Kinematic characteristics of the tennis serve from the ad and deuce court service positions in elite junior players

Janina Fett; Nils Oberschelp; Jo-Lâm Vuong; Thimo Wiewelhove; Alexander Ferrauti

doi:10.1371/journal.pone.0252650

. 2021 Jul 22;16(7):e0252650. doi: 10.1371/journal.pone.0252650

Kinematic characteristics of the tennis serve from the ad and deuce court service positions in elite junior players

Janina Fett ^1,^*, Nils Oberschelp ¹, Jo-Lâm Vuong ¹, Thimo Wiewelhove ¹, Alexander Ferrauti ¹

Editor: Marc HE de Lussanet²

PMCID: PMC8297898 PMID: 34292954

Abstract

Purpose

According to the official rules of the International Tennis Federation, players have to serve alternately from two different positions: the deuce (right, D) and the ad court (left, AD) side. This study aimed to compare body and ball kinematics of flat serves from both service sides.

Methods

In a controlled, semi-court laboratory setting, 14 elite male junior players served eight flat first serves to a target field directed to the receiver’s body from both service positions in a matched and counterbalanced order. An 8-camera-Vicon-System was used to capture the 3D-landmark trajectories.

Results

The mean service velocity was found to be similar on both sides (D: 151.4 ± 19.8 vs. AD: 150.5 ± 19.4 km/h), while multiple characteristics of the serve and ball kinematics differed significantly (p < .05). At starting, the front-foot angle relative to the baseline (D: 39.7±17.6° vs. AD: 31.1±17.4°) and lateral distance between the feet (D: 16.3 ± 12.9 cm vs. AD: 26.2 ± 11.9 cm) were significantly different. During the service, upper torso range of motion from maximum clockwise rotation until impact was significantly greater on the deuce court (D: 130.5 ± 19.8° vs. AD: 126.7 ± 21.1°). This was especially pronounced in foot-back technique players. Further, differences in the lateral ball impact location (D: 30.0 ± 24.1 cm vs. AD: 10.3 ± 23.3 cm) were observed.

Conclusions

Changing the service side affects the serve and ball kinematics in elite junior tennis players. Our results underline biomechanical differences regarding the starting position (feet and upper torso) as well as the movement and ball kinematics which could be relevant for skill acquisition, injury prevention and performance enhancement.

Introduction

In tennis, all points start with the serve, which has become the most important stroke and a key factor of game success [1–3]. This stroke allows players to gain points with very short rallies and percentage of points won after the first serve correspond to around 72–81% [4, 5]. Additionally, a successful first serve has evolved into a powerful tool to achieve direct points or to take instant initiative within a rally [6]. Nevertheless, it is also the most difficult stroke to master due to the complex combination and coordination of limb and joint movements required to summate and transfer forces from the ground up into the racquet head, reported as the kinetic chain [7]. Therefore, an appropriate skill acquisition (i.e., movement execution, technical skill, coordination of kinetic chain) is needed from beginners to elite junior players to ensure the best possible outcomes regarding a powerful and efficient serve.

According to the official rules of the International Tennis Federation (ITF), players must serve alternately from the two different positions, the deuce (right) and the ad court (left) side, into the diagonally opposite service boxes [8]. The percentage of serves performed per match from either side is almost identical (47.6% and 52.4% from AD and D, respectively) since only games ending after a 40:15 or 15:40 result in a higher number of serves from the deuce court side. Depending on the tactical strategy, different locations can be targeted: out wide, to the receiver’s body, and to the T (i.e., near the centre service line) [9].

Interestingly, from a biomechanical point of view, only one service model is mediated [7], not taking into account the service side. Usually, the serve is characterized by a corkscrew motion: after the ball toss, the serving arm moves behind the body, and the vertebral column is laterally flexed and hyperextended with a fully loaded lower body position [10]. Acceleration of the serving arm and racquet before ball impact is accompanied by a fast counter-rotation of the lumbar spine—from hyperextension to flexion, and from right twist to left twist [7, 10, 11]. Under the current recommendations for the starting position, players are instructed to put their front foot towards the right net post, and their rear foot parallel to the baseline (for a right-handed player). In an attempt to achieve stability, a common advice is to place the toes of the rear foot in line with the heel of the front foot [12, 13].

Due to its great importance, the serve has received significant biomechanical interest. A large number of investigations have examined the kinematic characteristics of lower and upper limb and trunk joint motion [1, 3, 10, 14–18], racquet and ball kinematics [2, 19–22], kinematics in relation to performance level, gender, age, and injury [10, 23–26] and effect of serve type (flat, kick, slice) [27, 28]. A small body of literature has focused on differences in serving locations [20, 22], but no study has examined serve kinematics according to the different service sides: the ad versus the deuce service side. It can be hypothesized that changing the service side can lead to different body and ball kinematics. To the best of our knowledge, this is the first study to examine serve kinematics from both the deuce and ad court sides. Thus, this study aimed to compare the body and ball kinematics of flat serves from the deuce and ad court side, targeting the centre of the service box (i.e., serving to the receiver’s body).

Materials and methods

Subjects

Fourteen male elite junior squad players of the German Tennis Federation (age: 14.6 ± 1.8 years, age at peak height velocity (APHV): 14.0 ± 0.7 years, maturity offset (MO): 0.6 ± 1.9, weight: 61.4 ± 16.3 kg, height: 176.0 ± 15.9 cm) participated in this study. Twelve players were right-handed and two left-handed. Considering the foot technique, there were six foot-up and eight foot-back technique players. Players were ranked on the national youth ranking list and had a weekly training volume (without competition) of 10.0 ± 2.6 hours on tennis specific training (i.e., technical and tactical skills). Participants were excluded if they had musculoskeletal injury (upper extremity surgery, shoulder, back, knee, ankle pain) within the past 12 months, conduct any sport-related rehabilitation during the 12 month prior the study or had any other kind of pain during the service execution.

This study was approved by the ethics committee of the Faculty of Sport Science of the Ruhr University Bochum (EKS-24072017), and all procedures conformed to the recommendations and guidelines of the Declaration of Helsinki. The players and parents were fully informed of all experimental procedures, and both players and parents provided written informed consent before participation.

Experimental design

Procedures

The investigation was conducted in a specific semi-court indoor tennis laboratory on a hard-court surface (Rebound Ace). Players had to serve against an absorption wall with a target field (direction to the receiver’s body from both sides). The position and size of the target field on the absorption wall were calculated such that it would be a valid serve on a full-size tennis court directed to the receiver’s body. For this calculation, triangulation was used to transfer the specific dimensions to the laboratory setting. A schematic screen is available as S1 Fig.

Each player was fitted with 86 retro-reflective markers placed on anatomical landmarks using double-sided tape according to the UWA full-body marker set [26], which is further characterized by a cluster method that saw three markers attached to each segment [16]. Additionally, five markers were fixed to the players’ racquets to create coordinate systems therein [26], and retro-reflective tape was placed on the ball in order to determine ball data [29]. To limit movement of the markers from their anatomical landmarks, all players wore tight shorts only. Familiarization with the testing surrounding and the landmark set has been implemented prior to the experiment.

All players completed a standardized warm-up protocol prior to testing. The standardized warm-up consisted of general movement preparation exercises, specific activation and mobilization with elastic tubes, and a set of submaximal serves with increasing velocities (total: 32; 16 at 50–70%, eight at 70–80%, and eight at 90–100% in accordance to their subjective feeling). For the testing protocol each player performed two sets of eight maximum “first flat” serves to the target field (direction: receiver’s body; size: 67 × 30 cm). The service execution was performed from both service sides (deuce and ad court) in a matched and counterbalanced order. After each set, there was a two minute rest period, between each serve there was a rest of 30s to prepare for the next service. Participants were instructed to serve the ball exactly as they would during a match with maximum power into the predefined target zone. To maximize ecological validity, players used their own racquets to feel as comfortable as possible during their serves [26].

The three fastest serves that landed in the target area were analysed [30]. Four players failed to make three valid attempts within eight serves. Because of that four players had to perform in total (both sides) three to four further serves to get three serves in on each side. Serve velocity was measured using a radar gun (Stalker Professional Sports Radar; Radar Sales, Plymouth, MN). The radar was located 2 m behind the server. It was aligned with the approximate height of ball contact (~ 3 m) and was aimed down the target zone. Peak velocity of each stroke was recorded.

Kinematic analysis

An 8-camera Vicon Vantage V5-System operating at 300 Hz was used to capture the three-dimensional (3D) landmark trajectories to reconstruct the service motion. After capturing all static and dynamic trials, the trajectories were reconstructed with Vicon Nexus software (Nexus, Vicon, Oxford, UK). The experimental data analysis was conducted on the basis of prior kinematic studies carried out by a research group of the University of Western Australia [17, 19, 21–23, 26]. According to the methods prescribed in their previous work [17, 22, 26], gaps were interpolated using a cubic spline; data were filtered using a Woltring filter and subsequently modelled with a customized version of the University of Western Australia model to calculate relevant anatomical, racquet, and ball data [16, 17, 19, 21–23, 26]. Ball data were held relative to the front foot. Therefore, the origin of the global coordinate system was translated to the position of the first metatarsal marker (prior to the initiation of each participant’s backswing) [22]. Negative x pointed lateral to the right along the baseline, negative y pointed towards to the net, and positive z pointed vertically upwards. The Euler Z-X-Y sequence was used to describe joint rotations, except at the shoulder where Y-X-Y decomposition was applied to estimate shoulder joint motion, as recommended by the International Society of Biomechanics [16, 17, 19, 31]. The kinematic data of left-handed players were inverted where appropriate [17, 26].

Kinematic variables

The service action was divided into different parts to analyse kinematic variables of interest: (1) starting position, (2) preparation phase, (3) propulsion, and (4) impact. Starting position was defined as when the player begins to initiate the ball toss. Time phase until peak shoulder external rotation was defined as the preparation phase; the phase from this key point to impact was defined as propulsion [17]. All variables of interest are shown in S1 Appendix.

During the starting position, the position of the feet and upper torso axis was measured relative to the baseline (x-axis of the global coordinate system). Upper torso rotation was calculated by a vector joining the two shoulder joint centres, which was expressed relative to the global x-axis [21]. From an overhead view, forward rotation (+) was counter-clockwise, and backward rotation (-) was clockwise for right-handed players. Foot axis was calculated by a vector joining the middle of the forefoot and the heel. In addition, the lateral distance between both toes (first metatarsal) was measured.

During the preparation phase, the analysed parameters were knee flexion, trunk extension, trunk tilt, external shoulder rotation, and elbow flexion. In addition, maximum upper torso position relative to the baseline and counter- upper torso rotation range of motion (ROM) was measured. Counter-upper torso ROM was defined as the ROM between upper torso starting position (relative to baseline) and maximum upper torso position (relative to baseline).

The peak angular velocities during the propulsion phase were focused on the knee extension, trunk flexion, trunk tilt, elbow extension, as well as shoulder internal rotation.

At the time of impact, knee extension, trunk extension, trunk tilt, upper torso position relative to baseline, upper torso ROM, shoulder abduction, and elbow extension were analysed. Upper torso ROM was defined as the maximum backward rotation (during preparation) to forward rotation until impact. The ball kinematics at impact were calculated relative to the first metatarsal of the front foot at the instant of starting position [19, 22].

S2 Appendix shows reliability data for all analysed variables. Overall, ‘good’ and ‘excellent’ reliability was found for nearly each variable (ICCs ranging from 0.76 to 0.99). Front knee extension velocity and shoulder internal rotation velocity during propulsion as well as front and back knee flexion at impact showed a ‘moderate’ reliability (ICCs ranging from 0.58 to 0.73) [32].

Anthropometric measurements

Participants body height and sitting height was measured to the nearest mm with a fixed stadiometer (Holtain Ltd., Crosswell, UK). Further, a purpose-built table was used for measuring sitting height. Body mass was recorded to the nearest 0.1 kg with a bioelectrical-impedance scale InBody770 (InBodyCo., Ltd., Gangnam-gu, Seoul, Korea). Status of maturity was calculated according to the maturity offset method as previously described [33, 34].

Statistical analysis

All data are presented as mean values and standard deviations (± SD). Paired sample t-tests were used to determine differences in kinematics between both sides, the deuce and ad court side. Additional, in case of non-normality the Wilcoxon-test was used. Further, the standardized difference or effect size (ES) of changes in each parameter between the two groups were calculated using the pooled standard deviation. Threshold values for Cohen’s d ES statistics were < 0.2 (small), 0.5 (moderate), and > 0.8 (large) [35]. For subgroup analysis of the total upper torso ROM (foot-up and foot-back players), a repeated-measures ANOVA was used. Intra-session reliability of all variable measures was calculated. Intraclass correlation coefficient (ICC 3,1) and standard error of measurement (SEM) (90% confidence limits) were assessed by the three testing trials and pooled between both service sides using the spreadsheets for analysis of validity and reliability of Hopkins [36]. Statistical significance was set at p ≤ .05. Data analyses were performed using the free statistical software JASP (version 0.11.1) and Microsoft Office 365 MSO (version 16.0.13029.20232).

Results

Serve kinematics

Mean values (± SD) of serve and ball kinematics for both conditions (deuce and ad court) are presented in Table 1. Comparing body kinematics on both service sides, there were significant differences in foot position as well as upper torso position at starting position. The front-foot angle (relative to baseline) was higher on the deuce court compared to the ad court side (deuce: 39.7 ± 17.6° vs. ad: 31.1 ± 17.4°; ES 0.49). The lateral distance between both feet was less on the deuce court (deuce: 16.3 ± 12.9 cm vs. ad: 26.2 ± 11.9 cm; ES -0.80) (Fig 1). Upper torso position at starting position was higher on the ad court (deuce: -60.9 ± 15.7° vs. ad: -69.6 ± 15.0°; ES -0.57).

Table 1. Peak and terminal kinematic variables of interest for the ad and deuce service side.

		Advantage court			Deuce court
	Unit	Mean		SD	Mean		SD	p	ES
Starting position
Front foot position to baseline	[deg]	31.1	±	17.4	39.7	±	17.6	.000	0.49
Back foot position to baseline	[deg]	12.0	±	9.8	12.0	±	6.6	.988	-0.01
Lateral feet distance §	[cm]	26.2	±	11.9	16.3	±	12.9	.000	-0.80
Upper torso position to baseline	[deg]	-69.6	±	14.6	-60.9	±	15.7	.002	-0.57
Preparation
Front knee flexion	[deg]	69.5	±	15.0	71.6	±	15.7	.025	0.14
Back knee flexion	[deg]	75.0	±	10.6	77.6	±	9.6	.067	0.26
Trunk extension	[deg]	-44.0	±	10.6	-44.2	±	10.3	.548	-0.02
Trunk tilt	[deg]	19.2	±	6.5	19.4	±	5.8	.649	0.03
Max. upper torso position	[deg]	-120.5	±	10.3	-105.6	±	9.5	.000	-1.50
Counter-upper torso rotation ^#	[deg]	50.9	±	16.1	44.7	±	15.3	.020	-0.39
Shoulder external rotation	[deg]	138.1	±	11.4	136.7	±	10.6	.125	0.13
Elbow flexion	[deg]	132.2	±	10.4	132.7	±	9.8	.553	0.05
Propulsion
Front knee extension ω	[deg/s]	0443.6	±	108.0	0447.2	±	99.1	.756	-0.03
Back knee extension ω	[deg/s]	0540.4	±	80.4	0517.9	±	101.9	.066	0.25
Trunk flexion ω	[deg/s]	0506.7	±	69.0	0493.2	±	71.2	.111	-0.19
Trunk tilt ω	[deg/s]	0421.0	±	99.6	0424.0	±	96.5	.696	-0.03
Shoulder internal rotation ω	[deg/s]	1970.9	±	275.9	2028.7	±	331.7	.203	-0.19
Elbow extension ω	[deg/s]	1546.5	±	303.1	1563.6	±	327.0	.584	-0.05
Wrist flexion ω	[deg/s]	1095.4	±	339.9	1070.9	±	299.5	.439	-0.08
Impact
Front knee flexion	[deg]	26.2	±	10.4	29.1	±	10.3	.003	0.28
Back knee flexion $1	[deg]	5.6	±	8.1	6.7	±	8.2	.474	0.13
Trunk extension	[deg]	-8.0	±	9.6	-7.6	±	9.5	.327	0.05
Trunk tilt	[deg]	-27.6	±	4.4	-27.2	±	4.1	.482	0.07
Upper torso position to baseline	[deg]	6.2	±	16.6	24.9	±	16.5	.000	1.13
Upper torso rotation (ROM)	[deg]	126.7	±	21.1	130.5	±	19.8	.008	0.18
Shoulder abduction	[deg]	114.5	±	6.4	114.0	±	6.4	.427	-0.07
Elbow flexion	[deg]	18.0	±	8.5	18.0	±	7.8	.998	0.00
Wrist extension $2	[deg]	-20.5	±	6.9	-20.3	±	6.2	.796	0.03
Ball kinematics
Ball velocity	[km/h]	150.5 ± 19.4	±	19.4	151.4	±	19.8	.505	0.04
Ball impact location X (lateral)	[cm]	10.3	±	23.3	30.0	±	24.1	.000	0.83
Ball impact location Y (forward)	[cm]	-49.9	±	17.7	-51.6	±	20.9	.552	-0.09
Ball impact location Z (upward)	[cm]	260.0 ± 23.1	±	23.1	258.0	±	22.7	.086	-0.08

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$1 non-normality distribution: Wilcoxon Test: p = .194; $2 non-normality distribution: Wilcoxon Test: p = .241

§ first metatarsal of front foot to first metatarsal of rear foot; # range of motion between upper torso starting position and maximum upper torso position during preparation; ω maximum angular velocity; ES effect size

During preparation, higher max. upper torso position (clockwise) was found on ad court side (deuce: -105.6 ± 9.5° vs. ad: -120.5 ± 10.3°; ES -1.50). Further, counter-upper torso ROM was higher on ad court side (deuce: 44.7 ± 15.3° vs. ad: 50.9 ± 16.1°; ES -0.39).

At impact, there was a higher knee flexion in the front leg on the deuce court side (deuce: 29.1 ± 10.3° vs. ad: 26.2 ± 10.4°; ES 0.28). Upper torso ROM from maximum clockwise rotation until impact was significantly greater on the deuce court side (deuce: 130.5 ± 19.8° vs. ad: 126.7 ± 21.1°; ES 0.18). Additionally, the alignment of the upper torso at impact was to be rotated significantly further forward on the deuce court side relative to baseline (deuce: 24.9 ± 16.5° vs. ad: 6.2 ± 16.6°; ES 1.13).

Additional subgroup analysis revealed descriptively slightly higher side differences in the upper torso ROM for foot-back players (n = 8, diff = 4.8° ± 4.8°, p = .026) compared to foot-up players (n = 6, diff = 2.4 ± 4.2°, p = .215). However, the side × subgroup interaction was unclear (p = .360) (Table 2 and Fig 2).

Table 2. Upper torso ROM in foot-up and foot-back players.

Upper torso rotation (ROM)		Advantage court			Deuce court			Difference				Side x group
Upper torso rotation (ROM)	Unit	Mean		SD	Mean		SD	Mean		SD	p	p
Foot up (n = 6)	[deg]	129.5 ± 23.1	±	24.1	131.9	±	23.2	2.4	±	4.2	.215	.360
Foot back (n = 8)	[deg]	124.7	±	20.0	129.5	±	18.4	4.8	±	4.8	.026	.360

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SD: standard deviation; ROM: range of motion

Ball kinematics

Mean service velocity was similar from both sides (deuce: 151.4 ± 19.8 km/h vs. ad: 150.5 ± 19.4 km/h; ES 0.04). Lateral ball impact location (X) was more leftwards on the deuce court (deuce: 30.0 ± 24.1 cm vs. ad: 10.3 ± 23.3 cm; ES 0.83) from a back-view position (Table 1).

Discussion

This is the first study to examine the effects of the deuce and the ad court side on body and ball kinematics during flat serves in elite junior tennis players. Obtaining knowledge about the respective serve characteristics is quite important since, according to the rules of the ITF, the distribution of serves performed from both sides is similar for almost every game score (except for a game win after 40:15 or 15:40). Our data demonstrate various significant differences regarding the starting position, as well as serve kinematics, when changing the service side. Specifically, these differences involve the foot starting position (front-foot angle to baseline), the upper torso position in relation to baseline, the total upper torso ROM until impact, the knee flexion of the front leg at impact and the lateral ball impact point. However, no differences were found for service speed (Table 1). These deviations are surprising, since in most coaches’ textbooks only one service technique is taught [7, 12, 13].

Regarding the basic foot position prior to the service movement, the front-foot and back-foot axis seem largely similar on the deuce and ad court side when comparing foot angle in relation to the baseline (Table 1 and Fig 1). A similar service basic position related to the baseline is recommended in the tennis literature (i. e. the front foot is aligned towards the right net post, and the rear foot is placed parallel to the baseline) [12, 13], with acceptable inter-individual variations [7] and is widely accepted by professional players and coaches. Interestingly, during stroke preparation, propulsion and ball impact, we found a significantly higher upper torso ROM until ball impact, with higher knee flexion of the front leg on the deuce court side (Table 1 and Figs 3 and 4). These changes were accompanied by a more leftwards ball impact point (Fig 4).

Fig 3 — A) red line: mirror image transferred from the ad court kinematics to the deuce court (total ROM: 126.7°, counter-rotation ROM: 50.9°); B) red line: mirror image transferred from the deuce court kinematics to the ad court (total ROM: 130.5°, counter-rotation ROM: 44.7°).

In general, our kinematic data are comparable to previous findings [2, 18, 21, 22]. Nevertheless, the small, but significant, kinematic differences found between both service sides need to be discussed in greater depth. As this is the first study to focus on this question, a comparison with other studies is limited. We therefore assume that the higher upper torso ROM on the deuce court side compared to the ad court side, as shown in our study (Table 1 and Figs 3 and 4), is mainly affected by the relationship between the basic foot and upper torso position and the respective target point in the diagonally opposite service box. Obviously, despite a (only) marginal adjustment of the starting position to a more open stance on the deuce court (Fig 1), this trend does not meet the complete requirements for an identical shaping of the service movement on both sides (i.e. similar upper torso ROM).

Regarding the underlying reasons for these surprising results, it has to be considered that the service is a highly asymmetric task from a perceptual point of view. The spatial–visual orientation differs significantly between the two service sides. During preparation, the visual perception is decreased on the deuce court side in terms of the target field during the early preparation phase which may have an effect on the player´s behaviour and the biomechanical outcome [37]. It further can be speculated that, for most players and coaches, the baseline seems to be the first point of reference for the foot position during serve preparation. Historically, various top players have even taken a parallel foot position entirely behind the baseline on both service sides (e. g. John McEnroe, USA). From a biomechanical point of view, and to simplify the motor learning process, a more open stance on the deuce court could be recommended. This change would adjust the total angle and body alignment towards the target, thereby leading to a more uniform and mirrored movement pattern on both service sides, as illustrated in Fig 3. This issue seems to be mainly relevant during skill acquisition in young players and might simplify the learning process in the early career stages. In the case of repeated and considerable weaknesses on one of the two service sides, even elite players could reflect a slight adjustment.

The individual service foot technique has to be considered as representing an important individual characteristic in this context. In general, there are two different types of foot techniques: the foot-up (during preparation, the back foot is moved forward next to the front foot) and the foot-back technique (players leave the back foot in relatively the same position) [38]. In our study, we noted a homogenous distribution of players using either foot technique. This is consistent with the general distribution in male elite junior players, where 52% of the players use the foot-back technique [39]. Regarding the side differences in the upper torso ROM, from a descriptive point of view, a stronger effect could be expected in the foot-back technique players because the basic foot position is neutralised early during the foot-up technique (Fig 2). Nevertheless, significant interactions (side × group) are missing.

A comparison of the lateral hitting positions on both sides reveals a more leftward ball impact point on the deuce court side, with a difference of 20 cm between the two service sides (back-view position) (Table 1 and Fig 4). The lateral hitting point from the deuce court in our study (30 cm) is consistent with the findings of Reid et al. (2010) [2]. Interestingly, older professional players impact the ball less leftwards than junior players do [20, 22]. This age-dependant difference might be due to a stronger spin serve by the juniors [22], compensating for body height differences, thereby resulting in an overall lower ball impact point and, thus, the need for a more curvilinear ball trajectory above the net [20, 40]. Regarding the side differences of the ball impact points they can be explained by the side specific change in the geometric spatial relationships between the starting position and the service target. This is in line with findings of Reid et al. [22] and Carboch et al. [20], who showed that the lateral displacement at impact is significantly further left in a wide than in a T serve. This effect seems to be inevitable to avoid players being able to anticipate the service direction. When serving from different sides, by contrast, players would be able to compensate for the side-specific spatial circumstances without any tactical disadvantages as mentioned above (Fig 3).

Side specific differences of the tennis serve should also be discussed regarding possible injury-risk implications. Serve production is a violent manoeuvre generating high recurring forces and places the greatest stress on the lower back among all strokes [40–42]. Consequently, the reported high prevalence of back pain in competitive junior and professional tennis players [23, 40, 43, 44] is not surprising, given the large loads in axial rotation [42]. The combination of repetitive rotational forces, coupled with trunk flexion and hyperextension, is particularly critical in the pathophysiology of lower back injuries [23, 42]. It is stated that players hit 50–150 serves during each of the approximately 60 matches played per season, without considering double matches and training sessions [25]. In light of our results, the higher upper torso ROM in general, compared to previous studies [18, 21], as well as the higher values on the deuce court side in comparison to the ad side (especially in foot-back technique players), could be considered as risk factors for back pain. This would underline the above-mentioned recommendation to reduce the upper torso rotation and rotational forces on the deuce court side. In this regard, it should be highlighted that we found no difference in serve velocity between the service sides, although kinematic serve differences were present. This illustrates that these side-related differences can be compensated regarding the service speed and have no effect on the power transmission to the ball. In this context, Kibler [45] stated that there are six to eight key positions during the serve, which are the most basic and which are required to be present in all motions. Further, there are multiple individual variations in other parts of the kinetic chain. It seems that small changes within the kinetic chain are compensated by subsequent body segments and their coordination.

From a practical point of view, our results clearly emphasise some modified kinematics between both service sides. Obviously, to a certain extent, the service has to be accepted as a biomechanically asymmetric and side-specific task. On the other hand, our data show some simple correction points (e. g. a change in the basic foot position) to adjust the serve kinematics which would come along several benefits. This includes a quicker and easier skill acquisition for beginners, which would allow them to generate a more stable movement execution. In tournament players, a beneficial time-saving cost–benefit ratio and a reduced injury risk can be expected.

Limitations

Some limitations of the study design must be considered. The study took place in an indoor laboratory that did not cover the real conditions of the tennis court, which could influence the spatial orientation of the players. Further, players were fitted with retro-reflective markers placed on anatomical landmarks. In addition, the player’s position along the baseline was restricted. These points could reduce the ecological validity of the investigation. Nevertheless, we used familiarisation procedures prior to the investigation in an attempt to minimise possible effects. Another limitation is that we did not perform a sample size estimation. However, the number of participants in this investigation was high compared with those of previous similar studies and the aim was to include only the best players from the respective age group. We also only analysed side-specific differences regarding serves directed to the middle of the service field (i.e. directed to the receiver’ body). Further investigations are recommended to verify the changes in body kinematics between all serve directions.

Conclusion

Changing the service side affects both the serve and the ball kinematics in elite junior tennis players. Our results underline biomechanical differences regarding the starting position (feet and upper torso), as well as movement and ball kinematics, which could be relevant for skill acquisition, injury prevention and performance enhancement. Since the serve is a highly complex and partly unsymmetrical task, the underlying reasons for these findings remain unclear. Nevertheless, a better kinematic adjustment on both sides might be an option that could economise the process of motor learning in young players. Tournament players and their coaches are recommended to explore slight adaptations in their service positions in cases of repeated and considerable weaknesses on one of the two service sides.

Supporting information

S1 Dataset. Original data.

(XLSX)

Click here for additional data file.^{(20.5KB, xlsx)}

S1 Appendix. Variables of interest.

(PDF)

Click here for additional data file.^{(102.7KB, pdf)}

S2 Appendix. Intra-session reliability statistics for each performance measure.

(PDF)

Click here for additional data file.^{(155KB, pdf)}

S1 Fig. Study setting.

(PDF)

Click here for additional data file.^{(67.3KB, pdf)}

Acknowledgments

The authors would like to thank Bruce Elliott and Jacqueline Alderson from the University of Western Australia (Perth, Australia), for assistance during data modelling to calculate relevant anatomical and ball kinematics during serves (University of Western Australia Model). Further, we would like to thank to all of the athletes for participating in the study. Also, we wish to thank Christoph Schneider for providing feedback on preparing and presenting the statistical analysis and data.

Data Availability

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

Funding Statement

The present study was funded by the German Federal Institute of Sport Science (http://www.bisp.de). The Grant number was AZ-072017/16. Funding was received by AF. We further acknowledge support by the Open Access Publication Funds of the Ruhr-Universität Bochum. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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PLoS One. doi: 10.1371/journal.pone.0252650.r001

Decision Letter 0

Marc HE de Lussanet

3 Feb 2021

PONE-D-20-36888

Title: Kinematic characteristics of the tennis serve from the ad and deuce court service positions in elite junior players.

PLOS ONE

Dear Dr. Fett,

In addition to the comments and suggestions of the two reviewers, I found a serious issue, i.e., your conclusions are not supported by the data. Unless you solve this issue, PLoS ONE cannot publish the work.

The problem is as follows. You assume, that the left and right (ad and deuce court) serve represent symmetric tasks, but playing a tennis serve is a highly asymmetric activity, not only biomechanically but also perceptually, because the head with the eyes are aside from the arms, and because the net is not perpendicular to the ball's trajectory.

1. Given the asymmetry of the player, the orientation of the net differs significantly from the player's perspective depending on the side.

2. In a real playing situation, the player also has to be prepared for the return, which might have effects on the optimal orientation of the body.

Second, the manuscript confuses "statistically significant results" with "biomechanically relevant factors". This is problematic, because if there are redundant degrees of freedom that do not affect the performance, the significant effects can well be biomechanically meaningless.

In addition to the issues raised by the reviewers, these effects need to be discussed and need to be reflected in the conclusions.

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Kind regards,

Marc H.E. de Lussanet, Ph.D.

Academic Editor

PLOS ONE

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Reviewer #2: Partly

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Comments to the Author

General comments:

This investigation aimed to analyze body and ball kinematics of flat serves from both service sides in junior tennis players. The study is well conducted and has a well-structured introduction with a thorough literature background. Procedures are described in high detail. Although as mentioned by the authors results could be affected by the sample size, data from elite junior players is interesting and adds information on the matter. I think this is a relevant article for publication in PLoS One.

Material and methods:

Subjects:

Lines 91-96: I would value more information regarding inclusion/exclusion criteria, especially concerning injury history and lower back pain. As mentioned in literature (Campbell et al., 2013, reference 22 here), players of similar characteristics as those analyzed in this study could show significant differences in some important kinematic variables depending on these issues.

Procedures:

Lines: 114-115: could this ‘center of the service field’ be considered as a flat serve to the receiver’s body? As mentioned in the discussion section this has certain importance in impact location and most likely other aspects. The whole rationale around the discussion section analyzes this specific laboratory layout. As a suggestion, since serving to different locations can significantly change kinematics, specifying that authors discuss a serve ‘to the body’ should be emphasized throughout the manuscript. Moreover, consider the possibility of including as a limitation or as future recommendation analyzing serves to other locations in other investigations.

Line 117; did the players rest between serves or only between sets?

Discussion:

Lines 246-248: I wouldn’t list differences found as surprising. You hypothesized there would be certain modifications due to changing service sides and players intuitively readjusting position in consequence.

Lines 330-332: consider clarifying this common instructional tip is not confirmed, as you say, only as long as initial position respective to the baseline is not modified in either serving side. It could be that the tip is useful as long as initial position changes.

I suggest authors include some information around possible explanations to similar serve velocities although certain kinematic differences were found between service sides. As well stated, contributors to velocity production as impact location or torso rotation may vary between positions yet serve velocity remained unaltered when comparing both sides.

Reviewer #2: - In this paper, a biomechanical analysis of two serve positions, the deuce (right, D) and the ad court (left, AD) side, is conducted. Although the paper seems to be well structured and written, and I only have several minor comments, my major concern is related to the main goal, hypothesis and practical applications.

- Since there are many confusing sentences along the paper, I would recommend a professional review of the paper.

- Although the biomechanical analysis of these two serve positions could be interesting in order to extract some practical applications regarding injury prevention programs (i.e., excessive trunk rotation, or problems in the kinetic chain), I don´t see any other key question to answer here. In fact, since there is a great variability in the serve among players, genders, and levels, it seems (at least in my point of view) to establish a general recommendation for these positions. At the end of the day, which one is more efficient under tournament conditions?. Can the authors provide information about this?

- For example, since the serve variability is clear, the authors only analyzed services targeting the center of the box. Can you please provide statistics to support this target and not others (i.e., out wide, to the receiver’s body, as you mentioned)?.

Introduction

- Line 50. Authors mentioned “skill acquisition” from beginner to elite…Can you mention some of those skills?.

- Lines 65-68. This is one of the things that confused me a lot. Where these recommendations come from?. If we take the mentioned reference, it would be non-reliable for me. What about ITF recommendations? National Tennis Federations suggestions, etc.?

- Line 68. Who assumes that?.

- Hypothesis: Can you please give some reference to support your hypothesis?.

- Hypothesis 2 is very subjective.

Materials and methods

- Ethics approval. Please check if this is the right place for it.

- Subjects. More information about the maturation status of the players would be needed (i.e., maturation offset).

- Lines 102-103. How this procedure was calculated?. It´s not easy to understand this “absorption wall” setting.

- Maybe some pics of the setting, as well as marker placement would help the reader to clearly understand the procedures.

- When the familiarization was conducted?. How many times the players repeated the experiment, and how was the reliability of the measures analyzed?

- Lines 113-114. I presume that this was a subjective feeling, right? Or was it related to the peak serve speed recorded?. If so, when?.

- About the serve protocol?. Can you please provide the average number of serves performed?. I assume that there were mistakes and not all the balls were placed exactly where the researches/players wanted. I would like to know the accuracy of the players in percentages. For example, if a player committed a lot of mistakes, a certain level of fatigue could be expected compared to other player who showed 95% of accuracy.

- Radar placement is rather high, taking into consideration that the average body height, isn´t it?.

- Maybe I´m wrong but the individual foot position and technique is not described (i.e., foot-up or foot-back?). This would definitely affect the results, right?.

- Kinematic variables: please see my previous comment. All the players followed the same foot position? (reference 15.)

Discussion

- Lines 247-251. Since the hypothesis is not really clear for me, these conclusions and suggestions are mainly speculative. In general, I found the discussion as very descriptive (repeating the results again) and speculative.

- I´m missing something in the discussion and it´s the relationship between these different techniques and the anthropometrical characteristics of the players. Did you check any relationship between modifications and for example, body height?. Shouldn´t be important in the serve?.

- Lines 261-263. This is like an impossible aim for me, since the variability in the serve is huge, and as I previously mentioned, the serve technique will depend on the players´ characteristics.

- Line 307: Ok, you mentioned for the fist time, the foot technique.

- There is no link between your kinematic data and previous studies analyzing the possible injury-risk implications (i.e., Review of tennis serve motion analysis and the biomechanics of three serve types with implications for injury. Abrams GD, et al. Sports Biomech. 2011; Upper limb joint kinetic analysis during tennis serve: Assessment of competitive level on efficiency and injury risks. Martin C, Bideau B, Ropars M, Delamarche P, Kulpa R. Scand J Med Sci Sports. 2014 Aug;24(4):700-7.). Maybe more information related to this point could be interesting for the reader.

Conclusions

- As I mentioned earlier, at the end I´m not really sure about the usefulness of these results, since the main goal of the serve (if I´m not wrong) is to generate high speed and being accurate. Regarding these factors, which were the differences between these two positions analyzed?. Moreover, are the differences reported related to an increase in the injury risk of these players?. If so, practical implications would be really interesting.

**********

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Reviewer #1: Yes: Joshua Colomar

Reviewer #2: No

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PLoS One. 2021 Jul 22;16(7):e0252650. doi: 10.1371/journal.pone.0252650.r002

Author response to Decision Letter 0

19 Apr 2021

Editor:

Dear Dr. Fett,

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. In addition to the comments and suggestions of the two reviewers, I found a serious issue, i.e., your conclusions are not supported by the data. Unless you solve this issue, PLoS ONE cannot publish the work.

1. Given the asymmetry of the player, the orientation of the net differs significantly from the player's perspective depending on the side.

2. In a real playing situation, the player also has to be prepared for the return, which might have effects on the optimal orientation of the body.

In addition to the issues raised by the reviewers, these effects need to be discussed and need to be reflected in the conclusions.

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,

Marc H.E. de Lussanet, Ph.D.

Academic Editor

PLOS ONE

Dear Dr. Marc H.E. de Lussanet,

We want to thank the editor and for their suggestions and constructive feedback. The abundance of partly contradicting stand points within international tennis experts shows that this topic should be discussed more intensively in tennis research and coaching practice. We have revised the manuscript according to the editors and reviewers’ comments and hope to answered all questions satisfactorily.

We completely agree that the tennis serve is a very complex and highly asymmetric task. This is in line with our results, showing that it is also asymmetric from a biomechanical point of view regarding the service sides. In the revised manuscript, we therefore included a more sophisticated approach of discussion including multiple reasons for our results (line 289ff). Next to methodological reasons in skill acquisition, individual characteristics, we agree that an important point can be perceptual factors. The service presents a highly asymmetric activity from a perceptual point of view. The spatial-visual orientation differs significantly between the two service sides. During preparation, players on the right court side have to turn away the head and eyes from the target field much more than on the ad court side. Thus, the visual perception is decreased on the deuce court side regarding the target field during early preparation phase. These spatial-visual determinants must be taken into account and may have an effect on the biomechanical response (Willams et al., 2000). These issues are reflected in the revised manuscript (discussion & conclusion) (line 289ff, 376).

Further, you are right, to mention that the anticipation of the receiver’s return may impact the server’s position along the baseline and the respective movement characteristics. During match play, from our point of view, this effect might be limited to changes of the server’s position along the baseline (e. g. leading to a shift of right-handed players more to the left when serving from the ad court side wide to the opponents backhand and increasing the chance to use the forehand inside-in as an effective answer to the return). In our study we therefore restricted the server´s lateral position to the maximum of 1.0m regarding the centre mark and we defined the target point in the service box (to centre of box meaning a serve to the opponent’s body). We assume that under these conditions the impact of strategical changes of the lateral serving position can be neglected. We would also defend our standpoint that under the restricted conditions of our study the trajectory of the ball is identical and symmetric in regard to the angle to the net and to the target zone as well as in regard to the height of the net. Nevertheless, we agree that these restrictions have to be discussed regarding their impact on the ecological validity of our study. Therefore, we extended this point as one major limitation of our study (line 358-362).

You also mentioned that the manuscript confuses "statistically significant results" with "biomechanically relevant factors". In this regard we cannot agree completely since the selected markers were chosen carefully according to the literature. Kibler (2014) stated that during the serve there are 244 possible degrees of freedom (DOF) in the body from the foot to the hand. Efficient mechanics in the kinetic chain can be improved by decreasing the possible DOF and most models of maximum efficiency in body motions find that limiting DOF to about six to eight maximises the total force output and minimise effort and load. These are called nodes and illustrates key positions which have been correlated with optimum force development and minimal applied loads. Of course, there are multiple variations in other parts of the kinetic chain, but these nodes and key positions are the most basic and the ones required to be present in all motions (Kibler, 2014). In our analysis we have focussed on those key positions (i.e., according to published data (Reid et al., 2014; Wagner et al., 2014; Whiteside et al., 2013)).

In consequence we do not believe that the results are biomechanically meaningless and we also think that “biomechanical relevance” is rather difficult to define. In this regard the missing difference in serve velocity does not contradict the practical relevance of the findings since players adapted over years to their technical solution. On the other hand, we allow to address the point that the process of motor learning, skill acquisition and training of two even slightly different movements takes more time and therefore seems to be less efficient. However, in the revised version we followed your suggestion by less emphazising those findings with low effect sizes (i.e., variable: upper torso ROM = discussion was shortened).

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Yes, you are right. A part of the presented data was presented at the ECSS Congress 2018 in Dublin and published in the conference proceedings (book of abstract), but they were not peer-reviewed. All submitted abstracts have been assessed for relevance and quality by an abstract review committee and the work was selected for oral presentation. However, the current manuscript covers a more extensive analysis and overview of the data. It presents new findings, and all contextual information (introduction, methods, findings and discussion) that were not possible to include in the mentioned conference abstract.

Reviewer 1

Reviewer #1: Comments to the Author

1) General comments:

We thank the reviewer for their time and the constructive feedback. We attempted to respond to and address all concerns raised by the reviewer. Please find our comments below.

2) Material and methods:

Subjects:

Thanks a lot for this comment. We have added more detailed information regarding maturation, training details as well as regarding exclusion criteria, especially concerning the injury history (line 89ff). None of the players had any injuries or complaints. We therefore assume no disturbing impact on kinematics.

3) Procedures:

Yes, that is right. We have named it ‚centre of the service box‘ but it seems not clear at all. We have emphasized this in the introduction (research question line 78-80)) as well as in the methods section (line101-102). In addition, it is established in the limitations to recommend investigations verifying changes between all serve directions (line 368ff).

4) Line 117; did the players rest between serves or only between sets?

Between each serve there was a rest of 30 s to prepare for the next one. This information is added in the section ‘methods’ (line 120).

5) Discussion:

You are right, we have changed accordingly (Introduction: 76; discussion line260 ff).

6) Lines 330-332: consider clarifying this common instructional tip is not confirmed, as you say, only as long as initial position respective to the baseline is not modified in either serving side. It could be that the tip is useful as long as initial position changes.

Yes, you are right. Nevertheless, due to restructuring the discussion we deleted this point.

7) I suggest authors include some information around possible explanations to similar serve velocities although certain kinematic differences were found between service sides. As well stated, contributors to velocity production as impact location or torso rotation may vary between positions yet serve velocity remained unaltered when comparing both sides.

We have added information regarding possible explanations for similar serve velocities although certain kinematic differences were found between service sides (line 341ff). Due to the cutback and restructuring of the discussion we have included this point, but not in detail as shown here.

The results show that the ball velocity does not change between the service sides although there are kinematic serve differences. In this regard the missing difference in serve velocity does not contradict the practical relevance of the findings since players adapted over years to their technical solution This illustrates that these side-related differences can be compensated regarding the service speed and have no effect on the power transmission to the ball. Kibler (2014) stated that during the service there are 6-8 key positions which have been correlated with optimum force development and minimal applied load and which are the most efficient methods of coordinating kinetic chain activation. Further, there are multiple individual variations in other parts of the kinetic chain (Kibler, Understanding the kinetic chain in tennis performance and injury, Aspetar Sports Medicine Journal, 2014, 3:492-497). In consequence, one possible reason could be the general high variability in the service execution so that small changes within the kinetic chain are compensated by subsequent body segments and their coordination. This reinforces the thesis that the results are primarily important for the training process and for facilitating techniqual acquisition (teaching and learning the service).

Reviewer #2:

1) - In this paper, a biomechanical analysis of two serve positions, the deuce (right, D) and the ad court (left, AD) side, is conducted. Although the paper seems to be well structured and written, and I only have several minor comments, my major concern is related to the main goal, hypothesis and practical applications.

We thank the reviewer for their time and constructive feedback. We attempted to respond to and address all concerns raised by the reviewer. Please find our detailed responses and comments below.

2) - Since there are many confusing sentences along the paper, I would recommend a professional review of the paper.

The paper underwent a professional proof-reading a second time. We hope to have solved the problem.

3) - Although the biomechanical analysis of these two serve positions could be interesting in order to extract some practical applications regarding injury prevention programs (i.e., excessive trunk rotation, or problems in the kinetic chain), I don´t see any other key question to answer here. In fact, since there is a great variability in the serve among players, genders, and levels, it seems (at least in my point of view) to establish a general recommendation for these positions. At the end of the day, which one is more efficient under tournament conditions?. Can the authors provide information about this?

Nowadays, tennis experts worldwide agree that the service quality seems to be crucial and increasingly important for successful tennis. Therefore, an increasing body of literature is focused on service training including the number of serves and the optimum within-session sequence (e. g. Fernandez-Fernandez 2020). No training advices are referring to the two different service sides yet and service quality tests are usually performed only from the deuce court side (e. g. Ferrauti & Bastiaens 2007). On the other hand, the ITF rules are leading to a more or less balanced quantity of serves from both sides (slightly more from the deuce court side because all games are starting there while not all games end with a serve from the ad court side). Furthermore, experiences from the practical field are clearly showing that players develop specific service side preferences which might be attributed to biomechanical aspects. We are therefore convinced that a more sophisticated analysis on the specific demands and particularities is of practical relevance.

In general, our main goal was to determine whether there were kinematic ball and serve differences between the two service sides. Contrary to your statement and opinion, we still believe that these results are of applied interest especially with regard to the skill acquisition during motor learning. We agree that there has always been a huge variability in the serve kinematics depending on various individual characteristics (e. g. parallel baseline stands of lefthanded John McEnroe completely closed to the ad service box and completely open to the deuce box). On the other hand, from an economical point of view, you may agree that junior players will take less time to optimize two similar techniques instead of two different movements. This fact should be considered when teaching and coaching the tennis serve, especially in the learning process (skill acquisition) in beginners. The adjustments according to the service side (and not the baseline orientation) would be one possible recommendation leading to a more uniform movement pattern. Consequently, the achievement of a faster learning success would be conceivable and lead to a more economic and beneficial time-saving cost–benefit ratio.

Of course, you are right, that there are still other important key questions to answer, i.e. the connection to injury risk and prevention. This issue was added and improved in the revised manuscript. Since this connection does not apply to all parameters, we focussed on selected parameters (line 329ff).

4) - For example, since the serve variability is clear, the authors only analyzed services targeting the center of the box. Can you please provide statistics to support this target and not others (i.e., out wide, to the receiver’s body, as you mentioned)?

The main reason was to adjust the target from both sides. The outcome would be most likely the same if we would have chosen the service box corners. But this would result in the double quantity of serves and therefore, from economic reasons, we decided to choose the centre of service box. We agree that this direction is less frequently used in professional tennis. Analyses of the 2009 French open show an approximately equal distribution of the serve directions from the deuce side (wide: 32.0%, body: 32.3%, T: 35.7%). More recent data show that in today’s tennis the directions to the wide and to the T are targeted more often (Kovalchik & Reid, J Sports Sci. Med. (2017) 16, 489-497; Krause et al, J Sports Sci. (2019) DOI: 10.1080/02640414.2019.1665245 published online). Nevertheless, the main reason for the target to the receiver’s body was a methodological one as mentioned before.

5) Introduction

- Line 50. Authors mentioned “skill acquisition” from beginner to elite…Can you mention some of those skills?.

i.e., movement execution, technical skill, coordination of kinetic chain (line 50-51)

6) - Lines 65-68. This is one of the things that confused me a lot. Where these recommendations come from?. If we take the mentioned reference, it would be non-reliable for me. What about ITF recommendations? National Tennis Federations suggestions, etc.?

The reference noted is from the online Coaching workbook from German Tennis Federation (TennisGate). These are typical recommendations especially for tennis players which are taught by coaches during skill acquisition in the early tennis training and learning process, especially for novice tennis players. Here, we have added more specific references for this, including also recommendations from the German tennis federation (Scholl (2014). Richtig Tennis Spielen – Optimales Training von Anfang an. BLV: München; Tina Hoskins (2003). The Tennis Drill Book, Human Kinetics: UK; Deutscher Tennis Bund (1981). Tennis-Lehrplan 2 Grundschläge. BLV: München). In the revised manuscript only the English written literature was added (line 69).

7) - Line 68. Who assumes that?.

Thanks, we deleted this sentence. It was too speculative.

8) - Hypothesis: Can you please give some reference to support your hypothesis?. Hypothesis 2 is very subjective.

Thanks for this comment, we changed hypothesis 1 and deleted 2.

Since this is the first study which compares serve kinematics from the ad and the deuce service side we cannot support our hypothesis by further literature. Nevertheless, statements and feedback from the practical setting (federations and coaches) encouraged our exploration. Nevertheless, you are right to call hypothesis 2 as very subjective. We have removed this sentence to keep the result more open (line 76).

9) Materials and methods

- Ethics approval. Please check if this is the right place for it.

Ethics statement was added to the section ‘Subjects’ (line 93).

10) - Subjects. More information about the maturation status of the players would be needed (i.e., maturation offset).

Thank you for the comment. We have added more detailed information regarding maturation, i.e. peak height velocity, maturity offset (line 84-85). Further, the procedures of these parameters were added to the section ‘Anthropometric measurements’ (line 178 ff)

11) - Lines 102-103. How this procedure was calculated?. It´s not easy to understand this “absorption wall” setting. Maybe some pics of the setting, as well as marker placement would help the reader to clearly understand the procedures.

The players' task was to target the direction of receiver’s body. We transferred the dimensions of a tennis court to the laboratory setting. Triangulation allowed us to calculate the height and width of the target field, since the dimensions in the laboratory are much smaller than on the tennis court (e.g. distance of the server to the net). Of course, the target fields are only an approximation, since the trajectory of the ball was assumed to be linear and the height of the ball impact point had to be estimated beforehand based on the literature and an estimated average size of the players.

The information on the methodical calculation was added (line 104-105).

To get a better overview we added a schematic screen of the laboratory setting (S1 Fig).

In general, we agree that this laboratory approach can lead to a loss in ecological validity. On the other hand, it was necessary to realize a proper motion capturing without any surrounding bias.

12) - When the familiarization was conducted? How many times the players repeated the experiment, and how was the reliability of the measures analyzed?

The familiarization was conducted on the same day of testing prior to the testing protocol. The participants were fitted with all retro-reflective markers before the warm up. In consequence, they got used to it during performing the general movement preparation exercises as well as during specific activation and mobilization with elastic tubes. Also, the warm-up program consisting of submaximal serves were done in the laboratory and not on the real tennis court to get used to the specific setting.

The players conducted the experiment once. Intra-session reliability of all variable measures was calculated. Intraclass correlation coefficient (ICC 3,1) and SEM (90% confidence limits) were assessed by the three testing trials and pooled between both service sides. Therefore, we used the spreadsheets for analysis of validity and reliability of Hopkins (Sportscience 19, 36-42, 2015 (sportsci.org/2015/ValidRely.htm).

S2 Appendix shows reliability data for all analysed variables. Overall, ‘good’ and ‘excellent’ reliability was found for nearly each variable (ICCs ranging from 0.76 to 0.99). Front knee extension velocity and shoulder internal rotation velocity during propulsion as well as front and back knee flexion at impact showed a moderate reliability (ICCs ranging from 0.58 to 0.73) (Koo & Li, 2015).

This information was added to the section “Materials and methods” (i.e., section ‘kinematic variables’ (line 173ff) and ‘statistical analysis’ (line 194ff).

13) - Lines 113-114. I presume that this was a subjective feeling, right? Or was it related to the peak serve speed recorded?. If so, when?.

Yes, you are right. It was not related to the peak serve speed recorded, it was in accordance to their subjective feeling. This information was added (line 116).

14) - About the serve protocol?. Can you please provide the average number of serves performed?. I assume that there were mistakes and not all the balls were placed exactly where the researches/players wanted. I would like to know the accuracy of the players in percentages. For example, if a player committed a lot of mistakes, a certain level of fatigue could be expected compared to other player who showed 95% of accuracy.

All players performed 8 serves on each side. Four players failed to make three valid attempts within eight serves. There was a serve accuracy of 36.2 % (SD 9.9). Because of that, four players had to perform in total (both sides) three to four further serves to get three serves in (on each side). Instead of a total of 16 serves these players performed on average 19.5 (SD 0.58) serves at all. Regarding the total serve performed during a real match (50-150 first and second serves (Reid et al. 2008)) we think that this increase (total of 16 to 19.5 serves) will not lead to a significant fatigue compared to the other players. These information were added (line 124-126).

15) - Radar placement is rather high, taking into consideration that the average body height, isn´t it?.

Radar placement was aligned with the approximate height of ball contact (~ 3m). Ball contact in our study was around 2.60m (SD 0.23m). In consequence, we think that this is still a valid placement. We have revised the sentence in the manuscript (line 128)

16) - Maybe I´m wrong but the individual foot position and technique is not described (i.e., foot-up or foot-back?). This would definitely affect the results, right?.

- Kinematic variables: please see my previous comment. All the players followed the same foot position? (reference 15.)

Yes, that is absolute important. That’s why we divided the sample and conducted further analysis regarding this effect (Table 2 & Figure 2).

The information about the players foot technique is described in the section ‘Subjects’ (line 86-87). In the first version it may have been insufficiently expressed. In the revision it is stated more clearly. There were six foot-up and eight foot-back technique players. Foot and upper torso position at starting position was determined before initiating the footwork. We believe that the foot technique can affect specific parameters (i.e., upper torso ROM), this is described in the section ‘discussion’ (line 304ff). From a descriptive point of view, the differences in upper torso ROM between the service sides could be affected by the foot-back technique players showing greater differences between the service sides with higher values on deuce court.

17) Discussion

Thanks for this comment. Yes, we completely agree but we were not able to change accordingly, since there is no scientific literature on this topic yet. Therefore, our discussion has to be less literature based but rather content related following plausible argumentations. Since, it is a first impulse to this field of research the discussion remains partly speculative. Nevertheless, we tried to make the discussion more precisely with more relation to the literature. In this context, we have completely restructured the discussion section (line 251ff).

18) - I´m missing something in the discussion and it´s the relationship between these different techniques and the anthropometrical characteristics of the players. Did you check any relationship between modifications and for example, body height?. Shouldn´t be important in the serve?.

Thank you very much for the comment. Our goal was to investigate side-specified differences in ball and serve kinematics and to discuss possible reasons for this. Of course, the questions listed here are very interesting considering the influence of anthropometric factors (Bonato et al. Relationship between anthropometric or functional characteristics and maximal serve velocity in professional tennis players. J Sports Med Phys Fitness, 2014 epub ahead of print; Vaverka & Cernosek. Association between body height and serve speed in elite tennis players. Sports Biomechanics, 2013, 12:1, 30-37). However, the influence of anthropometrics was beyond the scope of our study, so that we have unfortunately not included this point in the revised manuscript. Nevertheless, it is an interesting point and we will possibly work on it later in a further re-analysis of the data.

19) - Lines 261-263. This is like an impossible aim for me, since the variability in the serve is huge, and as I previously mentioned, the serve technique will depend on the players´ characteristics.

On the one hand we completely agree but we think that our data justify at least to re-think the current tennis coaches’ textbooks, the learning methods during skill acquisition, and, in some cases, even to reflect elite player´s technique in case they show repeated considerable weaknesses on one of the two service sides. This is mentioned accordingly in the conclusions.

20) - Line 307: Ok, you mentioned for the fist time, the foot technique.

Within the subject description, we provide information regarding the players’ foot technique. In the first version it may have been insufficiently expressed. In the new one it is stated more clearly (line 87). Because we think, that the foot technique is of great importance for the total upper torso rotation we conducted further calculation for this value (Table 2, Figure 2). These results outline that from a descriptive point of view the differences in upper torso ROM between the service sides could be affected by the foot-back technique players showing greater differences between the service sides with higher values on deuce court (line 304ff).

21) - There is no link between your kinematic data and previous studies analyzing the possible injury-risk implications (i.e., Review of tennis serve motion analysis and the biomechanics of three serve types with implications for injury. Abrams GD, et al. Sports Biomech. 2011; Upper limb joint kinetic analysis during tennis serve: Assessment of competitive level on efficiency and injury risks. Martin C, Bideau B, Ropars M, Delamarche P, Kulpa R. Scand J Med Sci Sports. 2014 Aug;24(4):700-7.). Maybe more information related to this point could be interesting for the reader.

Thank you for this comment. You are right, implications to injury risk are missing and we have improved this. Since this connection does not apply to all parameters, we focussed on selected parameters.

Line 329 ff:

Side specific differences of the tennis serve should also be discussed regarding possible injury-risk implications. Serve production is a violent manoeuvre generating high recurring forces and places the greatest stress on the lower back among all strokes (40–42). Consequently, the reported high prevalence of back pain in competitive junior and professional tennis players (23, 43, 44) is not surprising given the large loads in axial rotation (42). The combination of repetitive rotational forces coupled with trunk flexion and hyperextension is particularly critical in the pathophysiology of lower back injuries (23, 42). It is stated that players hit 50-150 serves during each of around 60 matches per season, not considering double matches and training sessions (25). In the light of our results the higher upper torso ROM in general compared to previous studies (18, 21) as well as higher values on the deuce court side in comparison to the ad side (especially in foot-back technique players), could be consider as a risk factor for back pain. This would underline the above-mentioned recommendation to reduce the upper torso rotation and rotational forces on the deuce court side. In this regard it should be highlighted that we found no difference in serve velocity between the service sides although there are kinematic serve differences. This illustrates that these side-related differences can be compensated regarding the service speed and have no effect on the power transmission to the ball.

22) Conclusions

Thanks for the comment. We hope by answering the above comments, we have been able to clarify the rational behind our main goal. We agree that it is important to generate high speed and accurate precision. In this respect, there is no difference between the two service sides (see Comment 7 reviewer 1 & line 341ff). But beyond that, it can still be important for the learning process. Learning the technical skill of the service could be easier and more economic. The relationship to injury mechanics was added to the revised manuscript (line 329ff).

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PLoS One. doi: 10.1371/journal.pone.0252650.r003

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Title: Kinematic characteristics of the tennis serve from the ad and deuce court service positions in elite junior players.

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Kinematic characteristics of the tennis serve from the ad and deuce court service positions in elite junior players

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PERMALINK

Kinematic characteristics of the tennis serve from the ad and deuce court service positions in elite junior players

Janina Fett

Nils Oberschelp

Jo-Lâm Vuong

Thimo Wiewelhove

Alexander Ferrauti

Roles

Abstract

Purpose

Methods

Results

Conclusions

Introduction

Materials and methods

Subjects

Experimental design

Procedures

Kinematic analysis

Kinematic variables

Anthropometric measurements

Statistical analysis

Results

Serve kinematics

Table 1. Peak and terminal kinematic variables of interest for the ad and deuce service side.

Fig 1. Mean foot position on ad and deuce court side immediately before ball toss.

Table 2. Upper torso ROM in foot-up and foot-back players.

Fig 2. Subgroup analysis of side differences in upper torso range of motion between foot-up and foot-back players (95% CI).

Ball kinematics

Discussion

Fig 3. Upper torso rotation at start, maximum backward rotation (clockwise), and impact on ad and deuce court side.

Fig 4. Mean ball displacement and upper torso rotation at impact.

Limitations

Conclusion

Supporting information

Acknowledgments

Data Availability

Funding Statement

References

Decision Letter 0

Marc HE de Lussanet

Roles

Author response to Decision Letter 0

Decision Letter 1

Marc HE de Lussanet

Roles

Acceptance letter

Marc HE de Lussanet

Roles

Associated Data

Supplementary Materials

Data Availability Statement

ACTIONS

PERMALINK

RESOURCES

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