International Journal of Sports Physiology and Performance, 2010, 5, 384-393
© Human Kinetics, Inc.
Effects of the Playing Surface on Plantar
Pressures During the First Serve in Tennis
O. Girard, J.-P. Micallef, and G.P. Millet
Purpose: This study aimed at examining the influence of different playing surfaces on in-shoe loading patterns in each foot (back and front) separately during
the first serve in tennis. Methods: Ten competitive tennis players completed
randomly five first (ie, flat) serves on two different playing surfaces: clay vs
GreenSet. Maximum and mean force, peak and mean pressure, mean area, contact
area and relative load were recorded by Pedar insoles divided into 9 areas for
analysis. Results: Mean pressure was significantly lower (123 ± 30 vs 98 ± 26
kPa; −18.5%; P < .05) on clay than on GreenSet when examining the entire back
foot. GreenSet induced higher mean pressures under the medial forefoot, lateral
forefoot and hallux of the back foot (+9.9%, +3.5% and +15.9%, respectively;
both P < .01) in conjunction with a trend toward higher maximal forces in the
back hallux (+15.1%, P = .08). Peak pressures recorded under the central and
lateral forefoot (+21.8% and +25.1%; P < .05) of the front foot but also the mean
area values measured on the back medial and lateral midfoot were higher (P <
.05) on clay. No significant interaction between foot region and playing surface
on relative load was found. Conclusions: It is suggested that in-shoe loading
parameters characterizing the first serve in tennis are adjusted according to the
ground type surface. A lesser asymmetry in peak (P < .01) and mean (P < .001)
pressures between the two feet was found on clay, suggesting a greater need for
stability on this surface.
Keywords: tennis serve, plantar loading, clay, GreenSet
The uniqueness of the game of tennis is the possibility to compete on various
playing surfaces; a point that is notably illustrated by the fact that the four Grand
Slam tournaments are played on different surfaces. Compared with hard courts
(eg, GreenSet), competing on clay has been usually associated with longer playing times (ie, greater exercise to rest ratio) and distance ran, a higher number of
strokes (topspin backhands and forehands) per game, lesser incidence of lower
extremity injuries as well as increased metabolic and cardio-respiratory load (ie,
higher blood lactate concentration, heart rate and oxygen uptake responses).1–4 In
investigating the influence of the playing surface on the amplitude and distribution
O. Girard is with ASPETAR, Qatar Orthopaedic and Sports Medicine Hospital, Doha, Qatar. J-P. Micallef
is with UPRES—EA 2991, Faculty of Sport Sciences, Montpellier, France. G.P. Millet is with ISSUL,
University of Lausanne, Lausanne, Switzerland.
384
Surface, Loading, Tennis Serve 385
of plantar pressures during tennis-specific movements, we have also reported that
clay induced lower loading in the hallux and lesser toes areas but higher relative load
on the medial and lateral midfoot.5 This is the result of a lower frictional resistance
on slow vs. fast surfaces, as evidenced by reduction in ball velocities associated to
higher and slower rebounds on clay.6
The serve in tennis is the predominant stroke, accounting for 45% (French
Open) to 60% (Wimbledon) of the total number of strokes during service games.3,4
A forceful lower limb drive is considered as the “starting point” of the kinetic chain
and influences directly serve efficiency.7 To achieve the different tactical objectives
of playing on various surfaces (ie, clay: baseline play; GreenSet: serve and volley
play),3 it is conceivable that players adapt their lower limb involvement by modifying
the nature (ie, magnitude and distribution) of in-shoe loading patterns. During the
first serve in tennis, the propulsion of the lower extremities is primarily orientated
upward to reach optimal racquet/ball impact heights; this proficiency increases
along with playing skill.7,8 Although of lesser magnitude, the forward propulsive
forces—that allow players to travel a approximately 50 cm distance into the court
with the first step, depending on the stance style being used—are known to affect
the production of linear and angular momentum.9,10 It is therefore likely that the
ground surface friction would influence the foot load repartition during the serve,
as observed in walking on different slippery surfaces.11 It is known that on slippery
(eg, oily, snowy or sandy) surfaces, humans tend to maximize the friction between
feet and surfaces by shifting the ground reaction forces (GRF) to a more vertical
direction.11 This is achieved by a subtle increase in toe grip (eg, higher pressures
in hallux or toes regions).
To date, however, the role of the tennis surface on loading patterns of the feet
during the serve remains unknown. This study aimed at examining the influence
of two different playing surfaces on in-shoe loading patterns in each foot (back
and front) separately during the first serve in tennis. We hypothesized that, for
stability reasons, pressure differences between the two feet would be lower on
clay than on GreenSet.
Methods
Subjects
Ten (7 males, 3 females) competitive tennis players (mean [SD] age 23.8 [6.0] y;
height 171.7 [8.1] cm; body mass 65.8 [8.7] kg) of International Tennis Number
3 (ITN 3) standing or higher volunteered to participate in the study. Subjects were
eligible to participate if they had no history of injury to the lower extremity within
the past year and no previous surgery. They signed a written consent before participation. Approval for the study was obtained from the local ethics committee.
Experimental Setup
After a standardized warm-up lasting 15 min (ie, submaximal run, sprinting,
practice serve with increasing speed), subjects performed five first (ie, flat with
minimum spin on the ball) serves from the “deuce” court on clay and GreenSet.
All serve trials were completed from the “deuce” or right court with a 30 s rest
386 Girard, Micallef, and Millet
between trials. Before initiating any serve action, subjects kept the starting position for 3 s. They were then requested to hit their serves to a 1 × 1 m target area
bordering the “T” of the right service box at match pace. Depending on their
own preference, physical and technical characteristics but also their style of play,
subjects assumed an individual stance— ie, between the foot-up and foot-back
techniques—in the starting position. All subjects were right-handed and used
their own racquet.
Instrumentation
Insole plantar pressure distribution was recorded using the X-Pedar Mobile System
(Novel GmbH, Munich, Germany). Each pressure insole consisted of a 2-mm-thick
array of 99 capacitive pressure sensors. Before commencement of data collection, the insoles were calibrated according to the manufacturer’s guidelines. This
involved loading the insoles to a range of known pressure values, which resulted
in an individual calibration curve for all sensors within the shoe (TruBlu Calibration, Novel GmbH, Munich, Germany). The insoles were placed bilaterally inside
the subject’s shoes. The data logger for data storage was in a harness on the chest
of the subject. Plantar pressures were sampled at 50 Hz via Bluetooth technology.
An excellent reproducibility has been reported for this device.12
Plantar Pressure Data
A regional analysis of each foot was performed utilizing nine separate “masks” or
areas of the foot, ie, medial and lateral heel, medial and lateral midfoot, medial,
central and lateral forefoot, hallux and lesser toes (Groupmask Evaluation, Novel
GmbH, Munich, Germany). The following parameters were determined for the
whole foot and the nine selected regions: maximum and mean force, peak and
mean pressure, mean area and contact area. In addition, the relative load in each
foot region was calculated as the force time integral (area under the force curve)
in each individual region divided by the force time integral for the total plantar
foot surface.5 For each trial, the analyzed period started at the onset of any foot
movement and ended at take-off. Analyses were performed with the appropriate
software (Novel Win, Novel GmbH, Munich, Germany).
Statistical Analyses
Mean (SD) was calculated for all variables. A two-way repeated-measures ANOVA
was used to examine the effect of playing surface on the overall plantar pressure
differences between front and back foot. In addition, for the front and the back foot
(separately), a two-way repeated-measures ANOVA was performed with playing
surface (clay vs GreenSet) and foot region (masks one to nine) as the repeated
factors and the foot loading parameters designated as dependent variables. A large
number of comparisons leads to an inflated Type I error in ANOVA test. Therefore
the Holm’s correction was applied.13 Briefly, it is a procedure recommended in
biomechanics that consists in a progressive adjustment in the alpha level based
on the number of comparisons and the desired experiment error rate. The statistical analyses were performed using SPSS for Windows (SPSS Inc., version 17.0,
Chicago, IL).
Surface, Loading, Tennis Serve 387
Results
Whole Foot
Independently of the playing surface, statistical analysis revealed higher (P < .05)
peak and mean pressures under the back compared with the front foot when examining the overall foot, but only a tendency to be higher for contact area and mean
forces (both P = .07) (Table 1). The interaction between type of foot and playing
surface was significant for peak (P < .01) and mean (P < .001) pressures, but not
for maximal force (P = .09).
Foot Pressure Distribution
All parameters displayed a foot region effect (P < .001) in both feet. ANOVA revealed
that playing surface had a significant effect on peak (P < .05) and mean (P < .001)
pressures of the front foot. The interaction of foot region and playing surface was
significant for the maximum (P < .01) and mean (P < .05) forces, mean pressure
(P < .001) and mean area (P < .01) of the back foot as well as for the front foot
peak pressure (P = .05). GreenSet induced higher mean pressures under the medial
forefoot, lateral forefoot and hallux of the back foot (+9.9%, +3.5% and +15.9%,
respectively; both P < .01) in conjunction with a trend toward higher maximal forces
in the back hallux (+15.1%, P = .08). Peak pressures recorded under the central and
lateral forefoot (+21.8% and +25.1%; P < .05) of the front foot as well as the mean
area values measured on the back medial and lateral midfoot were higher (P < .05)
on clay. A summary of plantar pressure parameters changes for each foot region
during the first serve between the two playing surfaces is presented in Figure 1.
Figure 1 — The main changes of the plantar loading parameters for front (left) and back (right)
feet during the first serve in tennis on clay and GreenSet in each of the nine areas of interest.
388
240 ± 81
87 ± 21
118 ± 27
74 ± 21
Peak pressure (kPa)
Mean pressure (kPa)
Contact area (cm )
Mean area (cm )
76 ± 13
119 ± 21
95 ± 21
277 ± 110
464 ± 111
921 ± 204
Clay
81 ± 19
134 ± 33
123 ± 30
400 ± 115
623 ± 131
1034 ± 163
GreenSet
Clay
89 ± 22
138 ± 23
98 ± 26
366 ± 109
587 ± 182
951 ± 244
Back Foot
(Right)
* P < .05 significantly different between front and back foot (independently of the playing surface).
** P < .01; *** P < .001 denote significant interaction between playing surface and foot type.
2
472 ± 212
Mean force (N)
2
845 ± 281
GreenSet
Front Foot
(Left)
Maximum force (N)
Parameters
+160
+36
−7.2 *
−18.5 *
+6.0
+4.7
+10.6
+13.2
+7
+16
+151
−7.1
−3.4
+9.5
+189
−6.7
+14.2
+16.2
GreenSet
+14
+19
+3 ***
+88 **
+123
+30
Clay
∆ Front vs Back Foot
(% Front Foot)
Back
Foot
Front
Foot
∆ GreenSet vs Clay
(% GreenSet)
Table 1 Plantar pressure parameters for the whole foot during the first serve in tennis on clay and GreenSet
under the front (left) and the back (right) foot
Surface, Loading, Tennis Serve 389
No significant interaction between foot region and playing surface on relative
load was found (Figure 2).
Figure 2 — Mean and standard deviation relative load (%) for each foot region under the
front (Panel A) and back (Panel B) feet during the first serve in tennis on clay and GreenSet.
390 Girard, Micallef, and Millet
Discussion
Several authors have published studies on the factors (eg, performance level, fatigue,
foot arrangement, ergogenic aids) affecting racket speed generation and accuracy
in the tennis serve.7–9,14 By influencing plantar pressures, ground reaction forces
and joint kinematics, shoe-surface frictional properties have also the potential to
modify the efficiency of tennis on-court movements. For instance, when different
types of playing surfaces were compared, evidence of lower relative load on the
medial and lateral midfoot5 in conjunction with lower peak vertical impact force
normalized to body weight15 has been found during tennis specific movements on
the hardest surface. In addition, it was reported that the surface with the lowest
mechanical cushioning resulted in the lowest vertical force magnitude during the
running tennis forehand impact phase.16 However, it is still unknown whether the
ground type surface would influence plantar loading during the serve, and provided
as the focus of this study.
Data from this study show that executing first serves on GreenSet vs clay
induced a 10% to 15% nonsignificant increase in plantar pressures (eg, maximum
force, peak and mean pressures) under the front foot, while loading under the back
foot was 7% to 18% lower. This would indicate that the above-mentioned in-shoe
loading characteristics are sensitive to changes in surface cushioning during tennis
first serves. Nevertheless, increasing the subjects’ sample is needed to confirm these
findings. When competing on fast surfaces, players generally develop a more aggressive game style (ie, net-rushers). The greater proportion of rallies played toward
the net on these particular surfaces confirms this viewpoint.3 When examining the
entire back foot, mean pressure values determined in this study were 18.5% higher
on GreenSet than on clay. The back foot plays a role in the initiation of the move
to the net on fast surfaces in getting the center of mass moving toward the net and
ahead of the front foot— that is, presumably by increasing mean pressure values
under the medial forefoot, lateral forefoot and hallux areas on GreenSet. In contrast,
the higher peak pressures recorded on clay under the central and lateral forefoot
of the front foot would help to create changes in the amount of linear and angular
momentums (eg, ball toss, impact height, racquet path), that will lead to a more
defensive game style (ie, long rallies from behind the baseline) on this surface.3
Nevertheless, research is needed to assess how kinematic adjustments might occur
when executing serves on different playing surfaces. It has been argued that the
greater number of strokes on clay could contribute to earlier fatigue.4 Since pedography analysis can be successfully used to detect fatigue-induced changes in plantar
surface loading characteristics, as did previously for treadmill running17—future
studies should determine whether prolonged tennis playing leads to significant
changes in magnitude and/or distribution of plantar pressures, which in turn can
increase the risk of overuse injury.
Several studies have investigated the GRF during the tennis serve,7,8,10,18 and
highlighted the paramount importance of the lower limb drive. In examining the
lower extremities activity that characterized the first serve of players of different
playing standards, Girard et al7 found elite players to generate higher vertical
GRFs than both intermediate and beginner players. However, none of the available source in the literature has differentiated the respective force of each foot, as
Impellizzeri et al19 did for the countermovement jump. Our results showing a higher
Surface, Loading, Tennis Serve 391
asymmetry in forces and pressures between the two feet on GreenSet vs clay is of
high interest. Theoretically, a fully symmetrical standing posture provides maximum stability, at least in healthy subjects,20 while with increasing weight-bearing
asymmetry, kinetic and postural regulation increase.21 Genthon and Rougier22
demonstrated the negative effects of an asymmetrical body weight distribution
on the control of undisturbed upright stance. They reported that the amplitudes of
the “center of pressure” under both feet increased with increasing weight-bearing
asymmetry in both the lateral and antero-posterior directions. More recently, it has
been highlighted that the overall plantar pressure of the preferred foot was higher
than that of the nonpreferred foot in four soccer-related movements.23 Specifically, higher pressure was found in the preferred foot during the take-off phase,
whereas this was found in the nonpreferred foot during the landing phase. This
has been interpreted as a tendency of the preferred foot for higher motion force
and of the nonpreferred foot for a greater role in body stabilization. Due to the
different nature of the movement, any comparison between results from this study
and our data remains anecdotal. Interestingly, however, it has been proposed that
the front foot (ie, nonpreferred for right-handed players) provides the stable post
to allow rotational momentum during the serve.24 Under this scenario, the back
foot should represent a large base for force production and the starting point for
kinetic chain developing from the ground to the leg, trunk, upper body segments
and finally the racquet. Further studies investigating the role of each leg on serve
efficiency are worthy of interest.
During explosive movements involving stretch-shortening cycle such as
countermovement jumps, it is known that the range of normal bilateral strength
asymmetry is ±15%.19 The asymmetries of forces measured in the current study
were more marked (eg, maximum force: +30% and +189% on clay and GreenSet,
respectively) indicating functional instead of postural differences between feet
during the serve in tennis. Nevertheless, it is unknown if the lesser asymmetry
in tennis serve plantar pressures observed on clay is related to the lower serve
efficiency (eg, average first serve ball velocity for men: 160 vs 185 km⋅h–1 at the
French Open vs Wimbledon, respectively) commonly reported for this surface.25
More tellingly, the lowest degree of asymmetry observed on clay may reflect an
increased need for stability (due to surface characteristics) that, at the same time,
could have detrimental effects on motor performance. In addition, one may argue
that the tactics adopted on clay would be, at least partly, under the influence of the
slippery characteristics of this surface and initiated from the ground—that is, lower
limb drive during the serve—and not due only to the longer time prior the next
stroke due to the slower surface.3 According to this standpoint, due, at least partly,
to lower stability and grip, serving on clay would have modified the magnitude and
orientation (to a more vertical direction, when compared with GreenSet) of GRF
and therefore the effective use of coordinated sequential movements through the
kinetic chain and the overall efficiency of the serve. Further research investigating
the effects of the friction coefficient of the surface with different players’ level
would help to answer the question of whether surface slipperiness is an influencing factor of the lower limb drive and of the serve efficiency in tennis and explain
the difference between ball velocities observed in professional tennis on various
Grand Slam tournaments.
392 Girard, Micallef, and Millet
Practical implications
This study is the first to report plantar loading differences during the first serve
between clay and GreenSet. The need to practice regularly on both surfaces
is emphasized, especially if players are continuously forced to compete on
various ground type surfaces.
Owing to the greater need for postural stability when serving on clay, we suggest
that a harder insert—which is known to increase the stability of the human
body26—should be used (ie, under the back foot to increase the possibility
to produce force from the ground) to attenuate the negative impact of a soft
surface (clay) on feet asymmetry.
Additionally, different fitness training programs could be used for each foot to
increase the bilateral difference in foot loading on a given surface.
Conclusion
In-shoe loading parameters characterizing the first serve in tennis are adjusted
according to the ground type surface to account for the different frictional properties
between clay and GreenSet. Moreover, the conservative behavior of the players on
clay might originate from a greater need for stability on this surface.
Acknowledgment
The authors thank Frank Eicher for his technical assistance during the study. There was no
financial support for this project.
References
1. Girard O, Millet GP. Effects of the ground surface on the physiological and technical
responses in young tennis players. In: Lees A, Kahn JF, Maynard IW, eds. Science and
racket sports 3. London: E & FN Spon; 2004:43–48.
2. Murias JM, Lanatta D, Arcuri CR, Laino FA. Metabolic and functional responses playing tennis on different surfaces. J Strength Cond Res. 2007;21:112–117.
3. O’Donoghue P, Ingram B. A notational analysis of elite tennis strategy. J Sports Sci.
2001;19:107–115.
4. Johnson CD, McHugh MP. Performance demands of professional male tennis players.
Br J Sports Med. 2006;40:696–699.
5. Girard O, Eicher F, Fourchet F, Micallef J-P, Millet GP. Effects of the playing surface
on plantar pressures and potential injuries in tennis. Br J Sports Med. 2007;41:733–738.
6. Nigg BM, Segesser B. The influence of playing surfaces on the load on the locomotor
system and on football and tennis injuries. Sports Med. 1988;5:375–385.
7. Girard O, Micallef J-P, Millet GP. Lower-limb activity during the power serve in tennis:
Effects of performance level. Med Sci Sports Exerc. 2005;37:1021–1029.
8. Girard O, Micallef J-P, Millet GP. Influence of restricted knee motion during the flat
first serve in tennis. J Strength Cond Res. 2007;21:950–957.
9. Bahamonde R, Knudson D. Ground reaction forces of two types of stances and tennis
serves. Med Sci Sports Exerc. 2001;33:s102.
10. Elliott BC, Marsh T, Blanksby B. A three-dimensional cinematographic analysis of the
tennis serve. Int J Sport Biomech. 1986;2:260–271.
Surface, Loading, Tennis Serve 393
11. Fong DT, Mao DW, Li JX, Hong Y. Greater toe grip and gentler heel strike are the
strategies to adapt to slippery surface. J Biomech. 2008;41:838–844.
12. Kernozek TW, Zimmer KA. Reliability and running speed effects of in-shoe loading
measurements during slow treadmill running. Foot Ankle Int. 2000;21:749–752.
13. Knudson D. Significant and meaningful effects in sports biomechanics research. Sports
Biomech. 2009;8:96–104.
14. Hornery DJ, Farrow D, Mujika I, Young WB. Caffeine, carbohydrate, and cooling use
during prolonged simulated tennis. Int J Sports Physiol Perform. 2007;2:423–438.
15. Stiles V, Dixon S. Biomechanical response to systematic changes in impact interface
cushioning properties while performing a tennis-specific movement. J Sports Sci.
2007;25:1229–1239.
16. Stiles VH, Dixon SJ. The influence of different playing surfaces on the biomechanics
of a tennis running forehand foot plant. J Appl Biomech. 2006;22:14–24.
17. Willson JD, Kernozek TW. Plantar loading and cadence alterations with fatigue. Med
Sci Sports Exerc. 1999;31:1828–1833.
18. Van Gheluwe B, Hebbelinck M. Muscle action and ground reaction forces in tennis.
Int J Sport Biomech. 1986;2:88–99.
19. Impellizzeri FM, Rampinini E, Maffiuletti N, Marcora SM. A vertical jump force
test for assessing bilateral strength asymmetry in athletes. Med Sci Sports Exerc.
2007;39:2044–2050.
20. Winter DA, Prince F, Frank JS, Powell C, Zabjek KF. Unified theory regarding A/P
and M/L balance in quiet stance. J Neurophysiol. 1996;75:2334–2343.
21. Anker LC, Weerdesteyn V, van Nes IJ, Nienhuis B, Straatman H, Geurts AC. The relation between postural stability and weight distribution in healthy subjects. Gait Posture.
2008;27:471–477.
22. Genthon N, Rougier P. Influence of an asymmetrical body weight distribution on the
control of undisturbed upright stance. J Biomech. 2005;38:2037–2049.
23. Wong PL, Chamari K, Chaouachi A, Mao DW, Wisloff U, Hong Y. Difference in plantar
pressure between the preferred and non-preferred feet in four soccer-related movements.
Br J Sports Med. 2007;41:84–92.
24. Bahamonde R, Knudson D. Linear and angular momentum in stroke production. In:
Elliott BC, Reid M, Crespo M, eds. Biomechanics of advanced tennis. London: ITF
Ltd; 2003:51–70.
25. Grand slam statistics. http://www.tennisserver.com/set/set_05_05.html (accessed 19
Nov 2009)
26. Robbins S, Gauw GJ, McClaran J. Shoe sole thickness and hardness influence balance
in older men. J Am Geriatr Soc. 1992;40:1089–1094.