A comprehensive biomechanical analysis of the barbell hip thrust

Adam Brazil; Laurie Needham; Jac L Palmer; Ian N Bezodis

doi:10.1371/journal.pone.0249307

. 2021 Mar 29;16(3):e0249307. doi: 10.1371/journal.pone.0249307

A comprehensive biomechanical analysis of the barbell hip thrust

Adam Brazil ^1,^*,^#, Laurie Needham ^1,^#, Jac L Palmer ^2,^#, Ian N Bezodis ^2,^#

Editor: Matti Douglas Allen³

PMCID: PMC8006986 PMID: 33780488

Abstract

Barbell hip thrust exercises have risen in popularity within the biomechanics and strength and conditioning literature over recent years, as a method of developing the hip extensor musculature. Biomechanical analysis of the hip thrust beyond electromyography is yet to be conducted. The aim of this study was therefore to perform the first comprehensive biomechanical analysis the barbell hip thrust. Nineteen resistance trained males performed three repetitions of the barbell hip thrust at 70% one-repetition maximum. Kinematic (250 Hz) and kinetic (1000 Hz) data were used to calculate angle, angular velocity, moment and power data at the ankle, knee, hip and pelvic-trunk joint during the lifting phase. Results highlighted that the hip thrust elicits significantly (p < 0.05) greater bilateral extensor demand at the hip joint in comparison with the knee and pelvic-trunk joints, whilst ankle joint kinetics were found to be negligible. Against contemporary belief, hip extensor moments were not found to be consistent throughout the repetition and instead diminished throughout the lifting phase. The current study provides unique insight to joint kinematics and kinetics of the barbell hip thrust, based on a novel approach, that offers a robust evidence base for practitioners to guide exercise selection.

Introduction

Strength training, known as the process of imposing physical loading to increase the capability of the neuromuscular-skeletal system to produce force, is recognised as an essential component in the physical preparation of athletes to enhance performance [1]. Therefore, a continuous aim of both research and practice is to identify the most effective training strategies to maximise performance enhancements [2, 3].

Recently, there has been an increase in the popularity of the barbell hip thrust, a type of bridging exercise performed against an external barbell resistance, used to develop the hip extensor musculature. Since its introduction to the literature by Contreras et al. [4], the hip thrust has gained popularity within the biomechanics and strength and conditioning communities due to evidence of superior gluteal activation characteristics compared with more conventional resistance training exercises such as the back squat or deadlift variations [5–8].

Due to the horizontally (anterior-posterior) loaded nature of the hip thrust, authors have speculated that this exercise requires a consistent hip extension moment throughout its range of motion [9, 10], and maximal muscular tension when hip joint reaches full extension [4, 5, 7]. In addition, the loading nature of the hip thrust elicits a horizontal orientation of the resultant ground reaction force vector relative to the athlete in the global coordinate system [11]. This relative orientation is suggested to enhance the transfer of training to athletic performances requiring horizontal force production (e.g. sprinting) based on the “force-vector hypothesis” [10, 12].

Studies have investigated the influence of hip thrust on sprint [10, 13, 14] or jump [10, 11] performance, as well as the relationships between the hip thrust and sprint performance measures [8, 12]. Based on the force-vector hypothesis, the hip thrust should provide a mechanical advantage over traditional standing barbell exercises that elicit a relative vertical orientation of the resultant force vector. Correlational analyses have supported these ideas by evidencing stronger relationships between hip thrust kinetic measures and sprint performance, compared with vertically oriented exercises [8, 12]. However, evidence from training studies is currently equivocal, with Contreras et al. [10] indicating sprint and vertical jump improvements were superior following hip thrust and front squat training interventions, respectively, whereas the hip thrust has recently been found to elicit equal improvements in vertical and horizontal jumping performance [11]. Further, Jarvis et al. [13] observed no significant improvements in sprint performance following a hip thrust training intervention. Differences in study populations, training parameters and performance tests likely provide some explanation for the current lack of consensus regarding the effectiveness of the hip thrust exercise, and signifies the requirement for further training interventions [15], but also comprehensive biomechanical analyses to better understand the mechanisms by which the hip thrust may influence performance [16].

Biomechanical analysis offers insight to the underlying kinematics and kinetics of a training exercise, providing coaches and athletes conceptual understanding to bring objectivity to sport-specific exercise selection [16]. Investigating the musculoskeletal demand placed on the lower limb and pelvic-trunk joints is fundamental to biomechanical analyses of strength training exercises [17–22], although has yet to be undertaken for the hip thrust. Specifically, for the hip joint, whilst authors have proposed that the hip thrust requires a consistent hip extension moment and greater muscular “tension” when hip joint is close to full extension [4, 5, 7, 8], there is currently no joint kinetic evidence to support these ideas.

Therefore, the aim of this study was to perform the first comprehensive biomechanical analysis the barbell hip thrust. Based on the current body of knowledge, it was hypothesised that the hip thrust would, 1) elicit a large hip extensor moment; and 2) hip extensor moments would remain consistent throughout the lifting phase. The purpose of the study was to provide scientists, coaches and athletes with information relating to the external and musculoskeletal demand of performing the barbell hip thrust, to help inform training practices that can positively impact upon athletic performance.

Materials and methods

Nineteen resistance trained males (age, 22.4 ± 3.1 years; mass, 78.8 ± 11.4 kg; height, 1.77 ± 0.09 m, hip thrust one-repetition maximum [1RM] = 189 ± 42 kg) gave written informed consent to participate in the current study following approval by the Cardiff Metropolitan University Ethics Committee. Participants were free from injury and regularly used the barbell hip thrust in their training routine.

Kinematic data were captured at 250 Hz with a 15 camera Vicon Vantage system (Vicon, Oxford, UK). A marker set comprising 26 individual markers were attached to each participant to facilitate the creation of an eight-segment model (bilateral feet, shanks and thighs, pelvis and thorax), at the following anatomical landmarks: (bilateral), acromion process, iliac crest, posterior superior iliac spine, anterior superior iliac spine, lateral and medal femoral epicondyles, lateral and medial malleoli, first and fifth metatarsal heads, calcaneus, head of the second toe; (unilateral), transverse process of the 7^th cervical and 10^th thoracic vertebrae, sternum, and xyphoid process. Rigid clusters comprising four-marker clusters were attached to the lateral aspect of the thigh and shank segments and used for segment tracking, and additional markers were attached to the barbell to track its position and orientation (Fig 1). Synchronised kinetic data were captured using three Kistler 9287CA force plates (Kistler, Winterhur, Switzerland) operating at 1000 Hz. Two force plates were located in standard in-ground dwellings, and were used to measure forces separately at each foot. The third force plate was mounted to a custom-built rig, specifically for measurement of the hip thrust. It was raised above the ground and angled at 20° to the horizontal, to facilitate accurate measurement of external force between the thorax and bench. Following pilot testing, a 15 mm medium density foam mat was secured to the top of the raised force plate to reduce participant discomfort. The rig was positioned such that the participant could comfortably perform the hip thrust with their feet located near the center of the in-ground plates. Participants performed a self-selected warm-up. Data collection comprised one set of three repetitions of the barbell hip thrust at 70% of 1RM determined from recent training data. Participants were asked to perform each repetition with their habitual technique and timing, with maximal intent in the lifting phase towards full hip extension, before controlling the barbell back to the starting position. The barbell came to a rest on the ground before the next repetition was completed, so that each repetition was performed from the same initial starting position (see S1 Appendix for example video trial).

After labelling and gap-filling of marker trajectories (Nexus, v2.6, Vicon, Oxford, UK), data processing was performed using Visual 3D software (v6, C-Motion Inc, Germantown, USA). Raw marker coordinates and force traces were low-pass filtered (4th order Butterworth) with cut-off frequencies of 3 and 30 Hz, respectively, determined through residual analysis [23]. Data from the raised force plate were rotated and resolved into the global coordinate system, defined as a right-handed orthogonal coordinate system of X (medio-lateral pointing right), Y (anterior-posterior pointing forwards) and Z (superior, pointing upwards). Analysis was undertaken on the lifting (bar-raising) phase of each repetition. The start of the lifting phase was defined when the vertical velocity of the barbell became positive and remained greater than 0 m·s^-1. The end of the lifting phase was defined as the point of maximum vertical barbell displacement.

For analysis of external kinetics, Y-Z resultant forces were calculated at the feet (combination of force data for each foot), thorax (force between the thorax and bench) and in total (sum of all forces), of which peak and average values were obtained for the lifting phase and normalised to bodyweight (BW). For joint level analyses, each segment’s coordinate system (SCS, defined with the same right-handed orthogonal coordinate system as the global reference) was defined using a static calibration trial. Joint angular velocity was the rate of change of the distal relative to the proximal SCS, described by an XYZ Cardan sequence. Inverse dynamic procedures were applied using Visual 3D software (v6, C-Motion Inc, Germantown, USA) [24] to calculate resultant moments resolved in the proximal SCS at the ankle, knee, hip and pelvic-trunk joints, using the default parameters for segment mass [25] and inertial characteristics [26] (Fig 2). The distal end of the foot was defined at the metatarsophalangeal joint, and the moment acting between the distal end of the foot and the ground was assumed to be zero. Due to the sagittal plane nature of the movement and the extensor demand of the task, x-axis (flexion-extension) data only are reported, with a focus on extensor properties that were defined as positive. Joint extensor (positive) impulse was calculated through integration of the respective moment-time curve using the trapezium rule. Joint power was calculated as the product of joint moment and joint angular velocity. The main phases of positive extensor joint power were identified, and joint work was calculated for each by integrating power-time curves (trapezium rule), to define positive extensor work performed (energy generation).

Joint kinetic data were normalised to body mass, and extensor characteristics were calculated. Ensemble mean and standard deviations were calculated for all discrete data and for external force, joint kinematic and kinetic time histories, which were time-normalised to 100% of the lifting phase using a cubic spline. All ankle, knee and hip joint data were initially averaged across the two limbs, to present data showing the distribution of loading at individual joints. Subsequently, joint kinetic variables of the hip and knee were each calculated as the sum of the two limbs to provide a valid comparison of musculoskeletal demand with the pelvic-trunk joint. This facilitated greater insight into the total bilateral loading at each joint that was required to raise the barbell.

Unpaired t-tests were used to analyse the differences in external resultant forces acting at the feet and thorax, and a one-way analysis of variance (ANOVA) was used to compare differences in joint kinematic and bilateral kinetic data between the knee, hip and pelvic-trunk joints. When a significant main effect was observed, Bonferroni post-hoc analyses were implemented to identify significant between-joint comparisons. T-tests and ANOVA were conducted using IBM SPSS Statistics (version 26, IBM, Armonk, USA), and statistical significance was accepted at p < 0.05. To accompany inferential statistics, Cohen’s d effect sizes and 95% confidence intervals were calculated using Estimation Statistics [27]. When confidence intervals of the effect size did not overlap zero, magnitudes of d were interpreted as small (0.2 ≤ d < 0.6), moderate (0.6 ≤ d < 1.2), large (1.2 ≤ d < 2.0), very large (2.0 ≤ d < 4.0), and extremely large (d ≥ 4.0) (equivalent scale used for negative values of d) [28].

In addition to discrete statistics, statistical parametric mapping (SPM) [29] was used to statistically compare joint moment waveforms between the knee, hip and pelvic-trunk joint. Specifically, a one-way ANOVA with post hoc test was used (α = 0.05). Post hoc testing was conducted using SPM independent t-test to provide the scalar output statistic, SPM{t}. Critical thresholds (t*) were adjusted using a Bonferroni procedure. All SPM analyses were done using open source spm1d code (v.04, www.smp1d.org) in Matlab (R2017a, The Mathworks Inc, Natick, USA).

Results

External kinematics & kinetics

During the lifting phase, group mean (± SD) vertical barbell displacement was 0.361 ± 0.042 m in a time of 0.828 ± 0.148 s (Table 1). Resultant forces at the feet, thorax and in total are presented in Fig 3. All force curves demonstrated a similar pattern, with an initial peak at approximately 20% of lift time followed by a steady decline and plateau towards the end of the lift. Magnitudes of peak (2.16 ± 0.52 BW vs. 1.69 ± 0.42 BW; d = 1.00) and average (1.72 ± 0.36 BW vs. 1.25 ± 0.29 N; d = 1.59) resultant force were significantly (p < 0.05) greater at the feet compared with the thorax, with moderate and large effect sizes, respectively (Table 1).

Table 1. External characteristics.

	Mean	±	SD		d	95% CI
Lift time (s)	0.828	±	0.148
Vertical barbell ROM (m)	0.361	±	0.042
Peak feet resultant force (BW)	2.16	±	0.52
Peak thorax resultant force (BW)	1.69	±	0.42	^*	1.00	0.64	to	1.40
Peak total resultant force (BW)	3.81	±	0.88
Average feet resultant force (BW)	1.72	±	0.36
Average thorax resultant force (BW)	1.25	±	0.29	^*	1.59	0.88	to	2.35
Average total resultant force (BW)	2.97	±	0.58

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*Denotes significant difference (p < 0.05) between feet and thorax force data. External forces have been normalised to bodyweight (BW).

Joint kinematics & kinetics

Continuous joint angle, angular velocity, moment and power data are presented for the ankle, knee, hip and pelvic-trunk joint in Fig 2. The ankle joint exhibited predominantly dorsiflexion throughout the lifting phase, with some individuals transitioning to a period of plantarflexion towards the end the lift. The direction of the net ankle moment demonstrated variation between plantar- and dorsi-flexor dominance, resulting in both positive and negative power to be observed amongst participants (Fig 4). Although variable in direction, magnitudes of ankle joint kinetics were negligible in comparison to other joints (Fig 4).

Fig 4 — Column 1 –ankle joint, column 2 –knee joint, column 3 –hip joint, and column 4 –pelvic-trunk joint. Row 1 –joint angles, row 2 –joint angular velocities, row 3 –joint moments, row 4 –joint powers.

The knee joint extended for most of the lifting phase, and the hip joint extended throughout the entire lifting phase, which was consistently observed across all participants (Fig 4). Whilst on average there was pelvic-trunk joint extension during the lift, both extension and flexion were observed amongst participants (Fig 4). ANOVA revealed a significant main effect for extensor range of motion, with the hip joint (75 ± 19°) showing significantly greater range of motion compared with the knee (21 ± 7°; d = 3.80, very large) and pelvic-trunk joint (12 ± 21°; d = 3.21, very large, Table 3).

Table 3. Joint characteristics post-hoc comparison effect sizes (d) and 95% Lower (L) and Upper (U) confidence intervals.

	Hip vs. Knee			Pelvic-Trunk vs. Knee			Hip vs. Pelvic-Trunk
	d	L	U	d	L	U	d	L	U
Extensor range of motion (°)	3.80	2.77	4.84	n/a			3.21	2.34	4.09
Time of peak extensor moment (%)	-3.98	-5.32	-2.84	-3.78	-5.14	-2.72	n/a
Joint angle at peak extensor moment (°)	2.03	1.38	2.78	8.60	6.21	11.04	-5.54	-6.82	-3.95
Peak extensor moment (N m·kg^-1)	4.58	3.38	5.73	1.46	0.58	2.38	1.91	1.17	2.55
Average extensor moment (N m·kg^-1)	3.75	2.69	4.67	1.34	0.54	2.08	1.98	1.20	2.67
Extensor impulse (N m·s·kg^-1)	3.16	2.33	3.90	1.32	0.53	2.21	1.83	1.21	2.44
Peak extensor power (W·kg^-1)	3.68	2.66	4.75	n/a			3.50	2.49	4.58
Average extensor power (W·kg^-1)	3.83	2.75	5.03	n/a			3.64	2.60	4.79
Extensor work (J·kg^-1)	4.42	3.09	5.79	n/a			4.19	2.83	5.55

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All comparisons are significant at p < 0.05 from Bonferroni post-hoc tests from one-way ANOVA. The direction of d indicates whether the joint on the left (positive) vs. right (negative) was of larger magnitude.

A net extensor joint moment dominated the lifting phase across the knee, hip and pelvic-trunk joints (Fig 4), with ANOVA revealing a significant main effect for all bilateral joint moment characteristic comparisons (Table 2). A similar moment pattern was observed between the hip and pelvic-trunk joint, with an initial peak at approximately 14% of movement time, before reducing and plateauing towards the end of the lifting phase. Joint moments at the knee reached peak magnitudes significantly later in the lifting phase (51.0 ± 12.6%; d = 3.98 and 3.78 for hip and pelvic-trunk comparisons, respectively), and again declined towards the end of the lift (Tables 2 and 3; Fig 4). The angle at which peak extensor moments occurred was significantly different between all joint comparisons (d = 2.03, very large, to 8.60, extremely large), with the knee (-108 ± 8°) and hip (-83 ± 16°) joints in flexed positions, whereas the pelvic-trunk joint (9 ± 18°) was in a more neutral position. Post-hoc analysis again revealed significant differences between all joint comparisons for the magnitude of peak and average moment, and joint impulse (Table 3). In all cases, the bilateral hip joint elicited the greatest magnitude of extensor kinetics, followed by the pelvic-trunk and knee joints (Table 2). Very large to extremely large effect sizes were observed between the hip and knee joint (d = 3.16 to 4.58), with large effect sizes found between the hip and pelvic-trunk (d = 1.83 to 1.91), and knee and pelvic-trunk joints (d = 1.32 to 1.46) (Table 3). The region of joint moment differences is highlighted in Fig 5, where significant SPM ANOVA main effects were observed. Post-hoc 1D analysis revealed statistically significant joint moment differences, as indicated by supra-threshold clusters, between the hip and knee joint for 100% of lift time, hip and pelvic-trunk joint between 0–78% of lift time, and knee and pelvic-trunk joint from 0–46% and 97–100% of lift time.

Table 2. Joint characteristics.

	Knee			Hip			Pelvic-Trunk
	Mean	±	SD	Mean	±	SD	Mean	±	SD
*Average of joints*
Extensor range of motion (°)	21	±	7	75	±	19	12	±	21	^*
Peak extensor moment (N m·kg^-1)	1.34	±	0.35	3.52	±	0.57
Time of peak extensor moment (%)	51.0	±	12.6	14.3	±	3.1	14.3	±	5.3	^*
Joint angle at peak extensor moment (°)	-108	±	8	-83	±	16	9	±	18	^*
Average extensor moment (N m·kg^-1)	1.02	±	0.31	2.39	±	0.42
Extensor impulse (N m·s·kg^-1)	0.82	±	0.25	1.97	±	0.45
Peak extensor power (W·kg^-1)	1.27	±	0.56	7.75	±	2.36
Average extensor power (W·kg^-1)	0.69	±	0.33	4.08	±	1.20
Extensor work (J·kg^-1)	0.43	±	0.18	3.23	±	0.88
*Sum of bilateral joints*
Peak extensor moment (N m·kg^-1)	2.65	±	0.71	6.97	±	1.13	4.39	±	1.53	^*
Average extensor moment (N m·kg^-1)	2.03	±	0.42	4.79	±	0.84	3.06	±	0.90	^*
Extensor impulse (N m·s·kg^-1)	1.64	±	0.50	3.95	±	0.90	2.46	±	0.72	^*
Peak extensor power (W·kg^-1)	2.66	±	1.25	15.39	±	4.72	2.39	±	1.33	^*
Average extensor power (W·kg^-1)	1.37	±	0.67	8.16	±	2.42	1.18	±	0.72	^*
Extensor work (J·kg^-1)	0.85	±	0.36	6.47	±	1.76	0.74	±	0.59	^*

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*Denotes significant main effect of one-way ANOVA.

Fig 5 — Shaded bars represent the SnPM{t} output statistic for each comparison. Intensity of shaded areas indicate the extent to which the critical threshold (t*) was exceeded during the movement phase with a p-value given for each supra-threshold cluster superimposed on top.

Extensor moments were observed at the knee and hip joint as they extended throughout the lifting phase. As such, both joints generated extensor energy as indicated by positive magnitudes of extensor power (Fig 4). Due to inter-individual variability in the direction of pelvic-trunk joint angular velocity, periods of positive (extensor energy generation) and negative (extensor energy absorption) power were shown (Fig 4). Significant ANOVA main effects were again found across all joint extensor power characteristics (Table 2), with post-hoc analyses revealing that the bilateral hip joint elicited the greatest magnitude of peak extensor power (d = 3.68 and 3.50, very large), average extensor power (d = 3.83 and 3.64, very large), and extensor work (d = 4.42 and 4.19), compared with the knee and pelvic-trunk joints, respectively. No significant post-hoc difference was observed between the bilateral knee and trunk for extensor power characteristics (Table 3).

Discussion

The aim of the current study was to perform the first comprehensive biomechanical analysis of the barbell hip thrust, quantifying the external and musculoskeletal demand of this exercise to offer scientists, coaches and athletes novel information to inform training practice. The results of this study demonstrated that the hip thrust places a large and significantly greater extensor demand bilaterally on the hip joint compared to the knee and pelvic-trunk joints, allowing the first hypothesis to be accepted. However, against contemporary belief [4, 5, 7, 10], the magnitude of hip extensor moment was not constant during the lift (Fig 4), but instead diminished throughout the hip extension range of motion and reached a local minimum as the hip joint approached full extension. The second study hypothesis was therefore rejected.

A unique aspect of the current study was the novel quantification of joint kinematic and kinetic data in order to explore the musculoskeletal demand placed on the lower limb and pelvic-trunk joints during the lifting phase of the hip thrust. Interestingly, joint kinetics at the ankle joint were small in magnitude and inconsistent in the direction of plantar- and dorsi-flexion across the nineteen participants (Fig 4). Inter-individual differences in the direction of the ankle joint moment were likely explained through subtle changes in the center of pressure location relative to the ankle joint, potentially influenced by initial foot position that was not controlled in this study as participants were encouraged to use their habitual lifting technique. The magnitude of ankle joint moment observed for the hip thrust is contrasting to standing exercises such as the back squat [16, 22], and deadlift [19] which have been shown to elicit plantarflexion moments in excess of 1 N m·kg^-1. The reduced demand to maintain upright posture through the foot-ground interaction due to the third point of contact (thorax and bench) may explain the low magnitude of joint kinetics observed at the ankle in the current study. This may offer an excellent practical solution for loading the hip (and to a lesser extent the knee) extensors when low ankle loading is desirable, for example during rehabilitation.

In contrast to the ankle joint, dominant extensor moments were observed at the knee, hip and pelvic-trunk joints throughout (Fig 4). Unsurprisingly given the nature of the exercise, magnitudes of extensor moment properties were significantly greater bilaterally at the hip joint compared with the knee and trunk-pelvic joints, whilst significantly greater magnitudes were also observed at the pelvic-trunk joint compared with the knee (Tables 2 and 3, Fig 5). Although the knee joint elicited the smallest magnitudes of extensor moment and impulse in the current study, average magnitudes of unilateral peak extensor moment (1.34 ± 0.35 N m·kg^-1, Table 2) were comparable with those previously observed during the straight- (0.80 N m·kg^-1) and hexagonal- (1.46 N m·kg^-1) bar deadlift against the same 70% 1RM external load [19, 20]. Therefore, whilst the hip thrust preferentially loads the hip extensors (as expected), there is also considerable demand placed on the knee extensors during the lift. This demand is supported by previous evidence demonstrating high levels of quadriceps muscle activation during the hip thrust exercise [7], and may indicate a requirement to stabilise the knee joint through a relatively small range of motion (20 ± 7°, Table 2).

Previous research into the hip thrust exercise has consistently speculated that there is a consistent hip extensor moment throughout hip extension [9, 10] or maximal “tension” at the hip joint at full extension [4, 5, 7], leading to the second study hypothesis. The current investigation challenged these notions for the first time by empirically demonstrating that the magnitude of hip extension moment decreased as the hip joint extended throughout the lift, reaching a local minimum near full extension (Fig 4) and permitting the second study hypothesis to be rejected. Peak unilateral hip extensor moment (3.52 ± 0.57 N m·kg^-1) during the hip thrust occurred near the onset of the lifting phase (14.3 ± 3.1%) at a joint angle of 83 ± 16° of hip flexion (Table 2). At the end of the lifting phase, the hip extension moment had decreased by approximately two-thirds (Fig 4). Whilst joint moments do not directly indicate tension in the muscle fibers, the current results offer biomechanical depth beyond EMG analyses to suggest that the extensor demand placed on the hip extensors is not maximised at the end range of motion in the hip thrust exercise.

Whilst the hip joint moment was not found to be consistent, it clearly remained extensor dominant throughout the entire lifting phase of the repetition (Figs 4 and 5). The maintenance of an extensor moment nearing full hip extension is in contrast with more traditional standing exercises such as the back squat, where hip extensor moments have been shown to be zero near the end range of motion [22]. Thus, the hip thrust may offer a mechanical advantage over traditional standing exercises such as the back squat or deadlift for loading the hip extensor musculature at joint angles closer to full extension, and offers some support for previous research suggesting the superiority of the hip thrust exercise based on increased gluteal activation characteristics [5–8]. Further research should seek to directly compare hip joint kinetics between the barbell hip thrust and more traditional strength training exercises in order to further understand the potential benefits of the hip thrust exercise for developing the hip extensor musculature.

Providing kinetic insight to the pelvic-trunk joint was a further novel aspect of the current study, which demonstrated an almost identical pattern of extensor moment compared with the bilateral hip joint but with a lower magnitude across the first 80% of lift time (Fig 5). The range of pelvic-trunk joint motion was small (12 ± 21°) and the extensor work done was more than 6-fold smaller than the hip joint (Table 2), indicating that the extensor moment at the pelvic-trunk joint was predominantly acting to resist pelvic-trunk flexion. The magnitude of peak pelvic-trunk extensor moment (4.93 ± 1.53 N m·kg^-1) was comparable with those reported during back squat and deadlift variations at similar relative loading conditions [19, 20]. However, whilst standing exercises such as the back squat will elicit both compressive and shear forces across the lumbar spine [22], the non-axial loading nature of the hip thrust may offer a means of training the hip extensor musculature with reduced compressive forces at the lumbar spine, although further investigation is required. Again, the need for future studies comparing kinetic data between the hip thrust and other commonly prescribed training exercises is required to more empirically inform exercise selection processes [16].

The novel biomechanical analysis performed in the current study was permitted by a custom-built force plate rig, specifically designed to measure external force between the thorax and bench, whilst traditional floor-mounted platforms measured external force between the feet and ground. Results demonstrated that the temporal pattern of resultant external force applied to the floor (feet) and bench (thorax) was similar (Fig 3), although a significantly greater proportion of the external force was applied to the floor (Table 1). From an external kinetics perspective, one proposed benefit of the hip thrust exercise is the horizontal orientation of the resultant ground reaction force vector relative to the athlete, which is proposed to elicit greater transference to athletic performances that rely on horizontal force production, such as accelerative sprinting, based on the “force-vector hypothesis” [4, 10, 12]. However, during tasks that require horizontal orientation of the ground reaction force vector relative to the global (real world) coordinate system (e.g. sprinting, jumping), the athlete typically adopts a forward leaning position, producing closer alignment between orientations of the athlete and resultant force vector in the athlete’s local coordinate system [11]. Therefore, when considered in the athlete’s local coordinate system, there may be greater disparity between the hip thrust compared with traditional standing exercises such as the back squat or deadlift. Future quantification of the orientation of the external force vector from the floor relative to the orientation of the pelvis could offer further insight to the “force-vector hypothesis” as well as delineating differences between horizontal (hip thrust) and vertical (squat and deadlift) loaded training exercises.

Previous investigations have suggested the hip thrust to be a superior training method for enhancing horizontally oriented performances (e.g. sprinting) [8, 12], although evidence from intervention studies remains inconclusive [10, 11, 13]. Novel biomechanical evidence from this study indicates a large potential for the hip thrust to develop force-producing capabilities of the hip extensor musculature, supporting its use within strength and conditioning practice. However, the current study provides mechanistic insight to explain equivocal evidence of hip thrust superiority, as hip extensor moments were not found to be consistent throughout the lift, which is often cited as a mechanism by which the hip thrust may offer superior benefit over traditional vertical-based resistance exercises [10]. The current study did not undertake EMG analysis, which can offer insight to the muscles contributing to the observed net joint kinetics [30]. From a hip extensor perspective, research has indicated dominant EMG activity in the gluteus maximus during the hip thrust [9] but biceps femoris during accelerative [31] and maximal velocity sprinting [32]. The different contribution of each muscle group toward hip extension between these tasks may offer additional challenge to the force-vector hypothesis, and offers an insightful avenue for future research.

Conclusions

The barbell hip thrust places extensor demand on the knee, hip and pelvic-trunk joints, with the largest bilateral demand placed on the hip extensor musculature. Whilst the hip thrust elicits a hip extensor moment throughout the full range of joint extension, the moment is not consistent and declines from the initial flexed position to full extension during the lifting phase. Whilst the current study has offered a unique dataset and novel biomechanical insight to the hip thrust exercise that offers practitioners with new information to guide exercise selection, future research is required to understand how hip thrust kinetics compare with more traditional training exercises, to understand the mechanisms through which this exercise may be a preferential training method for enhancing athletic performance.

Practical implications

The large demand placed on the hip extensor musculature during the hip thrust exercise observed in the current study supports the hip thrust exercise as a training means for developing the hip extensor musculature. The occurrence of a hip extensor moment as the hip joint approaches full extension is one mechanism through which the hip thrust may benefit tasks that require high hip extension force when the hip is near full extension, although further investigation is required.

Supporting information

S1 Appendix. Video file demonstrating example hip thrust trial in Visual 3D.

(MP4)

Click here for additional data file.^{(5.6MB, mp4)}

Acknowledgments

The authors would like to thank Mike Long, Ben Robson, Adam Tossell, Morgan Jones, Clémence Pouget and Jordan Lewis for their assistance with data collection and initial data processing.

Data Availability

All data files are available from figshare (DOI: https://doi.org/10.6084/m9.figshare.13582145.v2).

Funding Statement

The authors received no specific funding for this work.

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PLoS One. doi: 10.1371/journal.pone.0249307.r001

Decision Letter 0

Matti Douglas Allen

10 Dec 2020

PONE-D-20-27535

A comprehensive biomechanical analysis of the barbell hip thrust

PLOS ONE

Dear Dr. Brazil,

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Comments to the Author

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

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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 focus of the study is the biomechanics of the barbell hip thrust exercise and a research article that contributes to practice. However, there are a few general comments to be completed. Firstly, a free body diagram might be added to the materials and methods section. In addition, a figure can be used showing the experimental setup. Secondly, Newton-Euler inverse dynamic formulas might be presented in the method section.

Detailed comments about this study are below.

Abstract

The results of the study are presented clearly in abstract section.

Introduction

The reasons for studying the biomechanics of hip thrust exercise are presented in the introduction section. However, the hypothesis of the study has been not included in the introduction section. What is the hypothesis of the study? Please add hypothesis to the introduction.

Methods

Please, add a free body diagram to the materials and methods section. A considerable simplification can be made to understand the kinetic and kinematic dimensions of the hip thrust movement if the forces and moments in the pelvic-trunk joint and lower extremity joints are defined by the free body diagram. In addition, a figure that shows the marker set attached to the body can be added to method section, or a figure showing the experimental setup. As known, Euler’s 3D equations of motion are used for kinetic analyses of a segment Please, add to the text Newton-Euler inverse dynamic procedures used for calculation of moment. On the other hand, Euler's equations also require data such as angular acceleration and moments of inertia of lower extremity segments (thigh, shank and foot). It can be mentioned in the text if angular acceleration and moments of inertia of the lower limb segments data were used in calculation.

Results

External kinematics and kinetics and joint kinematics and kinetics were presented clearly in Table1, Table2, and Table3, and were supported with the figures.

Discussion

It was reported in the study that the hip extensor moment decreased during the lift contrary to the literatüre. This is the most important result of the study. The main results of the study, which contradicts with the literature, are very well discussed with fine details. I applaud the authors.

Page 14, line 284

Both dot and comma were used together in the end of the sentence. Please, remove the comma.

Page 14, line 303

Please, add a dot at the end of the sentence.

Reviewer #2: Manuscript Number: PONE-D-20-27535

Manuscript Title: A comprehensive biomechanical analysis of the barbell hip thrust

Comments to the Author

This paper attempts to identify the kinematic and kinetic characteristics of the hips-thrust exercise. The measurement method is original and interesting. While I find this manuscript and the topic interesting and of relevance to the readership of Plos One. These findings are expected to be useful for strength training. However, there are some shortcomings in the presentation.

------------------------------------------------------------------------------

Introduction

Page 4, Lines60-65

Clarifying the relevance of why previous studies(10,11,13) differ in their results and the need for comprehensive biomechanical analysis will make the significance of this study clearer. I recommend the author to revise it.

Page 4, Lines 71-72

The need and importance of investigating the musculoskeletal demands on the lower extremity and pelvic-trunk joints is not only cited, but also written, which makes the significance of analyzing the lower extremity and pelvic-trunk joints of the hip-thrust exercise in this study more clear. I recommend the author to revise it.

Page 4, Lines 77-81

Having a hypothesis often makes a paper easier to read. Authors are encouraged to state their hypothesis if they can derive it from the background already described.

Methods

Page 5, Line 85

Please indicate what the numbers indicate about the characteristics of the subjects: "age," "height," and "weight.

Page 5, Line 90

Add the details of the marker positions. This is important for the reproducibility of the experiments.

Page 5, Line 90

The setting of the measurement is one of the originalities of this study. I believe that illustrations or photographs of the measurements will help the reader's understanding and reproducibility of the experiment, and will enhance the value of the paper. It also helps the reader to understand by describing what new things this original setting allows us to know.

Page 5, Lines 103

The method for measuring the 1RM of hip thrust needs to be specified. It is also necessary to specify how the start and end postures and the speed of movement were controlled during the 1RM measurements and experiments. Since the motion and load conditions are not independent variables in this study, a description of the control of the motion is essential.

Page 5, Line 107

Although relatively slow motion is the target of the analysis, I feel that the cutoff frequency is low. I request the authors to add the evidence that the cutoff frequency of 3 Hz was appropriate for signal processing.

Page 6, Line115

With respect to the method of analysis, a description or literature on how to specifically define the local coordinate system of each segment is needed. Without it, the reader does not know the order of rotation of XYZ.

Page 6, 120

What model of inertia parameters was used in the inverse dynamics analysis? Or did you estimate inertia parameters based on morphometric measurements for each subject? Please clarify these.

Page 7 , Line114

Table 2 shows the negative d-value. I request the authors to maintain consistency with the description of the method.

Results

Page 9, Lines 177-179 & Page 13, Lines 274-276

Since Fig. 2 is an averaged figure, no information about individual differences is available in the figure. Request the authors to revise the figure or text.

Table 2 shows the negative d-value. I request the authors to maintain consistency with the description of the method.

Discussion

Pages 12-13, Lines 252-271

This section seems a bit redundant as a preamble to the results-based discussion. I recommend integrating it with the text at the end of the Discussion.

Page 13, Lines 276-279

As pointed out in the methods section, the presence or absence of movement control is one of the major factors influencing the results in the study of training movements. I request the authors to revise this text based on the revisions in the methods section.

Page 15, Lines 312-313

How much load is required for training depends on the objective. In addition, since this study has only one loading condition, what this sentence suggests seems to be an expanded interpretation of the results. Request the author to correct or delete the text.

Page 16, Liens 335-337

Future research will be aided by results-based conjecture about the possible risks of hip thrust versus squat, etc.

Conclusions

Page 17, Liens 362

What kind of movement selection can be shown in this study that has not been shown in the previous studies? (This study revealed changes in various parameters between joints and with time within the same movement. However, it does not show the superiority of hip thrust over other exercises)

Table 1

BW is not a proper unit of measurement. Please add a Note to the table or correct the units.

Table 2

No need to specify "±" in Table 2 where the results are not shown.

Fig 1

The legend in the figure does not match the title of the figure, so unification helps the reader to understand it.

Fig 2

It is recommended to add stick pictures to the figure to relate the data to the posture.

Add a horizontal line to the vertical axis of 0 to make it easier to see when the transition between flexion and extension is made.

Fig 3

I may be mistaken due to the low resolution of the figure, but what does 0.016 within Pelvic-Trunk vs Knee indicate? Please add explanations if necessary.

**********

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Reviewer #1: No

Reviewer #2: No

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Attachment

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Click here for additional data file.^{(65.7KB, pdf)}

PLoS One. 2021 Mar 29;16(3):e0249307. doi: 10.1371/journal.pone.0249307.r002

Author response to Decision Letter 0

21 Jan 2021

Editor Comments to the Author:

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming.

2. Please include captions for your Supporting Information files at the end of your manuscript, and update any in-text citations to match accordingly.

Response: Thank you for highlighting this. We have revised the manuscript, file names, and supporting information to meet PLOS ONES's style requirements.

Reviewer(s)’ Comments to the Author:

Reviewer 1

Comments to the Author

General Comments

On behalf of the authors I would like to thank you for your positive and helpful comments, which have contributed to the improvement of our manuscript. All comments have been carefully considered and where appropriate, amendments to the manuscript have been made. In particular, new Figures have been included to show the experimental setup (Figure 1) and a free body diagram (Figure 2).

Specific comments

Abstract

The results of the study are presented clearly in abstract section.

Response: Thank you for your positive comments.

Introduction

Response: Thank you for raising this, along with Reviewer 2. In the revised manuscript we have included hypotheses at the end of the introduction (line 86-88) and have altered the discussion narrative to reflect the addition of hypotheses

Methods

Response: We agree that including visual information in regard to the experimental setup and free-body diagram would benefit the manuscripts reach and readability. Therefore, new Figures have been included to show the experimental setup (Figure 1) and a free body diagram (Figure 2). The marker-set has been detailed in lines (102-106).

With respect to inverse dynamics procedures, we have revised the sentence structure in lines 148-151, to reflect and reference the use of standard procedures within Visual 3D software. The methods used for calculating joint kinetic data is publicly available on the Visual 3D website and based on the methods cited in ref [24]. The authors feel it is not necessary to also add equations to the manuscript.

Results

External kinematics and kinetics and joint kinematics and kinetics were presented clearly in Table1, Table2, and Table3, and were supported with the figures.

Response: Thank you for your positive comments on the results section. The results section has undergone minor amendments as per the recommendations of Reviewer 2 (please see below for details).

Discussion

It was reported in the study that the hip extensor moment decreased during the lift contrary to the literature. This is the most important result of the study. The main results of the study, which contradicts with the literature, are very well discussed with fine details. I applaud the authors.

Response: Again, thank you for your very positive comments. We have addressed your points below, and made further revisions based on the recommendations of Reviewer 2 (please see below for details).

• Page 14 Line 284. Both dot and comma were used together in the end of the sentence. Please, remove the comma.

Response: Amended in revised manuscript.

• Page 14 Line 303. Please, add a dot at the end of the sentence.

Response: Amended in revised manuscript.

Reviewer 2

Comments to the Author

General Comments

On behalf of the authors I would like to thank you for your helpful and positive comments, that has contributed to the improvement of our manuscript. All of your comments have been carefully considered, and where appropriate amendments to the manuscript have been made.

Specific Comments

Introduction

• Page 4, Line 60-65. Clarifying the relevance of why previous studies (10,11,13) differ in their results and the need for comprehensive biomechanical analysis will make the significance of this study clearer. I recommend the author to revise it.

Response: We have revised the structure of the manuscript around this sentence to help identify the significance of the study (line 71-76), before the original narrative that builds rationale for the biomechanical analysis conducted (line 77-84).

• Page 4, Line 71-72. The need and importance of investigating the musculoskeletal demands on the lower extremity and pelvic-trunk joints is not only cited, but also written, which makes the significance of analyzing the lower extremity and pelvic-trunk joints of the hip-thrust exercise in this study more clear. I recommend the author to revise it..

Response: We are not sure specifically what the reviewer is asking in this comment. The use of references 17-22 is to highlight that a breadth of research has been dedicated to performing joint kinetic analysis of strength training exercises, and that this effective approach has yet to be performed for the hip thrust. We believe this provides a strong rationale for the current study.

• Page 4, Line 77-81. Having a hypothesis often makes a paper easier to read. Authors are encouraged to state their hypothesis if they can derive it from the background already described.

Response: Thank you for raising this, along with Reviewer 1. In the revised manuscript we have included hypotheses at the end of the introduction (line 86-88) and have altered the discussion narrative to reflect the addition of hypotheses:

Methods

• Page 5, Line 85. Please indicate what the numbers indicate about the characteristics of the subjects: "age," "height," and "weight.

Response: This has been amended in the revised manuscript (line 94-95).

• Page 5, Line 90. Add the details of the marker positions. This is important for the reproducibility of the experiments.

Response: The revised manuscript now contains a list of maker positions used (line 102-106).

• Page 5, Line 90. The setting of the measurement is one of the originalities of this study. I believe that illustrations or photographs of the measurements will help the reader's understanding and reproducibility of the experiment and will enhance the value of the paper. It also helps the reader to understand by describing what new things this original setting allows us to know.

Response: Thank you for this comment. In line with comments from both reviewers, the revised manuscript contains two new figures in the methods section. One demonstrates the experimental setup (Figure 1), and the other a free-body diagram of the hip thrust exercise (Figure 2). We have also added new supplementary material alongside the manuscript, which shows a Visual 3D video of an example hip thrust trial.

• Page 5, Line 103. The method for measuring the 1RM of hip thrust needs to be specified. It is also necessary to specify how the start and end postures and the speed of movement were controlled during the 1RM measurements and experiments. Since the motion and load conditions are not independent variables in this study, a description of the control of the motion is essential.

Response: As the participants were well-trained individuals regularly performing the hip thrust exercise, 1RM was self-reported by the participants based on their most recent training performances. To maintain ecological validity, participants were asked to execute the movement using their habitual technique and timing, with the instruction to perform each repetition with maximum intent during the lifting phase to full hip extension, before controlling the barbell back to the starting position. Each repetition was initiated from this starting position, with the barbell coming to a rest on the ground between each of the three repetitions. This has been detailed in the revised manuscript (lines 119-124).

• Page 5, Line 107. Although relatively slow motion is the target of the analysis, I feel that the cut-off frequency is low. I request the authors to add the evidence that the cut-off frequency of 3 Hz was appropriate for signal processing.

Response: Thank you for your comment. The cut-off frequency used in the filtering process was based on residual analysis techniques (Winter, 2009). This statement and reference has been added to the revised manuscript (line 133-134). Similar studies in the area (below) have utilised cut-off frequencies in the 3-6 Hz range for kinematic data, aligning with the current study. Further, the frequency of the movement being analysed is well below 3 Hz, further justifying this as the cut-off frequency

Swinton PA, Lloyd R, Keogh JW, Agouris I, Stewart A. A Biomechanical Comparison of the Traditional Squat, Powerlifting Squat, and Box Squat. The Journal of Strength and Conditioning Research. 2012;26(7):1805-1816.

Bryanton, Megan A., et al. "Effect of squat depth and barbell load on relative muscular effort in squatting." The Journal of Strength & Conditioning Research 26.10 (2012): 2820-2828.

Gullett, Jonathan C., et al. "A biomechanical comparison of back and front squats in healthy trained individuals." The Journal of Strength & Conditioning Research 23.1 (2009): 284-292.

Southwell, Daniel J., et al. "The effects of squatting footwear on three-dimensional lower limb and spine kinetics." Journal of Electromyography and Kinesiology 31 (2016): 111-118.

• Page 6, Line 115. With respect to the method of analysis, a description or literature on how to specifically define the local coordinate system of each segment is needed. Without it, the reader does not know the order of rotation of XYZ.

Response: To provide clarity in the local coordinate system (SCS), the manuscript has been amended to include reference to the SCS being defined in the same right-handed orthogonal coordinate system as the global reference (line 145-146).

• Page 6, Line 120. What model of inertia parameters was used in the inverse dynamics analysis? Or did you estimate inertia parameters based on morphometric measurements for each subject? Please clarify these.

Response: The segment mass an inertial parameters used were consistent with the Visual 3D default (Dempster & Hanavan). The manuscript has been updated accordingly (line 148-151), referencing the source of the information and highlighting that all inverse dynamics procedures were completed using the default methods in Visual 3D (as with Southwell et al., 2016).

• Page 7, Line 124. Table 2 shows the negative d-value. I request the authors to maintain consistency with the description of the method.

Response: To address both directions of ‘d’, we have included two amendments to the revised manuscript. The first, is the addition of “equivalent scale used for negative values of d” in line 181. Secondly, in the footer of Table 3, we have now added the wording “the direction of d indicates whether the joint on the left (positive) vs. right (negative) was of larger magnitude.” To give context to the direction of ‘d’ for each comparison.

Results

• Page 9, Line 177-179/ Page 13, Line 274-276. Since Fig. 2 is an averaged figure, no information about individual differences is available in the figure. Request the authors to revise the figure or text.

Response: We feel that the point of discussion is valid based on the original construction of Figure 2 (now Figure 3) that shows the ensemble group mean and standard deviation. The shading in Figure 3 that covers both positive and negative values of ankle joint moment represents the variability between the nineteen participants in the study, and it is valid to interpret the standard deviation as inter-individual variation across the sample. Whilst the figure does not allow direct comparison of individual participants, the group standard deviations indicates that there was inter-individual variation in the nature of dorsi- / plantar-flexion moment during the lift.

In the revised manuscript, “inter-individual” has been removed from the results narrative (line 2010-211) and figure captions have been modified to explicitly state that data shown is ensemble group mean and standard deviation.

• Table 2. Shows the negative d-value. I request the authors to maintain consistency with the description of the method.

Response: Please see response above regarding the changes made in relation to this comment.

Discussion

• Page 12-13, Line 252-271. This section seems a bit redundant as a preamble to the results-based discussion. I recommend integrating it with the text at the end of the Discussion.

Response: Thank you for this comment. We agree that this section is better placed toward the end of the discussion and have moved the section accordingly in the revised manuscript (line 356-375).

• Page 13, Line 276-279. As pointed out in the methods section, the presence or absence of movement control is one of the major factors influencing the results in the study of training movements. I request the authors to revise this text based on the revisions in the methods section.

Response: In addition to the justification offered above, the focus of these lines was to offer mechanical explanation for the standard deviation overlapping dorsi- and plantar-flexion moments shown in Figure 2. Participants were encouraged to perform the movement with their habitual technique which likely resulted in the observed variation, and has been added to the manuscript (line 295).

• Page 15, Line 312-313. How much load is required for training depends on the objective. In addition, since this study has only one loading condition, what this sentence suggests seems to be an expanded interpretation of the results. Request the author to correct or delete the text.

Response: The aim of the sentence is to summarise one of the key findings of the study, which is that the hip joint moment was not maximal at full extension, which has been postulated in previous studies using EMG analysis. To keep the focus on the current results, and not speculate on advised range of motion to be used in training, the final part of the sentence has been deleted.

• Page 16, Line 335-337. Future research will be aided by results-based conjecture about the possible risks of hip thrust versus squat, etc.

Response: We feel as if the comment is addressed in the current narrative, reflecting that further investigation is required in regard to spinal loading, as well as highlighting the need for empirical studies comparing the hip thrust with other common training exercises.

Conclusions

• Page 17, Line 362. What kind of movement selection can be shown in this study that has not been shown in the previous studies? (This study revealed changes in various parameters between joints and with time within the same movement. However, it does not show the superiority of hip thrust over other exercises).

Response: Thank you for this comment. The reference to “exercise selection” in the manuscript is the process through which coaches/ athletes decide on which exercises to utilise within training. The new insight to the kinetic demands of the hip thrust exercise from the current study may offer rationale for including/ excluding the hip thrust based on its underlying mechanical features. For example, the study has highlighted that indeed the hip thrust does place a large demand on the hip extensors, but not a consistent demand throughout the movement, or at the end range of hip extension, which has been used as rationale for the exercises’ benefit through previous research. It is not the intention of the authors to suggest its superiority here, but to recognise that increased biomechanical knowledge of the exercise can inform the exercise selection process. We feel that the current wording in the manuscript reflects this explanation.

Tables & Figures

• Table 1. BW is not a proper unit of measurement. Please add a Note to the table or correct the units.

Response: A footnote to Table 1 has been added to the revised manuscript (line 200-201), detailing that external force data have been normalised to bodyweight (BW). This procedure has also been defined in the methods section (line 43) and is commonly used in sports biomechanics literature to report external force data.

• Table 2. No need to specify "±" in Table 2 where the results are not shown.

Response: Thank you for noticing this.

• Figure 1. The legend in the figure does not match the title of the figure, so unification helps the reader to understand it.

Response: Thank you for noticing this. Both the caption and legend for Figure 1 (now Figure 3) has been amended in the revised manuscript.

• Figure 2. It is recommended to add stick pictures to the figure to relate the data to the posture.

Add a horizontal line to the vertical axis of 0 to make it easier to see when the transition between flexion and extension is made.

Response: In the revised manuscript, a horizontal line at the y-axis 0-intercept has been added to better show the transition between positive and negative phases of moment/ power. The authors wish to exclude further information from the figure (e.g. stick pictures) to encourage the reader to consider the entire lifting phase, and not specific, discrete points when viewing this figure. In addition, stick figures are not commonly utilised in this type of analysis (e.g. Swinton et al., 2011; 2012, Southwell et al., 2016).

• Figure 3. I may be mistaken due to the low resolution of the figure, but what does 0.016 within Pelvic-Trunk vs Knee indicate? Please add explanations if necessary.

Response: The SnPM inference process returns a unique p-value for each supra-threshold cluster. The scalar output statistic (SnPm{t}) for the Pelvic-Truck vs Knee interaction exceeded the critical threshold at two unique time periods. As such the value in question represents the adjusted p-value for the second supra-threshold cluster. This has been clarified in the caption for Figure 3 (now Figure 5 in the revised manuscript).

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PLoS One. doi: 10.1371/journal.pone.0249307.r003

Decision Letter 1

Matti Douglas Allen

4 Mar 2021

PONE-D-20-27535R1

A comprehensive biomechanical analysis of the barbell hip thrust

PLOS ONE

Dear Dr. Brazil,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we are pleased to notify you your paper will be accepted at PLOS ONE pending the very minor revision suggested by the reviewer. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Once this revision is complete, we anticipate a very quick acceptance and initiation of the final stages in publication.

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Academic Editor

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Reviewers' comments:

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Reviewer #1: All comments have been addressed

Reviewer #2: (No Response)

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

Reviewer #1: Yes

Reviewer #2: Yes

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

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Reviewer #2: Yes

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

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Reviewer #2: Yes

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Reviewer #2: Yes

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6. Review Comments to the Author

Reviewer #1: (No Response)

Reviewer #2: Manuscript Number: PONE-D-20-27535R1

Manuscript Title: A comprehensive biomechanical analysis of the barbell hip thrust

Comments to the Author

Thanks to authors for making appropriate revisions. I believe that this research will develop resistance training and help athletes and coaches.

------------------------------------------------------------------------------

Mayor comments;

Figure 2

How did you handle the moment acting between the toe and the ground (I guess authors assumed that moment to be zero)? The details of the method should be clearly stated.

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Reviewer #1: No

Reviewer #2: No

PLoS One. 2021 Mar 29;16(3):e0249307. doi: 10.1371/journal.pone.0249307.r004

Author response to Decision Letter 1

5 Mar 2021

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.

Thank you for this comment. From the original submission there have been no references removed from the manuscript. Following the first revision, three references were added to the manuscript (23, 25, 26) to address reviewer comments, which resulted in changes to the numerical order of the in-text citations. The first revision of the manuscript highlighted the changes to the numerical order of in-text citations, but this did not reflect a removal/ change in references used. Since the original submission, reference 8 had been allocated a volume and page number, which was updated in the first revision of the manuscript.

Reviewer(s)’ Comments to the Author:

Specific comments

Figure 2. How did you handle the moment acting between the toe and the ground (I guess authors assumed that moment to be zero)? The details of the method should be clearly stated.

• In the current analysis, the foot was modelled as a single rigid segment with the distal endpoint at the metatarsophalangeal joint (MTPJ). The moment between the distal end of the foot and ground was not included in our analysis and was assumed to be zero. As the foot was modelled as a single segment, the joint moment acting at the MTPJ (between foot and toe) was also not considered in our analysis, which is consistent in the resistance training literature. Whilst the MTPJ has been shown to generate and absorb energy in locomotive tasks (e.g. sprinting), evidence has suggested (e.g. Bezodis et al., 2012. DOI: https://doi.org/10.1123/jab.28.2.222) that if the specific focus of the study is the kinetics of the ankle, knee and hip, the current method is appropriate.

In the revised manuscript, the following sentence has been included to the methods section for clarity (line 151-154):

“The distal end of the foot was defined at the metatarsophalangeal joint, and the moment acting between the distal end of the foot and the ground was assumed to be zero.”

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PLoS One. doi: 10.1371/journal.pone.0249307.r005

Decision Letter 2

Matti Douglas Allen

16 Mar 2021

A comprehensive biomechanical analysis of the barbell hip thrust

PONE-D-20-27535R2

Dear Dr. Brazil,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Matti Douglas Allen, MD, PhD

Academic Editor

PLOS ONE

PLoS One. doi: 10.1371/journal.pone.0249307.r006

Acceptance letter

Matti Douglas Allen

19 Mar 2021

PONE-D-20-27535R2

A comprehensive biomechanical analysis of the barbell hip thrust

Dear Dr. Brazil:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Matti Douglas Allen

Academic Editor

PLOS ONE

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

S1 Appendix. Video file demonstrating example hip thrust trial in Visual 3D.

(MP4)

Click here for additional data file.^{(5.6MB, mp4)}

Attachment

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Click here for additional data file.^{(65.7KB, pdf)}

Attachment

Submitted filename: Response to reviewers.docx

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Submitted filename: Response to Reviewers.docx

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Data Availability Statement

All data files are available from figshare (DOI: https://doi.org/10.6084/m9.figshare.13582145.v2).

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PERMALINK

A comprehensive biomechanical analysis of the barbell hip thrust

Adam Brazil

Laurie Needham

Jac L Palmer

Ian N Bezodis

Roles

Abstract

Introduction

Materials and methods

Fig 1. Image of experimental setup including location of force platforms, direction of resultant force vectors (blue lines), orientation and position of the body segments and barbell, and marker-set (green circles).

Fig 2. Representative two-dimensional free-body diagram of the barbell hip thrust.

Results

External kinematics & kinetics

Table 1. External characteristics.

Fig 3. Ensemble group mean (± SD gray shaded areas) external forces for the thorax-bench (thorax) and foot-ground (feet) interfaces.

Joint kinematics & kinetics

Fig 4. Group ensemble mean (± SD gray shaded areas) joint kinematic and unilateral kinetic time series data.

Table 3. Joint characteristics post-hoc comparison effect sizes (d) and 95% Lower (L) and Upper (U) confidence intervals.

Table 2. Joint characteristics.

Fig 5. Ensemble group mean (± SD gray shaded areas) joint moment time series data for the pelvic-trunk, bilateral hip and bilateral knee joints.

Discussion

Conclusions

Practical implications

Supporting information

Acknowledgments

Data Availability

Funding Statement

References

Decision Letter 0

Matti Douglas Allen

Roles

Author response to Decision Letter 0

Decision Letter 1

Matti Douglas Allen

Roles

Author response to Decision Letter 1

Decision Letter 2

Matti Douglas Allen

Roles

Acceptance letter

Matti Douglas Allen

Roles

Associated Data

Supplementary Materials

Data Availability Statement

ACTIONS

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RESOURCES

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