573

Journal of Applied Biomechanics, 2013, 29, 573-582 © 2013 Human Kinetics, Inc.

David Whiteside (Corresponding Author) and Machar Reid are with the School of Sport Science, Exercise and Health, Univer-sity of Western Australia, Crawley, Western Australia, Australia, and with the Sport Science and Medicine Unit, Tennis Australia, Victoria, Australia. Bruce Elliott and Brendan Lay are with the School of Sport Science, Exercise and Health, University of Western Australia, Crawley, Western Australia, Australia.

The Effect of Age on Discrete Kinematics of the Elite Female Tennis Serve

David Whiteside,1,2 Bruce Elliott,1 Brendan Lay,1 and Machar Reid1,2

1University of Western Australia; 2Tennis Australia

The importance of the flat serve in tennis is well documented, with an abundance of research evaluating the service technique of adult male players. Comparatively, the female and junior serves have received far less attention. Therefore, the aims of this study were to quantify the flat serve kinematics in elite prepubescent, pubescent, and postpubescent female tennis players. Full body, racket, and ball kinematics were derived using a 22-camera Vicon motion capture system. Racket velocity was significantly lower in the prepubescent group than in the two older groups. In generating racket velocity, the role of the serving arm appears to become more pronounced after the onset of puberty, whereas leg drive and “shoulder-over-shoulder” rotation mature even later in development. These factors are proposed to relate to strength deficits and junior players’ intentions to reduce the complexity of the skill. Temporally, coupling perception (cues from the ball) and action (body movements) are less refined in the prepubescent serve, presumably reducing the “rhythm” (and dynamism) of the service action. Practically, there appears scope for equipment scaling to preserve kinematic relevance between the junior and senior serve and promote skill acquisition.

Keywords: biomechanics, development, constraints, sport

The serve has been described as the most important stroke in tennis, as it provides a player with an oppor-tunity to gain the ascendency in a point, or even win it outright.1,2 It is used to start each point and, as a closed motor skill, is the only stroke that affords the player com-plete control over its execution. However, the mechanical complexity of the service action ensures that this is not an easy task. Research has attended to the importance of the serve through examinations of the kinematics3–5 and kinetics1,6–8 involved in the stroke. This work has historically centered on adult, male players4,9–11 and, consequently, the mechanics characterizing the female and junior serves are not as well understood. In these populations, instruction or development of the serve is underpinned by emulation, rather than the objective data that have guided instruction of the adult male serve.

The flat serve depends on the coordination of several body segments to generate commanding racket veloci-ties.12 Previous work in adult male players has high-lighted the importance of the lower limbs,6,10,11 trunk1,3 and serving arm4,7,8 in this process. The lower limbs are responsible for the initial propulsive action in the serve,

referred to as leg drive. Flexion at the knees has been documented as an important precursor to leg drive,13 though the quality of leg drive is thought to be an arti-fact of peak angular extension velocity at the knees.3,11 Transverse (twist), frontal (shoulder-over-shoulder or cartwheel) and sagittal plane (somersault) trunk rota-tions characterize the high performance male tennis serve1,14,15 and act to transfer momentum to the serving arm.12,16 Facilitated by the aforementioned leg drive, peak external rotation at the shoulder has approximated 170° in professional players when referenced relative to the horizontal,3,15 whereas values of ≈116–135° have been reported when referenced relative to the thorax.7 Subsequently, peak internal rotation velocity has been documented to be significantly higher in professional male players (2520°·s–1) compared with their female counterparts (1370°·s–1).3 This movement is the primary contributor to racket velocity in the flat serve, followed by wrist flexion.4,8 Rotations at the elbow are used to regulate the racket’s trajectory and orientation prior to impact.3,8

More recent studies have examined the role of the ball and racket in the male serve.2,17 This work has noted an impact location forward and lateral to the front foot in high performance male players. In the same popula-tion, the ball toss and racket swing have been shown to share a temporal association whereby ball zenith and racket high point occur simultaneously.2,18 The nature of the ball toss in the developing female serve has not been examined, and represents an interesting paradox

An Official Journal of ISBwww.JAB-Journal.comORIGINAL RESEARCH

574 Whiteside et al.

between science and practice, as the ball toss is an attri-bute of early instruction.

From the above, it is evident that the mechanics of the elite male serve have received considerable research attention; however, their application or relevance to the female and junior serves is largely unknown. Indeed, according to Newell’s constraints model19 it would appear logical to assume that these kinematics manifest differ-ently in female players.20 From a dynamical systems perspective, maximal performance is attained only when the necessary component parts of the movement system are fully developed.20 A component that is not fully developed or working at optimal capacity is considered a ‘rate limiter’, as it may inhibit optimal functioning of the system. Therefore, with stature, mass and strength known to undergo nonlinear increases throughout develop-ment,21–23 these factors may be considered rate limiters, as they can potentially inhibit the emergence of optimal movement patterns.24 With these rate limiters related to nonlinear improvements (and decrements) in performance during development,20 cross-sectional differences in the serve mechanics of female players of different ages are expected. Consequently, the aim of this study was to compare the lower limb, trunk, serving arm, racket, and ball toss kinematics in the flat serves of elite prepubescent, pubescent, and postpubescent female tennis players.

Materials and Methods

Subjects

Thirty-one elite female tennis players participated in this study, which was approved by the University of Western Australia’s (UWA) Human Ethics Committee. Before recruitment, informed consent was obtained and players completed a confidential questionnaire to determine the month and year of their first menses. Based on their age and response, players were recruited and assigned to one of three groups: prepubescent (aged 10–11 y), pubescent (aged 14–15 y) and postpubescent (aged 18+ y) (Table 1). In the prepubescent and pubescent groups (collectively referred to as the junior groups), players were ranked among the top 8 Australian players for their respective birth years, while the adult players possessed a profes-sional WTA ranking higher than 325.

Protocol

A full size tennis court was constructed at the Australian Institute of Sport indoor biomechanics laboratory. Sixty

retro-reflective markers, 14 mm in diameter were affixed to each player according to the UWA full body marker set.25,26 Three hemispherical markers, composed of ultra-light foam (radius 7 mm) were placed on each of the racket and ball to create coordinate systems therein. To maximize ecological validity, players used their own rack-ets and completed a ten-minute warm up with movement (five minutes) and serving (five minutes) components. Upon readiness, players performed maximal effort flat serves aiming for a 1 × 1 m target bordering the T of the service box (right-handers: deuce court; left-handers: advantage court). Five blocks of eight serves were per-formed with a 2-min rest period separating successive blocks. A 22-camera Vicon MX system (Vicon Motion Systems, Oxford, UK) operating at 500 Hz tracked three-dimensional (3D) marker trajectories. The junction of the baseline and center mark represented the global origin, in which positive x pointed rightward along the baseline, positive y pointed to the net and positive z pointed up. The five fastest serves landing in the target area were analyzed for each player.

Data Treatment

Gaps in the raw marker trajectories were interpolated using a cubic spline. To prevent impact accelerations from distorting the data, a second-order polynomial extrapolation estimated marker trajectories at impact.27 Data relevant to the calculation of postimpact ball flight variables were processed separately, where data before impact were cropped. Both sets of data were subse-quently filtered using a Woltring filter28 with the optimal mean squared error of 2 mm determined by a residual analysis. Filtered data were modeled using the UWA full body, racket and ball models.25,26,29 Joint rotations were expressed using the Euler ZXY sequence, except at the shoulder, where the International Society of Biomechan-ics’s recommended YXY decomposition was used.30 To preserve consistency in the statistical analyses, kine-matics for the left-handed players were inverted where appropriate such that all players could be considered together as right-hand dominant.

Variables and Time Points of Interest

Before analysis, all serves were time-normalized to 101 data points. The service action was deemed to begin at the instant the ball was released from the hand (Figure 1). Ball zenith represented the peak vertical displacement of the ball toss, while the subsequent nadir of vertical racket

Table 1 Age and physical and menarchial characteristics of participants

N Age (y) Height (cm) Mass (kg)Experienced

MenarcheTime Since Menarche

Prepubescent 12 10.5 ± 0.5 143.5 ± 5.9 36.5 ± 3.7 No N/A

Pubescent 11 14.6 ± 0.7 166.9 ± 4.7 56.7 ± 3.8 Yes 6–18 months

Adult 8 21.3 ± 3.8 169.2 ± 4.8 61.9 ± 4.2 Yes > 4 years

Kinematics of the Elite Female Serve 575

displacement was the racket low point. Impact represented the end of the analyzed service action.

The variables of interest were those considered important to velocity generation in extant literature and coach-led practice.15 Peak knee flexion indicated the mag-nitude of lower limb preparation before leg drive. “Triple extension” (the combined peak extension velocities at the ankle, knee and hip) are typical in jumping movements31 and were thus used to gauge the quality of propulsion in each lower limb. Peak vertical hip velocities determined the magnitude of leg drive. Peak separation angle between the hips and shoulders was measured, as was peak trunk tilt (Figure 2). Peak angular velocity of the trunk was mea-sured in the transverse (twist) and frontal (shoulder-over-shoulder) planes. The orientation of horizontal vectors joining the acromion processes and anterior superior iliac spines measured the transverse alignments of the trunk and pelvis respectively, where 0° was coincident with the global x-axis (ie, the baseline). At the shoulder, peak external rotation angle, internal rotation velocity and the abduction angle at impact were also considered. During the forward swing, peak extension and flexion velocities were measured at the elbow and wrist respectively. Finally, elbow flexion angle at impact was calculated.

Orientation of the racket at impact was expressed by its rotation (ie, backward tilt) about the global x-axis. At impact, the linear velocities of the racket about the three global axes were measured along with resultant racket velocity. Consistent with previous descriptions of the tennis serve, the 3D ball displacements at zenith and impact were expressed relative to the first metatarsal of the front foot.2,17 The spin axis and angular velocity of the ball described the rotational kinematics of ball flight.

Temporally, the occurrences of the key events and phases were expressed as a percentage of serve duration. The first peak vertical displacement of the racket head represented a body orientation referred to as the trophy position. The time margin separating the ball zenith and trophy position events was calculated to gauge the extent to which ball position was coupled with this posture during the preparation phase of the serve.

Statistical Analyses

One-way analyses of variance, with accompanying Bon-ferroni post hoc analyses assessed group differences. To reduce the risk of type I error associated with multiple comparisons, the alpha level was adjusted a priori to the more conservative level of P < .01.2,3,17

ResultsThe dynamism of leg drive was significantly greater in adult players, as evidenced by the triple extension velocities (Table 2). This is verified by the peak vertical velocities of the hip, which were also significantly greater in the adult group. Notwithstanding leg drive, peak knee flexion (front knee: ≈70°; back knee: ≈87°) was similar across all groups.

Figure 2 — Peak shoulder tilt, separation angle and trunk rotations in the serve.

Figure 1 — Key events and phases of the serve.

576

Tab

le 2

B

od

y ki

nem

atic

s

Pre

pu

bes

cen

tP

ub

esce

nt

Ad

ult

AN

OVA

Po

st H

oc

Var

iab

leU

nit

Mea

nS

DM

ean

SD

Mea

nS

DF

PG

1 vs

G2

G1

vs G

3G

2 vs

G3

Low

er L

imbs

Pe

ak F

ront

Kne

e Fl

exio

n A

ngle

deg

7510

657

698

3.82

7.0

34

Peak

Bac

k K

nee

Flex

ion

Ang

lede

g87

1087

888

8.0

25.9

75

Tri

ple

Ext

ensi

on in

Fro

nt L

ower

Lim

bde

g/s

1184

126

1367

241

1688

134

19.3

82<

.001

**

*

Tri

ple

Ext

ensi

on in

Bac

k L

ower

Lim

bde

g/s

1466

177

1596

191

1795

198

7.41

2.0

03*

*

Peak

Fro

nt H

ip V

ertic

al V

eloc

itym

/s1.

37.1

91.

47.1

11.

73.1

214

.566

<.0

01*

**

Pe

ak B

ack

Hip

Ver

tical

Vel

ocity

m/s

1.81

.25

1.94

.09

2.30

.11

19.3

10<

.001

**

*T

runk

Pe

ak S

epar

atio

n A

ngle

deg

307

256

1711

6.14

4.0

06*

*

Peak

Tru

nk T

ilt A

ngle

deg

3712

427

437

1.06

9.3

57

Pe

ak T

runk

Tw

ist ω

deg/

s72

319

071

585

715

145

0.01

1.9

89

Pe

ak S

houl

der-

Ove

r-Sh

ould

er ω

deg/

s–6

3546

–662

26–7

0055

5.52

8.0

09*

*

Pelv

is A

lignm

ent a

t Im

pact

deg

9410

7910

756

12.4

86<

.001

**

*

Shou

lder

Alig

nmen

t at I

mpa

ctde

g10

811

9810

877

11.9

25<

.001

**

T

runk

Tilt

at I

mp

deg

–25

7–3

98

–40

615

.643

<.0

01*

**

Serv

ing

Arm

Pe

ak E

xter

nal R

otat

ion

Ang

lede

g12

912

136

914

17

3.46

8.0

45

Pe

ak I

nter

nal R

otat

ion ω

deg/

s12

8836

521

6537

320

0029

719

.843

<.0

01*

**

Pe

ak E

lbow

Ext

ensi

on ω

deg/

s11

4718

515

9219

115

2414

420

.533

<.0

01*

**

Pe

ak W

rist

Fle

xion

ωde

g/s

1164

189

1581

184

1911

264

31.8

75<

.001

**

**

Sh

ould

er A

bduc

tion

Ang

le a

t Im

pact

deg

9513

102

1010

413

1.49

5.2

42

Elb

ow F

lexi

on A

ngle

at I

mpa

ctde

g42

1126

1127

88.

574

.001

**

*Te

mpo

ral

Pr

epar

atio

n as

Pro

port

ion

of S

erve

%42

1058

1260

79.

851

.001

**

*

Prop

ulsi

on a

s Pr

opor

tion

of S

erve

%42

1029

1227

78.

159

.003

**

*

Forw

ard

Swin

g as

Pro

port

ion

of S

erve

%8

45

16

22.

342

.115

T

ime

Mar

gin:

TP

to B

Zs

.17

.10

.07

.05

.03

.02

10.6

90<

.001

**

*

*Sig

nific

ant a

t P <

.01

leve

l.

Kinematics of the Elite Female Serve 577

Shoulder-over-shoulder rotation was reduced in prepubescent players who relied more on twist rotation of the trunk. Prepubescent players achieved a signifi-cantly greater peak separation angle, while peak trunk tilt was similar in all groups. Peak trunk twist velocity transcended age, however adults generated significantly higher peak shoulder-over-shoulder velocity (Prepubes-cent: –635°·s–1; Pubescent: –662°·s–1; Adult: –700°·s–1). At impact, prepubescent players had rotated their pelvises to a position virtually parallel to the net, while their trunk was rotated further still. These transverse plane rotations were reduced in the two older groups, who maintained more perpendicular orientations to the net at impact. The lateral tilt of the trunk at impact was significantly more pronounced in the two older groups (Prepubescent: –25°; Pubescent: –39°; Adult: –40°).

The two older groups generated significantly higher angular velocities at the joints of the serving arm (Table 2). Although peak external rotation at the shoulder was unaffected by age (approx. 135°), subsequent peak inter-nal rotation, elbow extension and wrist flexion velocities were all significantly higher in the two older groups. Shoulder abduction at impact was similar in all groups (≈100°), though the prepubescent elbow was significantly more flexed at this time.

Both racket orientation and racket velocity at impact appeared to be affected by age (Table 3). At impact, the racket was tilted significantly further backward in the prepubescent group (15°; Pubescent: 10°; Adult: 9°). Pre-pubescent racket velocity (29.9 m·s–1) was significantly lower compared with pubescents (40.7 m·s–1) and adults (43.4 m·s–1). The forward and lateral components of this velocity were also significantly lower in prepubescent players.

Players in the two older groups tossed the ball signifi-cantly further forward and impacted the ball significantly further into the court than prepubescent players (Table 3). Predictably, impact height was significantly lower in the prepubescent group, although when reported relative to the standing height of the player, it did not differ with age (≈1.5 × standing height). In the two older groups, postimpact ball rotation was significantly higher and the spin axis of the ball more upright (Figure 3).

The temporal sequence of the key events did not differ with age, nor did forward swing duration; how-ever, prepubescent players spent a significantly smaller proportion of the serve in the preparation phase (Figure 4). Conversely, propulsion represented a significantly smaller proportion of the serve in the two older groups. The time margin between trophy position and ball zenith was significantly lower in the two older groups.

DiscussionThere appears sufficient evidence to suggest that the female service action undergoes a significant evolution between prepubescence and adulthood. Most notably, the contributions of leg drive, shoulder-over-shoulder

trunk rotation, internal shoulder rotation and wrist flexion appear to increase with age, allowing older players to generate higher racket velocity. However, the postures that players assume during the preparation phase of the skill are extremely similar.

Peak knee flexion, a primary visual aid for coaches in assessing leg drive,32 did not differ with age and is comparable to what has been observed in adult males.11 On the contrary, the vertical hip velocities denote signifi-cantly greater leg drive in the adult cohort, suggesting that peak knee flexion is a poor indicator of leg drive. Adults also displayed enhanced triple extension in each lower limb during leg drive. Interestingly, when these exten-sion velocities were considered independently, extension was greater at the ankle than the knee or hip and was the best discriminator of the adults from the two younger groups. Thus, in the same way the plantar flexors are the primary contributors to forward propulsion in gait,33 this muscle group contributes critically to females’ leg drive. Therefore, with the feet fixed on the ground in the trophy position, the knee flexion that precedes leg drive appears as crucial to placing the plantar flexors on prestretch, as it is to the knee extensors. Ultimately, junior and senior players appear equally adept at prepar-ing the lower limbs to push-off; however, the potency of ensuing leg drive is reduced in junior players. Though strength was not directly assessed in this study (provid-ing a limitation herein), antecedent work suggests that the reduced leg drive in the junior groups may be due to strength deficits34 and the mechanical properties of developing musculature.23 These physical factors may be rate limiters and advocate an early instructional focus on the timing and coordination of leg drive, as opposed to its magnitude. Interestingly, the peak back hip velocities in the adult group (2.3 m·s–1) are comparable to previous descriptions of the male serve, which does not conform to established gender differences in peak lower extremity torque35 and jumping ability.36,37 This may be explained by the fact that, unlike peak torque generation and peak countermovement jumps, leg drive is not a maximal effort skill but rather optimal.

The peak separation angles denote how priming of the trunk was most pronounced in prepubescent players, however, peak twist velocity of the trunk was not affected by age. Peak trunk tilt also developed independent of age; however, the subsequent shoulder-over-shoulder velocity was significantly higher in the adult group. Considered alongside the racket kinematics, these findings support the positive relationship proposed to exist between shoul-der-over-shoulder rotation and racket velocity.1,9,16 The reduced shoulder-over-shoulder rotation in junior players may be due to the rate-limiting effect of trunk strength, which increases with age.22 Alternatively, the reduced vigor of junior leg drive may restrict the magnitude of shoulder-over-shoulder rotation later in the kinematic the kinematic chain.7,16 Practically, this implies that leg drive plays a crucial precursory role in the generation of potent frontal plane trunk rotation.

578

Tab

le 3

R

acke

t an

d b

all k

inem

atic

s

Pre

pu

bes

cen

tP

ub

esce

nt

Ad

ult

AN

OVA

Po

st H

oc

Varia

ble

Uni

tM

ean

SD

Mea

nS

DM

ean

SD

FP

G1

vs G

2G

1 vs

G3

G2

vs G

3

Rac

ket

R

acke

t Bac

kwar

d T

ilt a

t Im

pact

deg

153

102

93

11.0

98<

.001

**

*

R

acke

t Vel

ocity

at I

mpa

ct: X

m/s

–12

43

32

11.5

02<

.001

**

*

R

acke

t Vel

ocity

at I

mpa

ct: Y

m/s

293

403

433

65.1

83<

.001

**

*

R

acke

t Vel

ocity

at I

mpa

ct: Z

m/s

41

42

53

.133

.876

R

esul

tant

Rac

ket V

eloc

ity a

t Im

pact

m/s

303

413

433

73.1

76<

.001

**

*

Bal

l

Bal

l Pos

ition

at B

Z: X

cm3

141

11–3

13.4

68.6

31

B

all P

ositi

on a

t BZ

: Ycm

388

518

494

10.7

61<

.001

**

*

B

all P

ositi

on a

t BZ

: Zcm

311

2533

017

336

164.

289

.024

B

all P

ositi

on a

t Im

pact

: Xcm

–918

–812

–14

16.4

43.6

46

B

all P

ositi

on a

t Im

pact

: Ycm

4811

638

615

9.72

1.0

01*

**

B

all P

ositi

on a

t Im

pact

: Zcm

214

824

89

254

774

.740

<.0

01*

**

Bal

l Rot

atio

nde

g/s

3199

2045

7185

2532

6359

1746

10.7

06<

.001

**

*

B

all S

pin

Axi

s: E

leva

tion

Ang

lede

g47

1773

870

5.1

88<

.001

**

*

*Sig

nific

ant a

t P <

.01

leve

l.

Kinematics of the Elite Female Serve 579

Disparate trunk rotations produced markedly differ-ent trunk orientations at impact in the respective groups. In the transverse plane, the trunk and pelvis alignments at impact denote how twist rotation of the body was most pronounced in prepubescent players. In the frontal plane, increased trunk tilt at impact was indicative of heightened shoulder-over-shoulder rotation in the two older groups. Thus, it appears that the two older groups sacrificed ≈20° of transverse plane rotation to yield an extra ≈15° of trunk tilt at impact. A more tilted trunk effectively permits internal rotation at the shoulder to direct the racket toward the target at impact12 and may help to account for the differences in racket velocity.

Figure 3 — Elevation angle of the spin axis (server’s view from the baseline, looking toward the service box); Prepubescent: 47°, Pubescent: 73°, Adult: 70°. *Significant difference between the prepubescent group and the two older groups (P < .01).

Figure 4 — Key time points of the time-normalized serve. *Significant difference between the prepubescent group and the two older groups (P < .01).

These findings necessitate a practical appreciation for reduced frontal plane trunk rotation in prepubescent players and question the appropriateness of demanding shoulder-over-shoulder rotation in this population.

The magnitude of peak external rotation in all groups (≈130–140°) was comparable with what has previously been measured in male tennis players using this shoul-der definition7,38 and thus appears to transcend age and gender. The lack of age differences are interesting given that vigorous leg drive is considered to amplify external rotation.10 The racket parameters provide a possible exponent, whereby a relatively higher swing moment of inertia (due to age-related strength deficiencies) may

580 Whiteside et al.

passively contribute to the external rotation of the prepu-bescent shoulder. Internal rotation velocity was signifi-cantly higher in the two older groups (≈2000–2100°·s–1) compared with the prepubescent group (≈1300°·s–1), although still lower than what has been recorded in high performance males.4 This gender disparity intuitively deserves consideration in instructional settings, as does the fact that the contribution of internal rotation appears to increase after the onset of puberty in females. The rea-sons for the latter may be related to the shoulder dynamics and the aforementioned racket constraint in prepubescent players. In prepubescent players, the combined effects of developing internal rotators and a relatively higher racket swing weight may increase the transition period from external to internal rotation. Such a pause would dissipate stored elastic energy39 to the significant detri-ment of internal rotation velocity.40 With this in mind, appropriate racket and/or court scaling in prepubescent populations may reduce the potentially constraining effects of the equipment and immature musculature, permitting a more functional representation of the adult serve. In the absence of such scaling, coaches should expect reduced internal rotation in prepubescent players and tailor their expectations accordingly.

Their significantly lower elbow extension velocities, alongside a significantly more flexed elbow at impact point to reduced elbow involvement in prepubescent players. In maintaining a flexed elbow, these players conceded the increased impact height and ball spin that accompanies a more extended arm. Namely, vigorous elbow extension during the forward swing is considered fundamental to hitting “up and out”18, an action that is widely regarded as a key component of a powerful serve.41 This would therefore appear to support the contention that junior players do not commonly employ an up and out hitting action.42 However, a more flexed elbow is known to augment the contribution of internal rotation to racket velocity.4,43 Therefore, prepubescent players may have sacrificed elbow extension to enhance the contribution of internal rotation. With the elbow extension velocities in the two older groups matching previous descriptions of male and female professional players,3 it appears that elbow involvement (and, with it, an up and out racket trajectory) matures soon after the onset of puberty and may be independent of gender.

Previous work has proposed that flexion at the wrist is the second largest contributor to racket velocity at impact.4,8 This contention is supported by the cur-rent study in that peak wrist flexion velocity and racket velocity both trended significantly higher with age, where the adult players attained values (wrist flexion ω: 1900°·s–1; racket velocity: 43.4 m·s–1) comparable to those previously observed in professionals.3 It therefore seems logical to deduce that the contribution of wrist flexion to racket velocity increases as a player matures.

Shoulder abduction at impact appears an invariant feature of the serve. That is, no difference was found in this variable, which was between 95–105° in all groups and therefore consistent with previous descriptions of the

elite adult serve.3,7 Within this range, players are able to minimize shoulder loading, without compromising racket velocity.11 Consequently, it would appear that instruction of this aspect of the serve could proceed independent of age and gender.

In light of the varying angular joint velocities, it is not surprising that resultant racket velocity was sig-nificantly greater in the two older groups (Prepubescent: 29.9 m·s–1; Pubescent: 40.7 m·s–1; Adult: 43.4 m·s–1). The racket velocity also had a greater lateral (x) component, from left to right, at impact in the older two groups. This is likely a product of the differences in elbow extension velocity, as it is this motion that helps to produce lateral propulsion of the racket.8 Such a racket trajectory is analogous with “hitting across the ball,” thus explaining the increased ball spin and more vertical spin axis in the two older groups.

With impact height constraining the requisite ball trajectory for a successful serve,44 prepubescent play-ers responded by tilting the racket significantly further backward by a mean of ≈5° at impact, effectively increasing the projection angle of the ball. As racket tilt is presumably an upshot of wrist flexion, this may help to explain the lack of wrist flexion velocity observed in the prepubescent group. Accordingly, stature could be considered another rate limiter to serve performance, whereby developing players must use provisional move-ment patterns until they are tall enough to execute a more adult-like serve. However, appropriate scaling of the court dimensions and net height could control this effect and promote skill acquisition.45

Despite stature differences, toss height was not significantly affected age and infers that junior players require a relatively higher ball toss. Impact location was consistent with previous analyses of male players,2,17 whereby contact occurred forward and lateral to the front foot. Impact was significantly further into the court in the two older groups (Prepubescent: 48 cm; Pubescent: 63 cm; Adult: 61 cm), owing to their ball toss. Given the more vertical axis about which prepubescent players rotate, an impact location closer to the body is unsurpris-ing. In contrast, more pronounced shoulder-over-shoulder rotation intuitively produces an impact location further into the court, as was observed in the older players. Worth noting is the fact that impact height, when scaled to a percentage of stature, remained constant at 150% irrespective of age. This value supports previous tennis research5 and appears to be an invariant feature of the tennis serve, across gender, age and serve type.

A more pronounced lateral racket trajectory logically accounts for the significantly higher ball spin in the two older groups. This racket trajectory also accounts for the differences in the spin axis which, consistent with previ-ous work,46 was more upright in the two older groups (elevation: ≈70°) compared with the tilted axis observed in prepubescent players (elevation: 47°). These findings suggest a close relationship between the trajectory of the racket at impact and the spin axis of the ball and have applications in other serves.

Kinematics of the Elite Female Serve 581

Preparation constituted a greater proportion of the serve in the two older groups, who spent less time in propulsion. The opposite was true for prepubescent play-ers whose longer propulsion phase impeded an explosive action. This difference appeared to relate to the height of the ball toss and players’ ability to use the position of ball to regulate their swing path. The two older groups adjusted their swing such that their arrival in the trophy position coincided with ball zenith. However, prepubescent players appeared to initially move their body independently of the ball which, coupled with their relatively higher ball toss, resulted in them reaching the trophy position prematurely. In doing so, these players “held” this position, waiting for the appropriate moment to initiate leg drive, presum-ably negating the elastic potential of the muscles in the process.39 This inefficient stretch-shorten cycle presents another possible reason for the reduced ebullience of leg drive in the prepubescent cohort. An early instructional emphasis on coupling mechanics and perception (of ball position) may help to avert this issue and develop “rhythm.”

It is worth acknowledging that this study did not directly quantify the physical capacities that may dif-ferentiate the cohorts. Though these are implied through extant literature, their direct quantification may allow their relationship with performance to be more robustly explored. Similarly, heterogeneous instruction has the potential to account for age effects and may be limited in the future by tracking a prepubescent cohort longitu-dinally throughout development. The exclusive recruit-ment of either current or past members of the same development program (the Tennis Australia development pathway) was an attempt to control this.

In 2008, the International Olympic Committee declared that “more scientific research should be carried out to better identify the parameters of training the elite child athlete.”47 Accordingly, the current study provides a description of the kinematic parameters embodying the elite female tennis serve during development. Spe-cifically, the propulsive contributions of the serving arm become more pronounced after the onset of puberty, while leg drive and shoulder-over-shoulder rotation mature even later in development. These factors suggest that a heavy coaching emphasis on lower limb and trunk mechanics may not possess great value in junior environments. In addition, the activity of the degrees of freedom appears to increase with age, supporting the notion that junior players actively restrain certain movements (in this case elbow extension and wrist flexion) in an attempt to sim-plify complex movement skills.20 Consequently, attempts to indoctrinate refined movement patterns from a young age seem misplaced, as movement strategies inherently evolve as players mature and embrace more degrees of freedom. The formative years may be better spent refining those features of the serve that remain invariant through-out development such as key preparatory postures, or perception-action couplings that regulate “rhythm.” Equally, appropriate constraining of the task (equipment scaling) may act to preserve kinematic relevance between the junior and senior serve and promote skill acquisition.45

Acknowledgments

The authors wish to thank Tennis Australia’s Athlete Develop-ment Department and the Australian Institute of Sport for their support in the production of this work.

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