Coord and variability Whiteside, Elliott, Lay & Reid, 2014b
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Coordination and variability in the elite female
tennis serve
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Impact Factor: 2.25 · DOI: 10.1080/02640414.2014.962569 · Source: PubMed
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Coordination and variability in the elite female tennis
serve
a
a
a
David Whit eside , Bruce Clif f ord Elliot t , Brendan Lay & Machar Reid
b
a
School of Sport Science, Exercise and Healt h, Universit y of West ern Aust ralia, Crawley,
Aust ralia
b
Sport s Science and Medicine Unit , Tennis Aust ralia, Melbourne, Aust ralia
Published online: 30 Oct 2014.
To cite this article: David Whit eside, Bruce Clif f ord Elliot t , Brendan Lay & Machar Reid (2014): Coordinat ion and variabilit y in
t he elit e f emale t ennis serve, Journal of Sport s Sciences, DOI: 10. 1080/ 02640414. 2014. 962569
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Journal of Sports Sciences, 2014
http://dx.doi.org/10.1080/02640414.2014.962569
Coordination and variability in the elite female tennis serve
DAVID WHITESIDE1, BRUCE CLIFFORD ELLIOTT1, BRENDAN LAY1 & MACHAR REID2
1
School of Sport Science, Exercise and Health, University of Western Australia, Crawley, Australia and 2Sports Science and
Medicine Unit, Tennis Australia, Melbourne, Australia
Downloaded by [David Whiteside] at 12:37 03 November 2014
(Accepted 1 September 2014)
Abstract
Enhancing the understanding of coordination and variability in the tennis serve may be of interest to coaches as they work
with players to improve performance. The current study examined coordinated joint rotations and variability in the lower
limbs, trunk, serving arm and ball location in the elite female tennis serve. Pre-pubescent, pubescent and adult players
performed maximal effort flat serves while a 22-camera 500 Hz motion analysis system captured three-dimensional body
kinematics. Coordinated joint rotations in the lower limbs and trunk appeared most consistent at the time players left the
ground, suggesting that they coordinate the proximal elements of the kinematic chain to ensure that they leave the ground at
a consistent time, in a consistent posture. Variability in the two degrees of freedom at the elbow became significantly greater
closer to impact in adults, possibly illustrating the mechanical adjustments (compensation) these players employed to
manage the changing impact location from serve to serve. Despite the variable ball toss, the temporal composition of the
serve was highly consistent and supports previous assertions that players use the location of the ball to regulate their
movement. Future work should consider these associations in other populations, while coaches may use the current findings
to improve female serve performance.
Keywords: biomechanics, coaching, compensation, perception-action, motor control
Introduction
A proficient serve is a crucial part of a tennis player’s
stroke repertoire as it can be used to gain an advantage at the start of each point (McGinnis, 2013).
Skilled execution of the serve involves concatenation
of the lower limbs, trunk and serving arm to generate
racquet head speed and transfer momentum to the
ball (Bahamonde, 2000). While the notion of coordination has been explored variously in past tennis
research, variability in specific coordinative joint
rotations (e.g. concurrent extension of the hip,
knee and ankle during leg drive; concurrent upper
extremity joint rotations during forwardswing) is yet
to be comprehensively examined and may provide
coaches with useful information.
Investigations of the serve have primarily focused
on discrete values such as joint ranges, discrete kinematic peaks and phase durations. This research has
confirmed that flexion at the knee and ankle precedes the vigorous extension at these joints that propels the player into the air (Fleisig, Nicholls, Elliott,
& Escamilla, 2003; Reid, Elliott, & Alderson, 2008).
While the player is airborne, momentum is then
transferred to the serving arm through transverse,
frontal and sagittal plane trunk rotations
(Bahamonde, 2000; Elliott, 2006; Martin, Kulpa,
Delamarche, & Bideau, 2013). During the final
stages of the serve, vigorous internal rotation at the
shoulder and wrist flexion augments racquet head
speed (Elliott, 2006; Marshall & Elliott, 2000;
Tanabe & Ito, 2007), while pronation and extension
at the elbow act to orientate the racquet in a manner
befitting impact (Bahamonde, 2005; Elliott,
Marshall, & Noffal, 1995). Additionally, the direction of the ball toss ultimately produces an impact
location forward of, and lateral to, the front foot
(Chow et al., 2003; Reid, Whiteside, & Elliott,
2011). From a coaching perspective, these studies
provide relevant information regarding mechanics of
the service action and how players generate racquet
velocity and intercept the ball. There exists scope to
extend this work by exploring other aspects of the
service action, namely the role of coordinated joint
rotations and movement variability, as they relate to
performance.
Recent work has petitioned the exploration of
movement variability in sports biomechanics
research (Bartlett, Wheat, & Robins, 2007).
Correspondence: David Whiteside, School of Sport Science, Exercise & Health, University of Western Australia, 35 Stirling Highway, Crawley, Perth 6009,
Australia. E-mail: [email protected]
© 2014 Taylor & Francis
Downloaded by [David Whiteside] at 12:37 03 November 2014
2
D. Whiteside et al.
Documenting the variability of movement patterns
within the serve may uncover how tennis players
coordinate joint rotations to produce accurate, high
speed serves. Despite a long history of research
describing movement variability and its functional
relevance to human movement (Hatze, 1986;
Winter, 1984), movement variability in sporting
motions was often considered noise (Bartlett et al.,
2007). While this viewpoint has been tempered by
contemporary movement research, consistency in
some parameters related to performance is still considered critical to success.
In hitting and projectile tasks, the launch parameters critical to success relate to the trajectory and
orientation of the end-effector (i.e. the racquet, bat,
club, in hitting tasks, or the hand in throwing tasks),
as its terminal location, orientation and velocity will
ultimately determine the outcome of the task.
Indeed, a single study on the tennis serve has
reported that consistency in speed and location of
the serving hand around impact is positively related
to serve speed and accuracy (Antúnez, Hernández,
García, Vaíllo, & Arroyo, 2012). Other launch parameters that directly govern the outcome of the serve
include the impact height, ball projection angle and
racquet velocity, the combination of which are
thought to determine serve outcome (Whiteside,
Elliott, Lay, & Reid, 2013b). Though these launch
parameters are known to be important, the coordinated joint rotations that regulate their consistency
are not well understood.
Contemporary motor control research in sport
describes a particular form of coordinated joint rotation, referred to as mechanical compensation. In
human movement, this mechanism helps to regulate
the performance parameters that ultimately determine the outcome of the task (Dupuy, Mottet, &
Ripoll, 2000; Kudo, Tsutsui, Ishikura, Ito, &
Yamamoto, 2000; Smeets, Frens, & Brenner, 2002).
More explicitly, inadvertent variations in a given
execution parameter (e.g. a joint angle) are counteracted by the actions of other execution parameters
(Davids, Glazier, Araujo, & Bartlett, 2003). In this
way, errors that are introduced into the movement
system during performance can be managed, thus
preventing any negative influence on the task outcome that they would otherwise produce.
Mechanical compensation has been highlighted in
numerous projectile and striking sports including
golf drives (Horan, Evans, & Kavanagh, 2011), free
throw shooting in basketball (Button, MacLeod,
Sanders, & Coleman, 2003; Mullineaux & Uhl,
2010), underarm throwing (Dupuy et al., 2000;
Kudo et al., 2000), overarm throwing (Wagner,
Pfusterschmied, Klous, Von Duvillard, & Müller,
2012), table tennis forehands (Bootsma & Van
Wieringen, 1990), tennis forehands (Knudson,
1990), pistol shooting (Scholz, Schoner, & Latash,
2000) and dart throwing (Smeets et al., 2002). Since
mechanical compensation is an inherent biomechanical response to the demands presented when enacting
a particular movement task, it does not follow a consistent pattern. Therefore, variations in joint rotations
(or other mechanics) are no longer considered to be
indicative of movement system ineptitude. Rather,
movement variability is now considered critical to
the stabilisation of the performance parameters that
directly govern the outcome of the task (e.g. launch
parameters in the serve). In other words, variability in
the movement system is not detrimental so long as the
critical end point parameters remain stable. This
notion is reflected in recent tennis research, where
Whiteside et al. (2013b) showed that increased variability in coordinated elbow and wrist joint rotations
did not reduce serve accuracy. Given the whole-body
nature of the serve, other mechanics may also contribute to the compensatory process but are yet to be
characterised.
Movement variability in sporting actions is not
restricted to the magnitudes of discrete joint rotations. Research in other striking skills has highlighted
the importance of consistent temporal patterns (i.e.
the times at which specific movements occur) in
interceptive (batting) skills in cricket (Renshaw,
Oldham, Davids, & Golds, 2007) and baseball
(Katsumata, 2007), as well as the volleyball serve
(Davids, Kingsbury, Bennett, & Handford, 2001).
It has been suggested that these athletes utilise information from the incoming ball (i.e. its location) to
regulate the initiation of key propulsive movements,
hence ensuring that they intercept it at the appropriate moment. Similar strategies have been found in
the tennis serve, where players regulate their movements such that their arrival in the trophy position
coincides with ball zenith (Reid, Whiteside, &
Elliott, 2010; Whiteside, Elliott, Lay, & Reid,
2013a). Likewise, the timing of peak forward racquet
velocity is considered critical to accuracy in the first
serve (Antúnez et al., 2012). Ultimately, coordinated
joint rotations and/or temporal consistency may be
thought of as techniques that players exploit to simplify complex sporting movements. With this in
mind, it is expected that tennis players simplify
those aspects of the serve that are most important
to performance. At present, the particular methods
employed to do so are somewhat unclear as this
topic is yet to be comprehensively examined in the
tennis serve.
It has been suggested that, in learning movement
skills, inexperienced performers initially reduce the
skill complexity by restraining degrees of freedom
(Anderson & Sidaway, 1994; Stergiou & Decker,
2011). These degrees of freedom are then gradually
released as players become more familiar with the
Coordination and variability in the tennis serve
3
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Table I. Mean (± standard deviation) age, physical and menarchial characteristics of participants.
Group
N
Age (years)
Height (cm)
Mass (kg)
Experienced menarche
Time since menarche
Pre-pubescent
Pubescent
Adult
12
11
8
10.5 ± 0.5
14.6 ± 0.7
21.3 ± 3.8
143.5 ± 5.9
166.9 ± 4.7
169.2 ± 4.8
36.5 ± 3.7
56.7 ± 3.8
61.9 ± 4.2
No
Yes
Yes
N/A
6–18 months
>4 years
task and explore alternative solutions to the movement problem (Gentile, 2000; Newell, Deutsch,
Sosnoff, & Mayer-Kress, 2006). In this sense, children are often considered novices owing to their
motor inexperience (Guarrera-Bowlby & Gentile,
2004). Indeed, the popular “ten year (10,000 h)
rule” (Ericsson, Krampe, & Tesch-Römer, 1993)
virtually precludes any attempt to categorise children
as expert performers. This position is offered partial
support by Newell’s constraints model (Newell,
1986), which states that organismic factors (e.g.
anthropometry, strength, anticipatory ability) influence performance. As such, more physically and
mentally mature adults are expected to exhibit
more skilled performance than children. However,
“elite child athletes” present a unique cohort whose
motor proficiency may exceed their age-related
expectations and whose practice regimes are often
meticulous and intensive. Consequently, these athletes may not conform to traditional descriptions of
child motor performance and deserve investigation
in their own right. With movement patterns expected
to change throughout development, it seems necessary to separate performers according to level of
development when investigating movement in developing populations.
At present, there exists scope to supplement existing tennis literature with research focused on coordinated joint rotations and variability in the serve.
Therefore, the aims of this study were to examine
coordinated joint rotations and variability in the
lower limbs, trunk, serving arm and ball in the tennis
serves of elite female players at three stages of development. It was expected that functional movement
variability in the distal joints would manifest during
serve performance to preserve a successful outcome,
though its disposition would change with age.
However, the temporal composition of the serve
was expected to be comparatively more consistent
as players stabilised the timing of critical postures
with respect to impact.
Methods
Participants
The relevant Human Ethics Committee approved
this study prior to recruitment. Thirty-one elite
female tennis players provided informed consent
and were arranged into pre-pubescent, pubescent
and adult groups based on their age and menarchial
status (Table I). At the time of testing, players in the
pre-pubescent and pubescent groups held a top 8
national ranking for their respective age groups,
while the adult players possessed a professional
(Women’s Tennis Association: WTA) ranking
higher than 325. This cohort was the same as that
used in a previous study (Whiteside et al., 2013a).
Protocol
The protocol was completed at an indoor biomechanics laboratory, where a full-size tennis court was
constructed. Sixty retro-reflective markers, 14 mm
in diameter, were affixed to each player according an
established, calibrated anatomical systems (CAST)based, full body marker set (Besier, Sturnieks,
Alderson, & Lloyd, 2003; Chin, Elliott, Alderson,
Lloyd, & Foster, 2009; Lloyd, Alderson, & Elliott,
2000). Three hemispherical markers, composed of
ultra-light foam (radius 7 mm), were placed on the
racquet and three more on ball (Whiteside, Chin, &
Middleton, 2013) to create coordinate systems
therein. Prior to the serving protocol, dynamic calibration of a ≈5.5 (deep) 4 (wide) 4 (high) m
capture volume yielded a mean residual calibration
error smaller than 0.002 m.
Each player completed a standard 10 min warm
up and used their own racquet to complete the
protocol. Verbal confirmation of preparedness
prompted the initiation of the test protocol in
which players performed maximal effort flat (i.e.
“power”) serves. Players were instructed to aim for
a familiar 1 1 m target (Elliott et al., 1995;
Martin et al., 2013; Reid et al., 2008; Sakurai,
Reid, & Elliott, 2012) bordering the T of the service box (right-handers: deuce court; left-handers:
advantage court) and to hit all serves with the same
effect (i.e. to emphasise speed as opposed to spin).
Five blocks of eight serves were performed with a 2
min rest period separating successive blocks.
Three-dimensional (3D) marker trajectories were
recorded using a 22-camera VICON MX system
(VICON Motion Systems, Oxford, UK) operating
at 500 Hz. The global reference frame originated at
the centre of the baseline, where positive x pointed
rightward along the baseline, positive y pointed to
the net and positive z pointed up. Five of each
4
D. Whiteside et al.
player’s fastest serves landing in the target area were
selected for analysis.
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Data processing
Gaps in the raw marker trajectories were interpolated
using a cubic spline within the VICON Nexus software. A second-order polynomial extrapolation specific to tennis limited the distortion of kinematic data
around impact (Knudson & Bahamonde, 2001;
Reid, Campbell, & Elliott, 2012). Data were subsequently filtered using a Woltring filter (Woltring,
1986) with the optimal mean squared error of
2 mm determined by a residual analysis, and then
modelled using the University of Western Australia’s
full body (Besier et al., 2003; Lloyd et al., 2000),
racquet and ball (Whiteside et al., 2013) models.
Joint rotations were expressed using the Euler ZXY
sequence. Trunk rotations were expressed relative to
a virtual anatomical reference frame (x pointing forward towards the net, y pointing up and z pointing
right) originating at the global origin. To maintain
consistency in the statistical analyses, kinematics for
the left-handed players were inverted where appropriate such that all players could be considered
together as right-hand dominant (Campbell,
Straker, O’Sullivan, Elliott, & Reid, 2013;
Whiteside et al., 2013b).
Relevant events of the service action
Figure 1 denotes how the service action was deemed
to begin at the instant the ball was released from the
Figure 1. Key time points of interest in the tennis serve.
hand. Ball zenith represented the peak vertical displacement of the ball during its toss. The subsequent
nadir of vertical racquet displacement was the racquet low point, which is usually coincident with a
player leaving the ground (Bonnefoy, Slawinski,
Leveque, Riquet, & Miller, 2009). Impact was
defined as the frame (i.e. 0.002 s) prior to racquetball contact. The duration of the serve was considered as the time period between ball release and
impact. Leg drive was defined as the period from
ball zenith to racquet low point, while racquet low
point to impact was considered the forwardswing
phase of the serve.
Variables of interest
To gauge inter-limb coordination during leg drive,
the relative bilateral flexion-extension motion of the
ankles was compared, as was the relative bilateral
flexion-extension motion of the knees. The relative
transverse (twist) and frontal plane (shoulder-overshoulder) trunk rotations prior to impact indicated
how players manipulated the trunk during this time,
while relative extension and pronation denoted the
same for the elbow in the serving arm. The location
of the ball during the toss was expressed relative to
the front toe (Chow et al., 2003; Reid et al., 2011),
and its spatial variability measured at both ball zenith
and impact. Finally, the timing of both ball zenith
and racquet low point (expressed as a percentage of
the serve), and also duration of forwardswing provided an insight into the temporal pattern of the
service action.
Coordination and variability in the tennis serve
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Assessment of variability in joint mechanics and the ball
toss
Angle-angle plots provided a qualitative insight into
relative joint rotations. The variability of these traces
was quantified using the coefficient of correspondence – a vector coding technique that assesses the
repeatability of several variable–variable traces
(Tepavac & Field-Fote, 2001). Unlike alternative
vector coding methods that consider only vector
direction, the coefficient of correspondence is advantageous in that it also considers vector magnitude
(Wheat & Glazier, 2006). Ultimately, the coefficient
of correspondence quantified the magnitude of
variability in coordinated joint rotations using a
value between 0 and 1 (0 = no variability; 1 = maximum variability). Akin to previous work (Horan
et al., 2011), the average coefficient of correspondence value ±5% of the relevant event represented
the variability at that time point.
The 3D (xσ , yσ and zσ ) standard deviations of ball
displacement at ball zenith and impact were calculated for each player across their five trials. The
mean and standard deviations of these standard
deviations provided a gauge of ball toss variability
for each group (Davids et al., 2001; Reid et al.,
2010). Further, each standard deviation was doubled
(effectively creating error bar representing one standard deviation either side of the mean, in each
dimension) and then multiplied (2xσ 2yσ 2zσ )
to yield the “variability volume”: a singular quantity
of the ball’s 3D spatial variability for each player,
across their five serves.
5
Statistical analyses
Four split plot analyses of variance (SPANOVAs)
were used to examine between group (i.e. differences
between each group), within group (i.e. differences
between each time point) and interaction (i.e.
group time) effects in the coefficient of correspondence. The same procedure was used to compare the variability volumes in each group at ball
zenith and impact. The ranges of motion at the
ankles, knees, trunk and elbow were compared
between groups using one-way analyses of variance.
Where significant main effects existed, Bonferronicorrected post-hoc tests were employed to find the
source of the difference. In total, 13 analyses of
variance were performed, thus inflating the risk of
encountering type I errors. To account for this, the
significance level was adjusted sequentially for each
test according to the correction method proposed by
Holm (1979). The p-value was not sufficient to
reject the null hypothesis in the eleventh test, indicating that all significance in this study was reported
using a p-value of 0.0125. The temporal variability of
ball zenith, racquet low point and forwardswing
duration were interpreted descriptively.
Results
Ankle mechanics
The representative traces in Figure 2 represent how
all groups utilised simultaneous plantar flexion at
both ankles and simultaneous extension in both
Figure 2. Mean coefficients of correspondence and representative angle–angle plots for the ankles and knees.
Note: *Significant difference between ball zenith and racquet low point in all groups.
6
D. Whiteside et al.
During leg drive, the back knee extended through
a significantly (F2,28 = 15.954; P < .001) greater
range in the pubescent (MD: 22.46°; CI: 10.97–
33.95°; P < .001) and adult (MD: 22.59°; CI:
10.02–35.15°; P < .001) groups compared with the
pre-pubescent group, though range of motion
(RoM) at the front knee was not significantly different between groups (F2,28 = 4.364; P = .022).
Trunk mechanics
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Figure 3. Mean ranges of flexion-extension motion at the ankles
and knees during leg drive.
Note: *Significant difference between groups.
knees during leg drive. The coefficient of correspondence for ankle motion displayed a significant main
effect for group (F2,28 = 16.499; P < .001), which
post-hoc tests revealed to be a product of significantly
higher overall variability in the pubescent group
compared with the pre-pubescent group (mean difference (MD): 0.14; 95% confidence interval (CI):
0.08–0.20; P < .001). From a temporal perspective,
the bilateral coupling of ankle plantarflexion was
significantly more consistent at racquet low point
compared with ball zenith and independent of
group (F1,28 = 151.111; MD: 0.25; CI: 0.21–0.29;
P < .001).
The range of plantar flexion at the back ankle was
significantly (F2,28 = 10.913; P < .001) smaller during leg drive in the pre-pubescent group compared
with the pubescent (MD: 14.12°; CI: 5.10–23.14°;
P = .001) and adult (MD: 15.24°; CI: 5.38–25.11°;
P = .002) groups. Likewise, the range of plantar
flexion at the front ankle (F2,28 = 17.299; P < .001)
was significantly smaller during leg drive in the prepubescent group compared with the pubescent
(MD: 17.83°; CI: 8.12–27.53°; P < .001) and adult
(MD: 21.91°; CI: 11.29–32.52°; P < .001) groups
(Figure 3).
Knee mechanics
Regarding the coefficient of correspondence for
bilateral knee extension, the main effect for group
was not significant (F2,28 = .162; P = .852), although
a significant main effect for time revealed that relative knee extension was significantly more consistent
at racquet low point compared with ball zenith
(F1,28 = 86.376; MD: 0.26; CI: 0.20–0.31;
P < .001).
The variability of concurrent frontal and transverse
plane trunk rotations revealed a significant main
effect for group (F2,28 = 10.530; P < .001), wherein
relative trunk rotations were significantly more consistent in the pubescent (MD: 0.08; CI: 0.03–0.13;
P = .001) and adult (MD: 0.08; CI: 0.02–0.13;
P = .005) groups compared with the pre-pubescent
group. A post-hoc decomposition of the significant
main effect for time point (F2,28 = 67.672;
P < .001) revealed that relative trunk motion was
significantly different at all time points (most variable at ball zenith, most consistent at racquet low
point, in between at impact).
The range of trunk twist rotation did not differ
with age (F2,28 = .383; P = .686); however, the range
of shoulder-over-shoulder rotation was significantly
(F2,28 = 13.744; P < .001) larger in the pubescent
(MD: 20.12°; CI: 9.45–30.74°; P < .001) and adult
(MD: 18.04°; CI: 6.43–29.65°; P = .001) groups
compared with the pre-pubescent group.
Elbow mechanics
A main effect for group (F2,28 = 7.606; P = .002) was
discovered for the coefficient of correspondence of
the relative elbow joint rotations (flexion and radioulnar pronation). Further analyses revealed that the
pubescent group was significantly more consistent
than the adult group (MD: 0.12; CI: 0.04–0.21;
P = .002) over the period in question. Additionally,
post-hoc analyses of the significant time effect
(F2,28 = 70.069; P < .001) showed that variability
was significantly different at each of the three time
points (becoming progressively more variable
between ball zenith and impact). Importantly, an
interaction effect was also noted (F2,28 = 24.089;
P < .001) and was found to relate to the fact that
the coupled elbow joint rotations only became significantly more variable between racquet low point
and impact in the adult group (MD: 0.47; CI: 0.31–
0.64; P < .001) (Figure 4).
The range of flexion-extension at the elbow
between ball zenith and impact did not differ
between groups (F2,28 = .953; P = .398). However,
during the same period, the range of radio-ulnar
pronation was significantly (F2,28 = 12.116;
Downloaded by [David Whiteside] at 12:37 03 November 2014
Coordination and variability in the tennis serve
7
Figure 4. Mean coefficients of correspondence and representative angle–angle plots for the trunk and elbow.
Note: *Significant difference between the time points in all groups. † Significant difference between racquet low point and impact in the
adult group.
compared with ball zenith (MD: 1672 cm3; CI:
959–2386 cm3) (Table II). Descriptively, ball location was most consistent in the vertical direction at
impact compared with the left-right and forwardbackward directions (Table III).
Temporal variability of the ball zenith and racquet lowpoint events
Qualitatively, although the timing of racquet low
point was comparatively more consistent than ball
zenith in all groups, both events displayed impressive
temporal consistency. The duration of forwardswing
was equally consistent, displaying an average standard deviation
Coordination and variability in the elite female
tennis serve
ARTICLE in JOURNAL OF SPORTS SCIENCES · OCTOBER 2014
Impact Factor: 2.25 · DOI: 10.1080/02640414.2014.962569 · Source: PubMed
CITATIONS
READS
4
201
4 AUTHORS, INCLUDING:
Bruce C Elliott
Brendan Lay
University of Western Australia
University of Western Australia
297 PUBLICATIONS 4,526 CITATIONS
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Machar Reid
Tennis Australia
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All in-text references underlined in blue are linked to publications on ResearchGate,
letting you access and read them immediately.
Available from: Bruce C Elliott
Retrieved on: 11 March 2016
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Journal of Sports Sciences
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Coordination and variability in the elite female tennis
serve
a
a
a
David Whit eside , Bruce Clif f ord Elliot t , Brendan Lay & Machar Reid
b
a
School of Sport Science, Exercise and Healt h, Universit y of West ern Aust ralia, Crawley,
Aust ralia
b
Sport s Science and Medicine Unit , Tennis Aust ralia, Melbourne, Aust ralia
Published online: 30 Oct 2014.
To cite this article: David Whit eside, Bruce Clif f ord Elliot t , Brendan Lay & Machar Reid (2014): Coordinat ion and variabilit y in
t he elit e f emale t ennis serve, Journal of Sport s Sciences, DOI: 10. 1080/ 02640414. 2014. 962569
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Journal of Sports Sciences, 2014
http://dx.doi.org/10.1080/02640414.2014.962569
Coordination and variability in the elite female tennis serve
DAVID WHITESIDE1, BRUCE CLIFFORD ELLIOTT1, BRENDAN LAY1 & MACHAR REID2
1
School of Sport Science, Exercise and Health, University of Western Australia, Crawley, Australia and 2Sports Science and
Medicine Unit, Tennis Australia, Melbourne, Australia
Downloaded by [David Whiteside] at 12:37 03 November 2014
(Accepted 1 September 2014)
Abstract
Enhancing the understanding of coordination and variability in the tennis serve may be of interest to coaches as they work
with players to improve performance. The current study examined coordinated joint rotations and variability in the lower
limbs, trunk, serving arm and ball location in the elite female tennis serve. Pre-pubescent, pubescent and adult players
performed maximal effort flat serves while a 22-camera 500 Hz motion analysis system captured three-dimensional body
kinematics. Coordinated joint rotations in the lower limbs and trunk appeared most consistent at the time players left the
ground, suggesting that they coordinate the proximal elements of the kinematic chain to ensure that they leave the ground at
a consistent time, in a consistent posture. Variability in the two degrees of freedom at the elbow became significantly greater
closer to impact in adults, possibly illustrating the mechanical adjustments (compensation) these players employed to
manage the changing impact location from serve to serve. Despite the variable ball toss, the temporal composition of the
serve was highly consistent and supports previous assertions that players use the location of the ball to regulate their
movement. Future work should consider these associations in other populations, while coaches may use the current findings
to improve female serve performance.
Keywords: biomechanics, coaching, compensation, perception-action, motor control
Introduction
A proficient serve is a crucial part of a tennis player’s
stroke repertoire as it can be used to gain an advantage at the start of each point (McGinnis, 2013).
Skilled execution of the serve involves concatenation
of the lower limbs, trunk and serving arm to generate
racquet head speed and transfer momentum to the
ball (Bahamonde, 2000). While the notion of coordination has been explored variously in past tennis
research, variability in specific coordinative joint
rotations (e.g. concurrent extension of the hip,
knee and ankle during leg drive; concurrent upper
extremity joint rotations during forwardswing) is yet
to be comprehensively examined and may provide
coaches with useful information.
Investigations of the serve have primarily focused
on discrete values such as joint ranges, discrete kinematic peaks and phase durations. This research has
confirmed that flexion at the knee and ankle precedes the vigorous extension at these joints that propels the player into the air (Fleisig, Nicholls, Elliott,
& Escamilla, 2003; Reid, Elliott, & Alderson, 2008).
While the player is airborne, momentum is then
transferred to the serving arm through transverse,
frontal and sagittal plane trunk rotations
(Bahamonde, 2000; Elliott, 2006; Martin, Kulpa,
Delamarche, & Bideau, 2013). During the final
stages of the serve, vigorous internal rotation at the
shoulder and wrist flexion augments racquet head
speed (Elliott, 2006; Marshall & Elliott, 2000;
Tanabe & Ito, 2007), while pronation and extension
at the elbow act to orientate the racquet in a manner
befitting impact (Bahamonde, 2005; Elliott,
Marshall, & Noffal, 1995). Additionally, the direction of the ball toss ultimately produces an impact
location forward of, and lateral to, the front foot
(Chow et al., 2003; Reid, Whiteside, & Elliott,
2011). From a coaching perspective, these studies
provide relevant information regarding mechanics of
the service action and how players generate racquet
velocity and intercept the ball. There exists scope to
extend this work by exploring other aspects of the
service action, namely the role of coordinated joint
rotations and movement variability, as they relate to
performance.
Recent work has petitioned the exploration of
movement variability in sports biomechanics
research (Bartlett, Wheat, & Robins, 2007).
Correspondence: David Whiteside, School of Sport Science, Exercise & Health, University of Western Australia, 35 Stirling Highway, Crawley, Perth 6009,
Australia. E-mail: [email protected]
© 2014 Taylor & Francis
Downloaded by [David Whiteside] at 12:37 03 November 2014
2
D. Whiteside et al.
Documenting the variability of movement patterns
within the serve may uncover how tennis players
coordinate joint rotations to produce accurate, high
speed serves. Despite a long history of research
describing movement variability and its functional
relevance to human movement (Hatze, 1986;
Winter, 1984), movement variability in sporting
motions was often considered noise (Bartlett et al.,
2007). While this viewpoint has been tempered by
contemporary movement research, consistency in
some parameters related to performance is still considered critical to success.
In hitting and projectile tasks, the launch parameters critical to success relate to the trajectory and
orientation of the end-effector (i.e. the racquet, bat,
club, in hitting tasks, or the hand in throwing tasks),
as its terminal location, orientation and velocity will
ultimately determine the outcome of the task.
Indeed, a single study on the tennis serve has
reported that consistency in speed and location of
the serving hand around impact is positively related
to serve speed and accuracy (Antúnez, Hernández,
García, Vaíllo, & Arroyo, 2012). Other launch parameters that directly govern the outcome of the serve
include the impact height, ball projection angle and
racquet velocity, the combination of which are
thought to determine serve outcome (Whiteside,
Elliott, Lay, & Reid, 2013b). Though these launch
parameters are known to be important, the coordinated joint rotations that regulate their consistency
are not well understood.
Contemporary motor control research in sport
describes a particular form of coordinated joint rotation, referred to as mechanical compensation. In
human movement, this mechanism helps to regulate
the performance parameters that ultimately determine the outcome of the task (Dupuy, Mottet, &
Ripoll, 2000; Kudo, Tsutsui, Ishikura, Ito, &
Yamamoto, 2000; Smeets, Frens, & Brenner, 2002).
More explicitly, inadvertent variations in a given
execution parameter (e.g. a joint angle) are counteracted by the actions of other execution parameters
(Davids, Glazier, Araujo, & Bartlett, 2003). In this
way, errors that are introduced into the movement
system during performance can be managed, thus
preventing any negative influence on the task outcome that they would otherwise produce.
Mechanical compensation has been highlighted in
numerous projectile and striking sports including
golf drives (Horan, Evans, & Kavanagh, 2011), free
throw shooting in basketball (Button, MacLeod,
Sanders, & Coleman, 2003; Mullineaux & Uhl,
2010), underarm throwing (Dupuy et al., 2000;
Kudo et al., 2000), overarm throwing (Wagner,
Pfusterschmied, Klous, Von Duvillard, & Müller,
2012), table tennis forehands (Bootsma & Van
Wieringen, 1990), tennis forehands (Knudson,
1990), pistol shooting (Scholz, Schoner, & Latash,
2000) and dart throwing (Smeets et al., 2002). Since
mechanical compensation is an inherent biomechanical response to the demands presented when enacting
a particular movement task, it does not follow a consistent pattern. Therefore, variations in joint rotations
(or other mechanics) are no longer considered to be
indicative of movement system ineptitude. Rather,
movement variability is now considered critical to
the stabilisation of the performance parameters that
directly govern the outcome of the task (e.g. launch
parameters in the serve). In other words, variability in
the movement system is not detrimental so long as the
critical end point parameters remain stable. This
notion is reflected in recent tennis research, where
Whiteside et al. (2013b) showed that increased variability in coordinated elbow and wrist joint rotations
did not reduce serve accuracy. Given the whole-body
nature of the serve, other mechanics may also contribute to the compensatory process but are yet to be
characterised.
Movement variability in sporting actions is not
restricted to the magnitudes of discrete joint rotations. Research in other striking skills has highlighted
the importance of consistent temporal patterns (i.e.
the times at which specific movements occur) in
interceptive (batting) skills in cricket (Renshaw,
Oldham, Davids, & Golds, 2007) and baseball
(Katsumata, 2007), as well as the volleyball serve
(Davids, Kingsbury, Bennett, & Handford, 2001).
It has been suggested that these athletes utilise information from the incoming ball (i.e. its location) to
regulate the initiation of key propulsive movements,
hence ensuring that they intercept it at the appropriate moment. Similar strategies have been found in
the tennis serve, where players regulate their movements such that their arrival in the trophy position
coincides with ball zenith (Reid, Whiteside, &
Elliott, 2010; Whiteside, Elliott, Lay, & Reid,
2013a). Likewise, the timing of peak forward racquet
velocity is considered critical to accuracy in the first
serve (Antúnez et al., 2012). Ultimately, coordinated
joint rotations and/or temporal consistency may be
thought of as techniques that players exploit to simplify complex sporting movements. With this in
mind, it is expected that tennis players simplify
those aspects of the serve that are most important
to performance. At present, the particular methods
employed to do so are somewhat unclear as this
topic is yet to be comprehensively examined in the
tennis serve.
It has been suggested that, in learning movement
skills, inexperienced performers initially reduce the
skill complexity by restraining degrees of freedom
(Anderson & Sidaway, 1994; Stergiou & Decker,
2011). These degrees of freedom are then gradually
released as players become more familiar with the
Coordination and variability in the tennis serve
3
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Table I. Mean (± standard deviation) age, physical and menarchial characteristics of participants.
Group
N
Age (years)
Height (cm)
Mass (kg)
Experienced menarche
Time since menarche
Pre-pubescent
Pubescent
Adult
12
11
8
10.5 ± 0.5
14.6 ± 0.7
21.3 ± 3.8
143.5 ± 5.9
166.9 ± 4.7
169.2 ± 4.8
36.5 ± 3.7
56.7 ± 3.8
61.9 ± 4.2
No
Yes
Yes
N/A
6–18 months
>4 years
task and explore alternative solutions to the movement problem (Gentile, 2000; Newell, Deutsch,
Sosnoff, & Mayer-Kress, 2006). In this sense, children are often considered novices owing to their
motor inexperience (Guarrera-Bowlby & Gentile,
2004). Indeed, the popular “ten year (10,000 h)
rule” (Ericsson, Krampe, & Tesch-Römer, 1993)
virtually precludes any attempt to categorise children
as expert performers. This position is offered partial
support by Newell’s constraints model (Newell,
1986), which states that organismic factors (e.g.
anthropometry, strength, anticipatory ability) influence performance. As such, more physically and
mentally mature adults are expected to exhibit
more skilled performance than children. However,
“elite child athletes” present a unique cohort whose
motor proficiency may exceed their age-related
expectations and whose practice regimes are often
meticulous and intensive. Consequently, these athletes may not conform to traditional descriptions of
child motor performance and deserve investigation
in their own right. With movement patterns expected
to change throughout development, it seems necessary to separate performers according to level of
development when investigating movement in developing populations.
At present, there exists scope to supplement existing tennis literature with research focused on coordinated joint rotations and variability in the serve.
Therefore, the aims of this study were to examine
coordinated joint rotations and variability in the
lower limbs, trunk, serving arm and ball in the tennis
serves of elite female players at three stages of development. It was expected that functional movement
variability in the distal joints would manifest during
serve performance to preserve a successful outcome,
though its disposition would change with age.
However, the temporal composition of the serve
was expected to be comparatively more consistent
as players stabilised the timing of critical postures
with respect to impact.
Methods
Participants
The relevant Human Ethics Committee approved
this study prior to recruitment. Thirty-one elite
female tennis players provided informed consent
and were arranged into pre-pubescent, pubescent
and adult groups based on their age and menarchial
status (Table I). At the time of testing, players in the
pre-pubescent and pubescent groups held a top 8
national ranking for their respective age groups,
while the adult players possessed a professional
(Women’s Tennis Association: WTA) ranking
higher than 325. This cohort was the same as that
used in a previous study (Whiteside et al., 2013a).
Protocol
The protocol was completed at an indoor biomechanics laboratory, where a full-size tennis court was
constructed. Sixty retro-reflective markers, 14 mm
in diameter, were affixed to each player according an
established, calibrated anatomical systems (CAST)based, full body marker set (Besier, Sturnieks,
Alderson, & Lloyd, 2003; Chin, Elliott, Alderson,
Lloyd, & Foster, 2009; Lloyd, Alderson, & Elliott,
2000). Three hemispherical markers, composed of
ultra-light foam (radius 7 mm), were placed on the
racquet and three more on ball (Whiteside, Chin, &
Middleton, 2013) to create coordinate systems
therein. Prior to the serving protocol, dynamic calibration of a ≈5.5 (deep) 4 (wide) 4 (high) m
capture volume yielded a mean residual calibration
error smaller than 0.002 m.
Each player completed a standard 10 min warm
up and used their own racquet to complete the
protocol. Verbal confirmation of preparedness
prompted the initiation of the test protocol in
which players performed maximal effort flat (i.e.
“power”) serves. Players were instructed to aim for
a familiar 1 1 m target (Elliott et al., 1995;
Martin et al., 2013; Reid et al., 2008; Sakurai,
Reid, & Elliott, 2012) bordering the T of the service box (right-handers: deuce court; left-handers:
advantage court) and to hit all serves with the same
effect (i.e. to emphasise speed as opposed to spin).
Five blocks of eight serves were performed with a 2
min rest period separating successive blocks.
Three-dimensional (3D) marker trajectories were
recorded using a 22-camera VICON MX system
(VICON Motion Systems, Oxford, UK) operating
at 500 Hz. The global reference frame originated at
the centre of the baseline, where positive x pointed
rightward along the baseline, positive y pointed to
the net and positive z pointed up. Five of each
4
D. Whiteside et al.
player’s fastest serves landing in the target area were
selected for analysis.
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Data processing
Gaps in the raw marker trajectories were interpolated
using a cubic spline within the VICON Nexus software. A second-order polynomial extrapolation specific to tennis limited the distortion of kinematic data
around impact (Knudson & Bahamonde, 2001;
Reid, Campbell, & Elliott, 2012). Data were subsequently filtered using a Woltring filter (Woltring,
1986) with the optimal mean squared error of
2 mm determined by a residual analysis, and then
modelled using the University of Western Australia’s
full body (Besier et al., 2003; Lloyd et al., 2000),
racquet and ball (Whiteside et al., 2013) models.
Joint rotations were expressed using the Euler ZXY
sequence. Trunk rotations were expressed relative to
a virtual anatomical reference frame (x pointing forward towards the net, y pointing up and z pointing
right) originating at the global origin. To maintain
consistency in the statistical analyses, kinematics for
the left-handed players were inverted where appropriate such that all players could be considered
together as right-hand dominant (Campbell,
Straker, O’Sullivan, Elliott, & Reid, 2013;
Whiteside et al., 2013b).
Relevant events of the service action
Figure 1 denotes how the service action was deemed
to begin at the instant the ball was released from the
Figure 1. Key time points of interest in the tennis serve.
hand. Ball zenith represented the peak vertical displacement of the ball during its toss. The subsequent
nadir of vertical racquet displacement was the racquet low point, which is usually coincident with a
player leaving the ground (Bonnefoy, Slawinski,
Leveque, Riquet, & Miller, 2009). Impact was
defined as the frame (i.e. 0.002 s) prior to racquetball contact. The duration of the serve was considered as the time period between ball release and
impact. Leg drive was defined as the period from
ball zenith to racquet low point, while racquet low
point to impact was considered the forwardswing
phase of the serve.
Variables of interest
To gauge inter-limb coordination during leg drive,
the relative bilateral flexion-extension motion of the
ankles was compared, as was the relative bilateral
flexion-extension motion of the knees. The relative
transverse (twist) and frontal plane (shoulder-overshoulder) trunk rotations prior to impact indicated
how players manipulated the trunk during this time,
while relative extension and pronation denoted the
same for the elbow in the serving arm. The location
of the ball during the toss was expressed relative to
the front toe (Chow et al., 2003; Reid et al., 2011),
and its spatial variability measured at both ball zenith
and impact. Finally, the timing of both ball zenith
and racquet low point (expressed as a percentage of
the serve), and also duration of forwardswing provided an insight into the temporal pattern of the
service action.
Coordination and variability in the tennis serve
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Assessment of variability in joint mechanics and the ball
toss
Angle-angle plots provided a qualitative insight into
relative joint rotations. The variability of these traces
was quantified using the coefficient of correspondence – a vector coding technique that assesses the
repeatability of several variable–variable traces
(Tepavac & Field-Fote, 2001). Unlike alternative
vector coding methods that consider only vector
direction, the coefficient of correspondence is advantageous in that it also considers vector magnitude
(Wheat & Glazier, 2006). Ultimately, the coefficient
of correspondence quantified the magnitude of
variability in coordinated joint rotations using a
value between 0 and 1 (0 = no variability; 1 = maximum variability). Akin to previous work (Horan
et al., 2011), the average coefficient of correspondence value ±5% of the relevant event represented
the variability at that time point.
The 3D (xσ , yσ and zσ ) standard deviations of ball
displacement at ball zenith and impact were calculated for each player across their five trials. The
mean and standard deviations of these standard
deviations provided a gauge of ball toss variability
for each group (Davids et al., 2001; Reid et al.,
2010). Further, each standard deviation was doubled
(effectively creating error bar representing one standard deviation either side of the mean, in each
dimension) and then multiplied (2xσ 2yσ 2zσ )
to yield the “variability volume”: a singular quantity
of the ball’s 3D spatial variability for each player,
across their five serves.
5
Statistical analyses
Four split plot analyses of variance (SPANOVAs)
were used to examine between group (i.e. differences
between each group), within group (i.e. differences
between each time point) and interaction (i.e.
group time) effects in the coefficient of correspondence. The same procedure was used to compare the variability volumes in each group at ball
zenith and impact. The ranges of motion at the
ankles, knees, trunk and elbow were compared
between groups using one-way analyses of variance.
Where significant main effects existed, Bonferronicorrected post-hoc tests were employed to find the
source of the difference. In total, 13 analyses of
variance were performed, thus inflating the risk of
encountering type I errors. To account for this, the
significance level was adjusted sequentially for each
test according to the correction method proposed by
Holm (1979). The p-value was not sufficient to
reject the null hypothesis in the eleventh test, indicating that all significance in this study was reported
using a p-value of 0.0125. The temporal variability of
ball zenith, racquet low point and forwardswing
duration were interpreted descriptively.
Results
Ankle mechanics
The representative traces in Figure 2 represent how
all groups utilised simultaneous plantar flexion at
both ankles and simultaneous extension in both
Figure 2. Mean coefficients of correspondence and representative angle–angle plots for the ankles and knees.
Note: *Significant difference between ball zenith and racquet low point in all groups.
6
D. Whiteside et al.
During leg drive, the back knee extended through
a significantly (F2,28 = 15.954; P < .001) greater
range in the pubescent (MD: 22.46°; CI: 10.97–
33.95°; P < .001) and adult (MD: 22.59°; CI:
10.02–35.15°; P < .001) groups compared with the
pre-pubescent group, though range of motion
(RoM) at the front knee was not significantly different between groups (F2,28 = 4.364; P = .022).
Trunk mechanics
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Figure 3. Mean ranges of flexion-extension motion at the ankles
and knees during leg drive.
Note: *Significant difference between groups.
knees during leg drive. The coefficient of correspondence for ankle motion displayed a significant main
effect for group (F2,28 = 16.499; P < .001), which
post-hoc tests revealed to be a product of significantly
higher overall variability in the pubescent group
compared with the pre-pubescent group (mean difference (MD): 0.14; 95% confidence interval (CI):
0.08–0.20; P < .001). From a temporal perspective,
the bilateral coupling of ankle plantarflexion was
significantly more consistent at racquet low point
compared with ball zenith and independent of
group (F1,28 = 151.111; MD: 0.25; CI: 0.21–0.29;
P < .001).
The range of plantar flexion at the back ankle was
significantly (F2,28 = 10.913; P < .001) smaller during leg drive in the pre-pubescent group compared
with the pubescent (MD: 14.12°; CI: 5.10–23.14°;
P = .001) and adult (MD: 15.24°; CI: 5.38–25.11°;
P = .002) groups. Likewise, the range of plantar
flexion at the front ankle (F2,28 = 17.299; P < .001)
was significantly smaller during leg drive in the prepubescent group compared with the pubescent
(MD: 17.83°; CI: 8.12–27.53°; P < .001) and adult
(MD: 21.91°; CI: 11.29–32.52°; P < .001) groups
(Figure 3).
Knee mechanics
Regarding the coefficient of correspondence for
bilateral knee extension, the main effect for group
was not significant (F2,28 = .162; P = .852), although
a significant main effect for time revealed that relative knee extension was significantly more consistent
at racquet low point compared with ball zenith
(F1,28 = 86.376; MD: 0.26; CI: 0.20–0.31;
P < .001).
The variability of concurrent frontal and transverse
plane trunk rotations revealed a significant main
effect for group (F2,28 = 10.530; P < .001), wherein
relative trunk rotations were significantly more consistent in the pubescent (MD: 0.08; CI: 0.03–0.13;
P = .001) and adult (MD: 0.08; CI: 0.02–0.13;
P = .005) groups compared with the pre-pubescent
group. A post-hoc decomposition of the significant
main effect for time point (F2,28 = 67.672;
P < .001) revealed that relative trunk motion was
significantly different at all time points (most variable at ball zenith, most consistent at racquet low
point, in between at impact).
The range of trunk twist rotation did not differ
with age (F2,28 = .383; P = .686); however, the range
of shoulder-over-shoulder rotation was significantly
(F2,28 = 13.744; P < .001) larger in the pubescent
(MD: 20.12°; CI: 9.45–30.74°; P < .001) and adult
(MD: 18.04°; CI: 6.43–29.65°; P = .001) groups
compared with the pre-pubescent group.
Elbow mechanics
A main effect for group (F2,28 = 7.606; P = .002) was
discovered for the coefficient of correspondence of
the relative elbow joint rotations (flexion and radioulnar pronation). Further analyses revealed that the
pubescent group was significantly more consistent
than the adult group (MD: 0.12; CI: 0.04–0.21;
P = .002) over the period in question. Additionally,
post-hoc analyses of the significant time effect
(F2,28 = 70.069; P < .001) showed that variability
was significantly different at each of the three time
points (becoming progressively more variable
between ball zenith and impact). Importantly, an
interaction effect was also noted (F2,28 = 24.089;
P < .001) and was found to relate to the fact that
the coupled elbow joint rotations only became significantly more variable between racquet low point
and impact in the adult group (MD: 0.47; CI: 0.31–
0.64; P < .001) (Figure 4).
The range of flexion-extension at the elbow
between ball zenith and impact did not differ
between groups (F2,28 = .953; P = .398). However,
during the same period, the range of radio-ulnar
pronation was significantly (F2,28 = 12.116;
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Coordination and variability in the tennis serve
7
Figure 4. Mean coefficients of correspondence and representative angle–angle plots for the trunk and elbow.
Note: *Significant difference between the time points in all groups. † Significant difference between racquet low point and impact in the
adult group.
compared with ball zenith (MD: 1672 cm3; CI:
959–2386 cm3) (Table II). Descriptively, ball location was most consistent in the vertical direction at
impact compared with the left-right and forwardbackward directions (Table III).
Temporal variability of the ball zenith and racquet lowpoint events
Qualitatively, although the timing of racquet low
point was comparatively more consistent than ball
zenith in all groups, both events displayed impressive
temporal consistency. The duration of forwardswing
was equally consistent, displaying an average standard deviation