.9%) (Fig 5), while post hoc comparison did not find a significant step frequency difference between conditions in women (Table 3). Step velocity was significantly different between conditions after 15% of the distance for both, while it increased further with larger assisted loads and longer distances for women (70–80% of distance) than men (65–70% of distance). After reaching the maximal step velocity, maximal step velocity decreased again at 75% of the distance (45m) in a normal 60 m sprint in men to no reduction in step velocity with 5 kg assisted loads (Table 4 and Fig 3). In women, step velocity did not significantly decrease after reaching maximal step velocity in the normal and 4 kg assisted sprints, while it deceased at 85 and 90% with 3 and 5 kg assisted loads (Fig 3, Table 4).

Fig 3 — * indicates a significant difference between men and women for each of the sprint conditions. ʘ indicates where maximal velocity was reached for this condition. indicates where values decreased again for this condition.

Table 4. Maximal (±SD) step velocity and distance at which it is reached and decreases again, in each condition and per gender.

Gender	Normal	3 kg assisted	4 kg assisted	5 kg assisted
Step velocity
Women (m/s)	8.37±0.39	8.73±0.38^*	8.87±0.40^*	9.13 ±0.42^*
Men (m/s)	9.57±0.36	9.82±0.41^*	10.00±0.43^*	10.14±0.40^*
Distance maximal velocity reached
Women (%)	65	75^*	75^*	80^*
Men (%)	65	65	70^*	70^*
Distance decrease maximal velocity
Women (%)	100	85^*	100	90^*
Men (%)	75	80^*	80^*	100^*

Open in a new tab

* indicates a significant difference with the normal 60m condition on a p<0.05 level.

Fig 4 — * indicates a significant difference between men and women for each of the sprint conditions. † indicates a significant difference between these two sprint conditions. ʘ indicates where maximal/minimal values were reached before plateau for this condition. indicates where maximal values increased for this condition. indicates where values decreased again for this condition.

Step length increased significantly for both genders until 45–50% of the distance. From then on it mainly plateaued in all condition in both men and women. Until the step length increased again during the last 5% (Fig 4). Step frequency reached maximal at 0–15% of the distance and decreased again at the last 5% of the distance for both genders.

Contact time decreased until 20–35% of the distance in women and 35–50% of the distance in men where it plateaued for the rest of the distance (Fig 5). Flight time increased until 45–50% of the distance for all conditions in men where it plateaued until the last 5% and increased again. In women flight time increased after 10%, until 35–45% of the distance. It flattened and increased with the 3 kg assisted load in the last 15% of the distance and with the other loads in the last 5% (Fig 5).

Discussion

The main findings of the present study were that using increasing assisted loads resulted in shorter 60 m times, because of higher step velocity mainly caused by longer step lengths in both genders, and by shorter contact times in women. Men had longer step lengths, longer contact times, and shorter flight times than women. However, the assisted loads had a greater effect on women than on men. These time differences with gender were the result of greater and a longer duration of increase in maximal step velocity with increasing assisted loads for women (70–80% of distance) than men (65–70% of distance). This was mainly caused by shorter contact times, and more increased step lengths in women compared to men.

The kinematics in the present study are in line with the findings of Mero and Komi [9] in elite sprinters. The men have higher step velocity, longer steps or less steps in 60 m (Table 2), longer contact time and shorter flight times, but the same average step frequency compared with the women (Table 3). This could be partially explained by the body height of the men (which was on average 0.14 m (8%) higher) and thereby their longer limb length. As maximal stride length correlates very well with step length (r = 0.59–0.70) [19, 23–25], the taller males had 12% longer step length and thereby on average 4 steps less over 60 m (Table 2) than the females. Beside the anthropometric differences between genders, the longer step length is probably also caused by a lower capacity for women to produce horizontal force at high running velocities [18] as women have a lower leg muscle mass relative to their total body mass than men [26]. In addition, contact and flight times were different between the men and women as men had longer contact time than women, but also shorter flight times, which are also probably caused by the body height and mass differences. The higher body mass and height in males creates a greater moment of inertia, especially in the first steps of the sprint, and when the athletes accelerate this effect will decrease and therefore men decrease contact time over a longer distance than women. Since the flight time in women is higher than in the men, the step frequency is not different between genders (Table 3).

The effect of the assisted load was comparable with earlier studies [10, 15–17, 27, 28] that showed increases in maximal velocity varying from 2.7% [17] to 11.5% [9]. In these studies loads were used from 28 [16] to 50.6 N [17] that resulted in higher maximal velocities. Furthermore, the increases in maximal velocity differ much between the studies, which is caused by the type of pulling system; some used an elastic or rubber rope that reduced in pulling force during the sprints [9, 16], while in others a pulley system that gave constant pulling forces during the whole assisted sprint [10, 15, 17, 28] was used. Furthermore, the level of the athletes and their sprint experience influenced the enhancement of sprint times during the assisted loads. This meant that lower level female and male sprinters had greater advantage (increase in maximal velocity) from the assisted load than elite sprinters [9].

In the present study, the assisted load had a larger effect upon the women (-6.7%) than the men (-4.2%) in 60 m sprint times (Table 1). The changes in time were followed by an increase in maximal step velocity, and by maintaining this step velocity longer as the assisted load is higher. Since the absolute assisted load was the same and the women were lighter than the men the relative propulsive force was higher and therefore could pull women longer to a relatively higher step velocity (increase from 65% in normal run to 85% of total distance with 5 kg, Table 4) than men (from 65% to 70% of total distance, Fig 3, Table 4). This higher step velocity was the result of increased step length with each assisted load for women (+3–7.9%) and for men only between normal and 3 kg (+2.1%) and for 5 kg assisted loads (+4.8%) (Fig 4). However, the distance at which the maximal step length was reached did not change much (45–55% of total distance) with increasing assisted loads. This coincided with reaching the maximal flight times in both men and women. Also, the shortest contact time was reached in men at around 50% of the total distance after which it reached a plateau. This plateau might be indicative of the ‘second transition phase’ during acceleration as referred to previously by Plamondon and Roy [29] and Nagahara et al. [30]. This transition is characterized by a change from a leaning position to a more stable upright position and thereby termination, in which athletes adopt different lower limb mechanics around this point in a sprint effort in preparation for upright sprinting [30]. This transition is reported to occur at around step 14–16 (28–32 m) during a regular sprint [29, 30]. Based upon the reported step kinematics with the different load it seems that the assisted load does not have much influence on this transition point.

In women the occurrence of the shortest contact time changed from 25 to 35% of the distance and decreased significantly when pulled with 5 kg assisted force. This indicates that the assisted load helped the acceleration phase more in women than in men due to the greater relative assisted load. Since the women also had a longer flight phase the pulling force helped them to increase step length more than the men.

The assisted load had most influence after 15% of the distance (after 9 m) as no significant differences in kinematics were observed in earlier phases. This was probably caused by the positioning of the athletes as they leaned forwards for producing maximal horizontal forces [1, 31, 32], without breaking the steps. That it occurred from this point on (after 15% of distance) is in accordance with the “first transition” during the acceleration phase when sprinting [28–30]. Previous studies have attributed this to the foot beginning to contact the ground in front of center of gravity and the knee joint starting to flex during ground contact [28, 30]. The pulling force attached to the waist could prohibit the athletes from leaning forward more in the first steps and therefore changes in kinematics are not possible in the first steps.

With increasing assisted loads, the maximal step velocity increased relatively more and for longer for women (70–80% of distance) than men (65–70% of distance). After reaching the maximal step velocity, it decreased again at 75% of the distance in normal 60 m sprint conditions in men to no reduction in maximal step velocity with 5 kg assisted loads. In women the maximal step velocity did not decrease in the normal and 4 kg assisted sprints but deceased at 85% and 90% with 3 and 5 kg assisted loads respectively (Fig 3), indicating the absolute assisted load had a different effect on maintaining the maximal step velocity. In men it seems that with 5 kg assisted loads they can maintain their maximal step velocity, while in women when the load is too great, they will decrease their maximal step velocity at the end of the total distance. These differences in development can be explained by more braking during foot placement that occurs during assisted sprints with larger loads [9].

There were some limitations in this current study. Firstly, the results are only investigated as the mean step kinematics for men and women and not the individual adaptations to the changing assisted loads. The individual athletic level of the men and women was not the same as they ranged from 10.27–11.97 s (men) and 11.74–13.07 s (women). Therefore, the strength to adjust with increasing assisted load could be different, besides the anthropometric differences between and within gender. It is possible that the faster sprinters adapt differently to the pulling force than the slower sprinters as observed in the study of Mero and Komi [9] in which the faster male sprinters increased step frequency when performing assisted sprints, while the female sprinters and slower male sprinters increased step length. A possible explanation for these findings is that the athletes were simply not able to increase limb velocity to the degree required to achieve greater step frequency under the assisted sprint condition. Most athletes had some experience with assisted sprint (elastic band, pulley system), but not with this specific system. It is possible that with greater exposure to these different assisted loads, athletes may learn to make the necessary adjustments to allow them to increase step frequency alongside the increases in step length as Mero and Komi [33] observed in elite female sprinters, who increased step frequency with assisted sprints compared to normal sprints. Kristensen et al. [13] observed an increase of step frequency after 6 weeks of assisted sprint training in sport students.

The present investigation did not conduct any direct or indirect assessment of kinetics, such as ground reaction forces or torques generated at each joint segment, or 2- or 3-dimensional joint kinematical analyses. Clearly, the absence of these measurements is a limitation, which could have given more information about acute changes in muscle recruitment, muscle stiffness and possible altered joint kinetics and kinematics [27]. Future investigations should include these measurements of detailed muscle activation, kinetic and/or kinematic analysis of lower limb joints and limb segments to get a better understanding of the alterations that occur with increasing assisted load. Furthermore, detailed anthropometrics (fat percentage and muscle mass) and muscle strength should be included, which could help in explaining step kinematics changes within and between men and women. In addition, the pulling force should be individualized to investigate if similar enhancements in sprinting times were caused by similar adaptations (step length, step frequency) in men and women and at different experience levels, or if these adaptations are individually different [34].

In conclusion, the assisted loads had a greater effect on women than on men, as shown by their larger decrease in sprint times. These time differences in gender were the result of greater and longer duration increase in maximal step velocity with increasing assisted loads for women (70–80% of distance) than men (65–70% of distance). This was mainly caused by shorter contact times, and more increase in step lengths in women compared to in men. The practical implication for trainers and athletes is that assisted sprints make it possible to reach higher step velocities and hold this maximal step velocity for a longer time when the assisted load is high enough (> 30 N). This could be a training impulse to move the speed barrier upwards. However, when the assisted load is too high, step kinematic alterations will not help in maintaining this maximal step velocity at this level, as seen with 5 kg loads in women. Therefore, individual assisted loads should be applied to increase maximal step velocity and keep this velocity, without introducing too much braking in the steps. Based upon the findings of the present investigation, assisted loads of 4–5 kg can be applied depending upon the anthropometrics and experience level of the athlete.

Supporting information

S1 Data

(PDF)

Click here for additional data file.^{(888.8KB, pdf)}

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

The authors received no specific funding for this work.

References

1.Mann R, Murphy A. The mechanics of sprinting and hurdling: 2013 edition. Lexington, KY2013.
2.Haugen TA, Tnnessen E, Hisdal J, Seiler S. The Role and Development of Sprinting Speed in Soccer. International Journal of Sports Physiology & Performance. 2014;9(3):432–41. doi: 10.1123/ijspp.2013-0121 . [DOI] [PubMed] [Google Scholar]
3.Higham DG, Pyne DB, Anson JM, Eddy A. Physiological, Anthropometric, and Performance Characteristics of Rugby Sevens Players. International Journal of Sports Physiology & Performance. 2013;8(1):19–27. . [DOI] [PubMed] [Google Scholar]
4.Hill AV. First and last experiments in muscle mechanics. Cambridge: Cambridge University Press; 1970. [Google Scholar]
5.Delecluse C, van Coppenolle H, Willems E, van Leemputte M, Diels R, Goris M. Influence of high-resistance and high-velocity training on sprint performance. Med Sci Sports Exerc. 1995;27(8):1203–9. [PubMed] [Google Scholar]
6.Zafeiridis A, Saraslanidis P, Manou V, Ioakimidis P, Dipla K, Kellis S. The effects of resisted sled-pulling sprint training on acceleration and maximum speed performance. J Sports Med Phys Fit. 2005;45(3):284–90. SPHS-. [PubMed] [Google Scholar]
7.Spinks CD, Murphy AJ, Spinks WL, Lockie RG. The effects of resisted sprint training on acceleration performance and kinematics in soccer, rugby union and Australian football players. J Strength Cond Res. 2007;21(1):77–85. doi: 10.1519/00124278-200702000-00015 . [DOI] [PubMed] [Google Scholar]
8.Martinopoulou K, Argeitaki P, Paradisis G, Katsikas C, Smirniotou A. The effects of resisted training using parachute on sprint performance. Biology of Exercise. 2011;7(1):7–23. . [Google Scholar]
9.Mero A, Komi PV. Effects of supramaximal velocity on biomechanical variables in sprinting. Int J Sport Biomech. 1985;1(3):240–52. doi: 10.3382/ps.2010-00694 . [DOI] [PubMed] [Google Scholar]
10.van den Tillaar R, Gamble P. Comparison of step-by-step kinematics of resisted, assisted and unloaded 20-m sprint runs. Sports Biomech. 2018:1–14. Epub 2018/03/27. doi: 10.1080/14763141.2018.1442871 . [DOI] [PubMed] [Google Scholar]
11.Lahti J, Jiménez-Reyes P, Cross MR, Samozino P, Chassaing P, Simond-Cote B, et al. Individual Sprint Force-Velocity Profile Adaptations to In-Season Assisted and Resisted Velocity-Based Training in Professional Rugby. Sports. 2020;8(5). Epub 2020/05/30. doi: 10.3390/sports8050074 . [DOI] [PMC free article] [PubMed] [Google Scholar]
12.Paradisis GP, Cooke CB. Kinematic and postural characteristics of sprint running on sloping surfaces. J Sports Sci. 2001;19(2):149–59. doi: 10.1080/026404101300036370 . [DOI] [PubMed] [Google Scholar]
13.Kristensen GO, van den Tillaar R, Ettema GJ. Velocity specificity in early-phase sprint training. J Strength Cond Res. 2006;20(4):833–7. Epub 2006/12/30. doi: 10.1519/R-17805.1 . [DOI] [PubMed] [Google Scholar]
14.Majdell R, Alexander MJL. The effect of overspeed training on kinematic variables in sprinting. J Hum Mov Stud. 1991;21(1):19–39. SPHS-610833. [Google Scholar]
15.van den Tillaar R, Von Heimburg E. Comparison of different sprint training sessions with assisted and resisted running: effects on performance and kinematics in 20 m sprints. Hum Mov. 2017;18(2): 21–9. [Google Scholar]
16.Clark DA, Sabick MB, Pfeiffer RP, Kuhlman SM, Knigge NA, Shea KG. Influence of towing force magnitude on the kinematics of supramaximal sprinting. J Strength Cond Res. 2009;23(4):1162–8. doi: 10.1519/JSC.0b013e318194df84 . [DOI] [PubMed] [Google Scholar]
17.Kratky S, Buchecker M, Pfusterschmied J, Szekely C, MÜLler E. Effect of ady-weight supporting kite on sprint running kinematics in well-trained sprinters. J Strength Cond Res. 2016;30(1):102–8. doi: 10.1519/JSC.0000000000001070 . [DOI] [PubMed] [Google Scholar]
18.Slawinski J, Termoz N, Rabita G, Guilhem G, Dorel S, Morin JB, et al. How 100-m event analyses improve our understanding of world-class men’s and women’s sprint performance. Scand J Med Sci Sports. 2017;27(1):45–54. doi: 10.1111/sms.12627 . [DOI] [PubMed] [Google Scholar]
19.Brechue WF. Structure-function relationships that determine sprint performance and running speed in sport. Int J Appl Sports Sci. 2011;23(2):313–50. . [Google Scholar]
20.Rakovic E, Paulsen G, Helland C, Eriksrud O, Haugen T. The effect of individualised sprint training in elite female team sport athletes: A pilot study. J Sports Sci. 2018;36(24):2802–8. Epub 2018/05/10. doi: 10.1080/02640414.2018.1474536 . [DOI] [PubMed] [Google Scholar]
21.Mangine GT, Huet K, Williamson C, Bechke E, Serafini P, Bender D, et al. A resisted sprint improves rate of force development during a 20-m sprint in athletes. J Strength Cond Res. 2018;32(6):1531–7. Epub 2018/05/23. doi: 10.1519/JSC.0000000000002030 . [DOI] [PubMed] [Google Scholar]
22.Cohen J. Statistical power analysis for the behavioral sciences. Hillsdale, NJ, England: Lawrence Erlbaum Associates; 1988. 174 p. [Google Scholar]
23.Malina RM, Harper AB, Campbell DE, Avent HH. Physique of female track and field athletes. Med Sci Sports Exerc. 1971;3(1):32–8. SPH10134. [PubMed] [Google Scholar]
24.Kumagai K, Abe T, Brechue WF, Ryushi T, Takano S, Mizuno M. Sprint performance is related to muscle fascicle length in male 100-m sprinters. J Appl Physiol. 2000;88(3):811–6. Epub 2000/03/10. doi: 10.1152/jappl.2000.88.3.811 . [DOI] [PubMed] [Google Scholar]
25.Abe T, Fukashiro S, Harada Y, Kawamoto K. Relationship between sprint performance and muscle fascicle length in female sprinters. J Physiol Anthropol Appl Human Sci. 2001;20(2):141–7. Epub 2001/06/02. doi: 10.2114/jpa.20.141 . [DOI] [PubMed] [Google Scholar]
26.Perez-Gomez J, Rodriguez GV, Ara I, Olmedillas H, Chavarren J, González-Henriquez JJ, et al. Role of muscle mass on sprint performance: gender differences? Eur J Appl Physiol. 2008;102(6):685–94. doi: 10.1007/s00421-007-0648-8 . [DOI] [PubMed] [Google Scholar]
27.Mero A, Komi PV. Electromyographic activity in sprinting at speeds ranging from sub-maximal to supra-maximal. Med Sci Sports Exerc. 1987;19(3):266–74. Epub 1987/06/01. . [PubMed] [Google Scholar]
28.van den Tillaar R, Gamble P. Comparison of step-by-step kinematics and muscle activity of resisted, assisted and unloaded 30 m sprint runs in experienced sprinters. Trans Sports Med. 2018;1(4):151–9. [Google Scholar]
29.Plamondon A, Roy B. Kinematics and kinetics of sprint acceleration. Can J Appl Sport Sci. 1984;9(1):42–52. Epub 1984/03/01. 6705128. [PubMed] [Google Scholar]
30.Nagahara R, Matsubayashi T, Matsuo A, Zushi K. Kinematics of transition during human accelerated sprinting. Biol Open. 2014;3(8):689–99. Epub 2014/07/06. doi: 10.1242/bio.20148284 ; PubMed Central PMCID: PMC4133722. [DOI] [PMC free article] [PubMed] [Google Scholar]
31.Nagahara R, Mizutani M, Matsuo A, Kanehisa H, Fukunaga T. Association of Sprint Performance With Ground Reaction Forces During Acceleration and Maximal Speed Phases in a Single Sprint. J Appl Biomech. 2018;34(2):104–10. Epub 2017/09/28. doi: 10.1123/jab.2016-0356 . [DOI] [PubMed] [Google Scholar]
32.Nagahara R, Amini E, Marcon KCC, Chen PW, Chua J, Eiberger J, et al. Influence of the Intention to Lean the Body Forward on Kinematics and Kinetics of Sprinting for Active Adults. Sports. 2019;7(6). Epub 2019/06/05. doi: 10.3390/sports7060133 ; PubMed Central PMCID: PMC6628312. [DOI] [PMC free article] [PubMed] [Google Scholar]
33.Mero A, Komi PV. Force-, EMG-, and elasticity-velocity relationships at submaximal, maximal and supramaximal running speeds in sprinters. Eur J Appl Physiol Occup Physiol. 1986;55(5):553–61. Epub 1986/01/01. doi: 10.1007/BF00421652 . [DOI] [PubMed] [Google Scholar]
34.Salo AI, Bezodis IN, Batterham AM, Kerwin DG. Elite sprinting: are athletes individually step-frequency or step-length reliant? Med Sci Sports Exerc. 2011;43(6):1055–62. Epub 2010/10/29. doi: 10.1249/MSS.0b013e318201f6f8 . [DOI] [PubMed] [Google Scholar]

PLoS One. doi: 10.1371/journal.pone.0255302.r001

Decision Letter 0

Dragan Mirkov

29 Mar 2021

PONE-D-21-02963

Comparison of development of Step-Kinematics of Assisted 60 m Sprints with different pulling forces Between Experienced Male and Female Sprinters

PLOS ONE

Dear Dr. van den Tillaar,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by May 13 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Dragan Mirkov, Ph.D.

Academic Editor

PLOS ONE

Journal Requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. We noticed you have some minor occurrence of overlapping text with the following previous publication(s), which needs to be addressed:

https://onlinelibrary.wiley.com/doi/abs/10.1002/tsm2.130

https://commons.nmu.edu/isbs/vol36/iss1/64/

https://www.tandfonline.com/doi/abs/10.1080/14763141.2018.1442871?journalCode=rspb20

In your revision ensure you cite all your sources (including your own works), and quote or rephrase any duplicated text outside the methods section. Further consideration is dependent on these concerns being addressed.

3. In your Data Availability statement, you have not specified where the minimal data set underlying the results described in your manuscript can be found. PLOS defines a study's minimal data set as the underlying data used to reach the conclusions drawn in the manuscript and any additional data required to replicate the reported study findings in their entirety. All PLOS journals require that the minimal data set be made fully available. For more information about our data policy, please see http://journals.plos.org/plosone/s/data-availability.

Upon re-submitting your revised manuscript, please upload your study’s minimal underlying data set as either Supporting Information files or to a stable, public repository and include the relevant URLs, DOIs, or accession numbers within your revised cover letter. For a list of acceptable repositories, please see http://journals.plos.org/plosone/s/data-availability#loc-recommended-repositories. Any potentially identifying patient information must be fully anonymized.

Important: If there are ethical or legal restrictions to sharing your data publicly, please explain these restrictions in detail. Please see our guidelines for more information on what we consider unacceptable restrictions to publicly sharing data: http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions. Note that it is not acceptable for the authors to be the sole named individuals responsible for ensuring data access.

We will update your Data Availability statement to reflect the information you provide in your cover letter.

Additional Editor Comments (if provided):

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: No

Reviewer #2: No

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: GENERAL COMMENTS

The author aimed to explore how different magnitudes of pulling forces influence kinematic variables during 60 m sprint performance in experienced men and women sprinters. Although this is an important question for many practitioners, the study quality needs to be improved significantly in order to be considered for publication. Firstly, the English language is poor, especially in the introduction section (see specific comments below). Secondly, result section is too long, with too many tables and figures (8 in total) and the information overlaps between text, tables and figures. Therefore, I propose merging the information into either tables or figures, or simply creating a single table with most important findings for all kinematic variables and keeping all the figures. Please report only main results in the text. And finally, discussion section carries the most problems. It is simply too long (more than 6 pages, 11 paragraphs) and not well organised. I propose following organisation (or other that will enable more concise discussion): Main findings (paragraph 1), The influence of assisted load on step velocity (paragraph 2), contact and flight time (paragraph 3), step length and frequency (paragraph 4), limitations (paragraph 5), and conclusions (paragraph 6). Please lower down the number of words in discussion.

SPECIFIC COMMENTS

INTRODUCTION

The general organisation of the paragraph and the information provided in each of the paragraphs are very good. However, there is excess of using the pointing words (“this”, “that”, “these”) which decreases the readability of the manuscript. It is essential to rewrite the bellow indicated sentences.

Line 55: remove “this” before assisted since you have not specified any way of providing assisted force until this part.

Lines 56-60: Please rephrase these sentences due to the poor English: “Some pulley systems, such as elastic bands or being pulled by a partner [9] are not easy to control the amount of assistance exactly over the whole sprint distance. Thereby, these systems are often not suited for research purposes in which assisted load must be controlled to know what exact loads are necessary and for the possibility of replicating the study”

Line 68: The reference 16 was not cited correctly in the text (here and throughout of the text).

Line 68: Please put “between 35 and 45 m” in brackets, since it is very confusing for the readers if it stays like this without brackets or signs of interpunction.

Line 69: Please change “to maximal velocity” by “until reaching maximal velocity”

Line 77: Who are “They” at the beginning of the sentence? Please specify.

Line 77-78: Please rewrite the sentence due to the poor English.

Line 78: “This was mainly due to their shorter step length…” What exactly? Be more specific.

Lines 80-82: Which effect? What kind of assisted effect? You need to specify which effect in this sentence.

Lines 89-93: Please rephrase these sentences due to the poor English: “As not much is known about the acute effect of assisted sprints with different loads on these kinematics for men and women, more detailed information about the acute effect of several different loads upon sprinting kinematics for men and women could help in prescribing training programs involving resisted sprints that are specific for gender.”

METHOD

The Design section needs to be improved. For example: Male and female sprinters cannot be “used in…”. They might perform, run, etc. The length of the distance (60m) is repeated several times. You have not stated the type of study design (i.e., Cross-sectional study design in your case).

Why do you report their personal best at 100m in the Participant section? I think it would be of greater value to report their personal best at 60m.

In the second sentence of the Procedure section, you reported three times the word “best”. Please rephrase this sentence.

The picture of DynaSpeed and actual testing setup would be of value here (Line 130).

Did the subjects perform familiarisation session?

Use either subject or participant systematically, but I think that according to the instructions for authors provided by the PlosOne the use of the word Subject is mandatory. Please check this.

Why you used standing start for such a short sprinting discipline?

Lines 141-146: The sentence is too long and difficult to understand. Please rephrase it.

In the study design you are mentioning only assisted sprints, however, in the Statistical analysis you are mentioning that you performed ANOVA for both resisted and assisted sprints. How do you explain this?

RESULTS

Result section is very difficult to be understand. There are too many tables and figures, and the information is repeated in the text as well. Only the most important information needs to stay in the text, while I propose merging the information from the tables and figures, or at least lowering down the information reported in the tables. The quality of the figures needs to be increased, as well as the figure size.

Line 198: Please check part of this sentence: “in both and on step frequency and contact time”

DISCUSSION

The discussion is extremely long (more than 6 pages). Too many different issues are discussed. I propose next organisation: Main findings (paragraph 1), The influence of assisted load on step velocity (paragraph 2), contact and flight time (paragraph 3), step length and frequency (paragraph 4), limitations (5), and conclusions (paragraph 6). Please lower down the number of words used in discussion.

298: This sentence carries no useful information: “Lower pulling forces did not lead to increased sprint velocity.” In which group? In this study? Is it opposite to your findings?

309: Please change “holding” by “maintaining” or other more appropriate word.

Reviewer #2: GENERAL COMMENTS:

Author made a good effort in addressing the effects of pulling forces (assisted training) on step-kinematics of male and female sprinters. The paper is generally well written except at some points that I addressed in specific comments.

SPECIFIC COMMENTS

Abstract

Page 2, Line 40-41 – you state “…which could be a training impulse to move the speed barrier upwards”. This seems to be a bit of an overreach with your results as you did not investigate the effects. Maybe using MAY instead of COULD would solve this.

Introduction

Page 4, Line 73 – Suggestion: Omit “all the” before the “studies” - Studies of van den Tillar…

Page 4, Line 77 – To what study do you refer by the sentence “They found that women achieve peak velocity earlier than men and that this was lower than in men”?

Page 4, Line 73-80 – Improve the clarity of this part of the test as it is not clear who did what, and which reference refers which statement.

Page 4, Line 80-83 – I’m not sure what is the purpose of the sentence: “Mero and Komi (9)…”. I does not seem to fit with the previous one or the following one. Try to make this and the following sentence more generic and more connected to the previous text.

Page 5, Line 96-97 – Shouldn’t you add WOULD after the “step velocity”?

Page 5, Line 98 – The sentence: “This higher maximal velocity is the result of longer step length, shorter contact times, and higher step rate as found in earlier studies [9, 10, 16, 17]” does not seem to belong here. This was supposed to be cleared out earlier in Introduction. Here just clearly state your hypotheses.

Page 5, Line 99-102 – You state: “In addition, it was expected that 60 m times in women would decrease relatively more than in men…” Was this a hypothesis as well?. After that, you continue with: “due to the fact that the pulling force was equal for men and women and therefore women experience more pulling force relative to their body mass”. Shouldn’t this be part of the methods/results or the discussion? Also, this could be mentioned earlier in introduction.

Methods

Design

You did not explain what kind of study design you used.

Page 5, Line 107 – What does “international” refer to? This means that they are competing internationally or they are coming from different countries?

Page 5, Line 109 – I do not think you need to state here which pulley system you used. You will explain it in procedures.

Participants

Were there participants younger than 18 years as men age of males was 22 ± 6 years? If yes, how did you provide the consent from them? Could you add age ranges?

What was the length of training history?

What part of the season was it?

What about avoiding other types of training for 48 hours? Also, what did they do prior these 48 hours as this could also be a sufficient time for the acute effects of supercompensation on performance, depending on the part of the season were in.

Procedures

General comment: Procedures could be explained in more details. Researchers and trainers may have difficulties replicating, thus reducing the applicability and external validity of this study.

Could you structure this part paragraphs instead of single long paragraph for easier reading, better clarity and understanding?

Although I understand why the individualized warmup was performed, a general information about the structure of warmup would be beneficial for the readers of this manuscript. Like this, it seems that 24 different warmups could be performed without any guidance or without any specificity relative to tasks that participants were warming up for. Given that you emphasize that they used individualized warmup does not make it less important, nor does it explain much.

Did you perform the testing as a training session or it was planned within the athletes’ schedule?

Did participants performed familiarization trials?

What was the rest period between the conditions?

Why the 3, 4, and 5 kg trials were not randomized?

Could you provide the reliability and validity of each device used?

Page 6, Line 129-130 – Add more details on how this works or cite the research that explains the procedure in details.

Page 6, Line 135 – I am not sure if I understand how you measured speed. More details may be needed so the study is replicable to a trainer or researcher less familiar with this device.

Results

Page 8, Line 178-181 – This is too long sentence. It would be clearer if you divide it into 2 sentences.

Page 9, Table 1 – This table has not been mentioned in results section, yet it is positioned before the Table 2 that you mentioned.. Indicate significant results in this table, same as you did in other tables. In table title, you used (±SD), but this was not how you used it in the table itself.

Page 10, Line 197 – check for the accuracy of “assistance condition”. Should this be “assisted conditions”?

Page 10, Line 196-200 – This sentence contains too much information. Make it two sentences.

Page 11, Line 218-222 – This should be 2 sentences. It would make it easier to fallow.

Page 11-12, Line 224-225 – “post hoc comparison did not find a significant step frequency difference between conditions in women” – to my understanding, this was already shown in Table 3?

Page 12, Line 226-227 – I suggest replacing “step velocity” with “it”, “more” with “further”, and “increasing” with “larger”.

Page 12, Line 228-230 – This statement should be clearer and more accurate. I’m not so sure what you wanted to say here.

Discussion

Page14, Line 280 – Maybe using “compared” instead “then” women would be more appropriate here.

Page 14, Line 280 – Consider replacing “This is explainable” with “This could be partially explained”. Also, this statement should be referenced.

Page 14, Line 282 – You start the sentence with “As maximal stride length correlates very well with step length (r = 0.59–0.70) [19, 23-25]. How does this relate to the previous sentence or even with the continuance of this very sentence? I am not really sure what you wanted to say. You also continue this sentence with “the taller males had 12% longer step length and thereby on average 4 steps less over 60 m”. Does this mean that taller males would have 12% longer step when compared to shorter males? Also, here you used males, while through the text you used man. These are not the synonyms. In my opinion, using sex (females and males) instead of gender (women and men) is more appropriate for this research as you are investigating differences performance produced by two biological sexes. However, I do not know whether this fits journal requirements and policies.

Page 14, Line 277-284 – Consider making this part better connected with the following of this paragraph. Transition from anthropometrics to muscular potential to exert forces should be smoother and better linked. Although you may not have it in your sample, males and females occasionally may be of the same height. However, it does not necessarily mean that they would perform the same (i.e., have the same step length and velocity).

Page 15, Line 208-309 – Consider replacing “mainly caused” by “followed”. How are you sure that the changes in 60-m sprint time were caused by an increase in maximal step velocity and by holding this step velocity longer and not the other way around. By pulling the athletes, you are actually the one increasing their sprinting speed and then you follow what is going on with the kinematics and dynamics. Whether a repeated application of your conditions produce acute increase the maximal step velocity and prolong it so sprinters become faster was not investigated here.

Page 15, Line 310-313 – Make this argument clearer. From the stated here, I am not sure what are you trying to say.

Page 16, Line 334-335 – Should you add “in earlier phases” at the end of this sentence?

Page 16, Line 335-337 – You state: “This was probably caused by the positioning of the athletes as they leaned forwards for producing maximal horizontal forces [1, 31, 32], without breaking the steps”. How does the positioning explain non-significant effects of pulling forces in the early phase of sprint?

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Filip Kukić

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

Attachment

Submitted filename: Review.docx

Click here for additional data file.^{(21.1KB, docx)}

PLoS One. 2021 Jul 27;16(7):e0255302. doi: 10.1371/journal.pone.0255302.r002

Author response to Decision Letter 0

3 May 2021

Dear editor and reviewers,

We would like to thank the reviewers for doing a great job on improving our manuscript. We have now changed the manuscript according to the comments of the reviewers to improve the readability of the final manuscript. We now think that this manuscript is ready for publication in your journal.

Comments to the reviewers

GENERAL COMMENTS:

SPECIFIC COMMENTS

Abstract

Changed it in may

Introduction

Page 4, Line 73 – Suggestion: Omit “all the” before the “studies” - Studies of van den Tillar…

Deleted

Page 4, Line 77 – To what study do you refer by the sentence “They found that women achieve peak velocity earlier than men and that this was lower than in men”?

Slawinski et al. (2017) and Brechue (2011) found this. This is now changed in the text.

Page 4, Line 73-80 – Improve the clarity of this part of the test as it is not clear who did what, and which reference refers which statement.

We have included the names of the references to the text to improve clarity.

We fully agree with the reviewer that it does not fit with the previous or following one. We have rewritten it in:

When comparing the effect of assisted sprints upon step kinematics between men and women only Mero and Komi (9) investigated this, … We think it connects the sentences better now.

Page 5, Line 96-97 – Shouldn’t you add WOULD after the “step velocity”?

Included to the text.

We have changed the sentence that it is a clear expected hypothesis based upon the findings from earlier studies.

It is also a hypothesis. We have rewritten the next sentence as we tested several assisted loads and explain what we expect between the sexes when using those different loads. We hope these changes are ok for the reviewer.

Methods

Design

You did not explain what kind of study design you used.

We have deleted the word design to avoid confusion and just wrote what we have done to answer the research question.

Page 5, Line 107 – What does “international” refer to? This means that they are competing internationally or they are coming from different countries?

Some of them were competing at international level and some were also from other countries. However, we have changed it in experienced to avoid confusion.

Page 5, Line 109 – I do not think you need to state here which pulley system you used. You will explain it in procedures.

We have changed it in a motorized pulley system.

Participants

Were there participants younger than 18 years as men age of males was 22 ± 6 years? If yes, how did you provide the consent from them? Could you add age ranges?

There were four subjects younger than 18 but we have the consent from their parents. We have included this in the methods now. We could add a age range. However, with the standard deviation it is already visible what the main range is and it seems for us a bit weird to write range on age, but not an the rest of the anthropometric variables.

What was the length of training history?

This we did not ask, but all participants had several years of sprint training behind them even the athletes under 18 had years of experience.

What part of the season was it?

It was during the indoor season, a few weeks before the national championships. We have included this to the text now.

The participants were instructed to avoid undertaking any maximal sprint training in the 48 hours prior to testing. We have included this to the text.

Procedures

General comment: Procedures could be explained in more details. Researchers and trainers may have difficulties replicating, thus reducing the applicability and external validity of this study.

Could you structure this part paragraphs instead of single long paragraph for easier reading, better clarity and understanding?

We have split up the paragraph in several and include some more information in the methods part to give the reader more information about the methods.

As it is an individualized warm-up it is difficult to come with more details, because as you said yourself 24 different warm-ups could be performed. In a sprint competition it is also not expected that everybody does the same warm-up before their sprint. But all the warm-ups can lead to maximal performance for each subject, which is the most important for this study. We have added that the warm-up was around 30-45 minutes long. But as said before it is difficult to specify this, since some used jogging, static stretching, drills, while others did no stretching or dynamic exercises. For the point of the present study it is not so important , we think.

Did you perform the testing as a training session or it was planned within the athletes’ schedule?

It was planned as a training session that was scheduled within the athletes’ schedule.

Did participants performed familiarization trials?

The participants did not have familiarization trials, but as already written in the text: All subjects had some experience with assisted sprints, but not with this device. It is difficult to perform a familiarization trial in the same session as this could influence the next run. In another study we had athletes perform a familiarization session, but the sprint results were approximately the same.

What was the rest period between the conditions?

The rest between each trial was 10-12 minutes to avoid fatigue. We have include this to the text now.

Why the 3, 4, and 5 kg trials were not randomized?

We wanted to randomize them. However, since we did not know (no earlier studies on this area), we wanted for the safety of the athletes (avoid possible injuries) start with this order since it was also a few weeks before the nationals. Some trainers were a bit sceptic beforehand. After the session this was gone by the trainers.

Could you provide the reliability and validity of each device used?

I the present study we have not investigated the reliability and validity of the devices used. However, we have investigated the step kinematics between this IMU+laser system with 50 force plates and found that the step kinematics were approximately the same for male sprinters between the systems. We are writing the article now and have not finished all analyses. However, no statistically significant differences between any of the step kinematics between the two systems were found. We can’t refer to this study yet since it is not finished.

Page 6, Line 129-130 – Add more details on how this works or cite the research that explains the procedure in details.

We have included some more information about the system in the following sentences.

Page 6, Line 135 – I am not sure if I understand how you measured speed. More details may be needed so the study is replicable to a trainer or researcher less familiar with this device.

The laser measured distance over time which then automatically by the software was calculated in velocity at the different steps. This was all done automatically and since it was a commercial product, they don’t want to share the exact details for the filtering and smoothing of the data. However, the system seems to be accurate since it was compared with a system of 50 force plates (gold standard) in which no significant differences in step kinematics were found between the systems. That study is not finished yet, so we cannot refer to it yet. Sorry. We hope that the reviewer understands that the producer of the system does not give away these details.

Results

Page 8, Line 178-181 – This is too long sentence. It would be clearer if you divide it into 2 sentences.

It is divided in two sentences now.

Page 9, Table 1 – This table has not been mentioned in results section, yet it is positioned before the Table 2 that you mentioned. Indicate significant results in this table, same as you did in other tables. In table title, you used (±SD), but this was not how you used it in the table itself.

We have now mentioned table 1 in the text. We have already written under the table that all numbers were significantly different between sexes and between conditions. Therefore, we did not included * since this would be at all numbers and thereby creating a chaos in our opinion.

Page 10, Line 197 – check for the accuracy of “assistance condition”. Should this be “assisted conditions”?

This is changed in assisted conditions

Page 10, Line 196-200 – This sentence contains too much information. Make it two sentences.

We have split the sentence in two.

Page 11, Line 218-222 – This should be 2 sentences. It would make it easier to fallow.

Sentence is split in two.

Page 11-12, Line 224-225 – “post hoc comparison did not find a significant step frequency difference between conditions in women” – to my understanding, this was already shown in Table 3?

We found a significant effect of step frequency in women as written in line 208. However, post hoc comparison did show a significant difference between the conditions. This was shown in table 3 and not in figure 4. We have changed this in the text.

Page 12, Line 226-227 – I suggest replacing “step velocity” with “it”, “more” with “further”, and “increasing” with “larger”.

We have changed it according to the comments of the reviewer.

Page 12, Line 228-230 – This statement should be clearer and more accurate. I’m not so sure what you wanted to say here.

We have rewritten the statement to be more accurate.

Discussion

Page14, Line 280 – Maybe using “compared” instead “then” women would be more appropriate here.

We have changed it in “compared with”

Page 14, Line 280 – Consider replacing “This is explainable” with “This could be partially explained”. Also, this statement should be referenced.

We have changed it according to the comment of the reviewer. It is referenced in the following sentence.

It refers to the previous sentence and is related to the males compared to the females. We have added this now to the sentence to avoid confusion.

We agree that with the same anthropometry it is perhaps possible to have the same step kinematics. We have changed the transition sentence a bit to get a better connection to the previous part.

Page 15, Line 308-309 – Consider replacing “mainly caused” by “followed”. How are you sure that the changes in 60-m sprint time were caused by an increase in maximal step velocity and by holding this step velocity longer and not the other way around. By pulling the athletes, you are actually the one increasing their sprinting speed and then you follow what is going on with the kinematics and dynamics. Whether a repeated application of your conditions produce acute increase the maximal step velocity and prolong it so sprinters become faster was not investigated here.

We fully agree with the comment of the reviewer. We increase their velocity, but we don’t know if it will help their sprint capacity in normal runs. That is in the future a study we have planned to conduct.

Page 15, Line 310-313 – Make this argument clearer. From the stated here, I am not sure what are you trying to say.

The absolute assisted load was the same, while women are lighter and therefore the relative propulsive force was higher for women which resulted in a relatively peak velocity reached later of the total distance. We have rewritten the text and hope it is clear now.

Page 16, Line 334-335 – Should you add “in earlier phases” at the end of this sentence?

This is added to the text now.

As written and explained in the following sentences. The pulling force attached to the waist could prohibit the athletes from leaning forward (positioning) more in the first steps and therefore changes in kinematics are not possible in the first steps.

Attachment

Submitted filename: comments to the reivewer(b).docx

Click here for additional data file.^{(33.9KB, docx)}

PLoS One. doi: 10.1371/journal.pone.0255302.r003

Decision Letter 1

Dragan Mirkov

22 Jun 2021

PONE-D-21-02963R1

Comparison of development of Step-Kinematics of Assisted 60 m Sprints with different pulling forces Between Experienced Male and Female Sprinters

PLOS ONE

Dear Dr. van den Tillaar,

Please submit your revised manuscript by Aug 06 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

We look forward to receiving your revised manuscript.

Kind regards,

Dragan Mirkov, Ph.D.

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: (No Response)

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: (No Response)

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #1: (No Response)

Reviewer #2: No

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #1: (No Response)

Reviewer #2: Yes

**********

6. Review Comments to the Author