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Tóm tắt nội dung (trích từ tài liệu gốc): See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/353719322 The Effects of Knee Flexion on Tennis Serve Performance of Intermediate Level Tennis Players Article in Sensors � August 2021 DOI: 10.3390/s21165254 CITATIONS READS 0 92 6 authors, including: Thales R Souza Joana Hornestam Federal University of Minas Gerais Federal University of Minas Gerais 74 PUBLICATIONS 647 CITATIONS 3 PUBLICATIONS 8 CITATIONS SEE PROFILE SEE PROFILE Thiago R T Santos Fabr�cio Magalh�es Federal University of Minas Gerais Federal University of Minas Gerais
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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/353719322
The Effects of Knee Flexion on Tennis Serve Performance of Intermediate
Level Tennis Players
Article in Sensors � August 2021
DOI: 10.3390/s21165254
CITATIONS READS
0 92
6 authors, including: Thales R Souza
Joana Hornestam Federal University of Minas Gerais
Federal University of Minas Gerais 74 PUBLICATIONS 647 CITATIONS
3 PUBLICATIONS 8 CITATIONS
SEE PROFILE
SEE PROFILE
Thiago R T Santos
Fabr�cio Magalh�es Federal University of Minas Gerais
Federal University of Minas Gerais 34 PUBLICATIONS 311 CITATIONS
47 PUBLICATIONS 218 CITATIONS
SEE PROFILE
SEE PROFILE
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sensors
Article
The Effects of Knee Flexion on Tennis Serve Performance of
Intermediate Level Tennis Players
Joana Ferreira Hornestam 1, Thales Rezende Souza 1, Fabr�cio An�cio Magalh�es 1, Mick�el Begon 2,
Thiago Ribeiro Teles Santos 1 and S�rgio Teixeixa Fonseca 1,*
1 Graduate Program in Rehabilitation Sciences, Universidade Federal de Minas Gerais,
Belo Horizonte 31270-901, Brazil; hornestam@ufmg.br (J.F.H.); thalesrs@ufmg.br (T.R.S.);
fanicio@ufmg.br (F.A.M.); trtsantos@ufmg.br (T.R.T.S.)
2 School of Kinesiology and Exercise Sciences, Faculty of Medicine, Universit� de Montr�al,
Montreal, QC H3C 3J7, Canada; mickael.begon@umontreal.ca
* Correspondence: sfonseca@ufmg.br; Tel.: +55-31-3409-7405
Citation: Hornestam, J.F.; Souza, Abstract: This study aimed to investigate the effects of knee flexion during the preparation phase
T.R.; Magalh�es, F.A.; Begon, M.; of a serve on the tennis serve performance, using inertial sensors. Thirty-two junior tennis players
Santos, T.R.T.; Fonseca, S.T. The were divided into two groups based on their maximum knee flexion during the preparation phase
Effects of Knee Flexion on Tennis of serve: Smaller (SKF) and Greater (GKF) Knee Flexion. Their racket velocity, racket height, and
Serve Performance of Intermediate knee extension velocity were compared during the tennis serve. Inertial sensors tracked participants'
Level Tennis Players. Sensors 2021, 21, shank, thigh, and racket motions while performing five first, flat, and valid serves. Knee flexion
5254. https://doi.org/10.3390/ was analysed during the preparation phase of serve, knee extension velocity after this phase, racket
s21165254 velocity just before ball impact, and racket height at impact. Pre-impact racket velocity (mean
difference [MD] = 3.33 km/h, p = 0.004) and the knee extension velocity (MD = 130.30 /s, p = 0.012)
were higher in the GKF than SKF; however, racket impact height was not different between groups
(p = 0.236). This study's findings support the importance of larger knee flexion during the preparation
phase of serve-to-serve performance. This motion should be seen as a contributor to racket velocity.
Keywords: biomechanics; inertial sensors; leg drive; lower limb drive; racket sport; serve speed
Academic Editors: Mark Robinson 1. Introduction
and Jacqueline Alderson
The serve is one of the most frequent and essential strokes in a tennis match [1]. The
Received: 10 June 2021 serve speed is the most used parameter to evaluate tennis serve performance [2�5]. A faster
Accepted: 30 July 2021 serve reduces the time for the opponent to respond and may hamper the return. Another
Published: 4 August 2021 parameter used to measure tennis serve performance is the racket impact height [6�8].
Hitting the ball at a higher position may improve the viewing area of the target zone and
Publisher's Note: MDPI stays neutral increase the available target window, increasing the chances to hit faster and more valid
with regard to jurisdictional claims in serves [2,9]. The upper limb's angular motion is well known as a major contributor to
published maps and institutional affil- serve speed [10,11]. However, the influence of the lower limb's angular motion is still
iations. controversial and limited to adult tennis players [5�7,12]. More studies are necessary
to address the effects of lower limb motion on serve performance, especially in young
Copyright: � 2021 by the authors. tennis players.
Licensee MDPI, Basel, Switzerland.
This article is an open access article There is anecdotal evidence that the lower limbs are at the base of the tennis serve's
distributed under the terms and kinetic chain. Thus, the lower limb motion would be important to initiate energy generation
conditions of the Creative Commons and transfer to the trunk, upper limb, and then to the racket [13]. Greater knee flexion
Attribution (CC BY) license (https:// during the serve preparation phase allows reaching greater knee extension velocity (since
creativecommons.org/licenses/by/ acceleration is applied during a longer period), producing a more effective lower limb drive
4.0/). as more mechanical energy is added to the body. However, studies that investigated the
impact of knee flexion on serve performance found inconsistent results [5,6]. Sgro et al. [6]
found that advanced tennis players had greater knee flexion during the preparation phase,
greater serve speed, and greater racket impact height than beginners. Moreover, they found
Sensors 2021, 21, 5254. https://doi.org/10.3390/s21165254 https://www.mdpi.com/journal/sensors
Sensors 2021, 21, 5254 2 of 10
a positive association of maximum knee flexion with serve speed and racket impact height.
Conversely, Elliott et al. [5] did not find differences in serve speed when comparing tennis
players with different lower limb kinematics (greater and smaller knee flexion) during their
tennis serve. In addition, studies that induced immediate reduction in knee flexion found
it to negatively impact serve performance [7,12]. Artificially restricting maximum knee
flexion at 10 with an orthosis [7] or asking players to intentionally reduce knee flexion [12]
led to decreases in serve speed and impact height. However, these findings may have been
significantly influenced by an unnatural serve motion due to the immediate induction of
knee flexion reduction. Therefore, it is still necessary to investigate the influence of the
magnitude of knee flexion on tennis serve performance. To the best of our knowledge,
no study has compared the serve performance of junior tennis players with different
magnitudes of knee flexion in natural serve conditions.
Inertial measurement systems (IMS) are becoming widely used in sports motion
analysis [14�20]. As they enable three-dimensional (3D) motion tracking in sport-specific
settings (in-field), they can reveal more realistic results than optoelectronic systems that
require laboratory settings. Thus, evaluating the tennis strokes directly on the tennis
court using IMS would be interesting to maintain the characteristics of the sport's natural
movement. No previous study has investigated the impact of lower limb motion during
serve on tennis serve performance using IMS. Therefore, the current study aimed to
investigate on court the effects of the lower limb drive on the tennis serve performance in
junior players of intermediate level, using wearable inertial sensors. More specifically, we
investigated the effects of the knee flexion magnitude on pre-impact racket velocity and
racket�ball impact height during serve in a more ecological condition. Once this effect was
confirmed, this study investigated the effect on knee extension velocity as an indicator of
lower limb drive effectiveness during the serve. It was hypothesized that junior players of
similar levels with greater serve knee flexion would have greater knee extension velocity,
higher racket velocity, and higher impact height than those with less knee flexion.
2. Materials and Methods
2.1. Participants
The sample size was calculated in G*Power software based on a pilot study with
10 participants, considering the differences of the mean pre-impact racket resultant velocity
between two groups with different maximum knee flexion during the tennis serve. This
calculation indicated a minimum total sample size of 18 participants, considering a power
of 90% and alpha of 0.05. Thirty-two junior competitive intermediate-level tennis players,
aged between 13 and 17, volunteered to participate in this study. The level of play was
defined based on athletes' International Tennis Number (ITN), which characterizes interme-
diate players classified as 5 to 7, on a 1�10 scale [21]. By including only intermediate-level
tennis players, we limited the tennis level's influence on serve performance. Indeed, players
of different levels could show distinct performances regardless of their knee flexion during
the serve [6]. Participants and their legal guardians signed an informed consent form. All
players were asymptomatic and none had been injured in the previous six months, had any
orthopaedic surgery, or had knee passive range of motion limitations [22]. The participants
were equally divided into two groups, with a division cut-off based on the median of the
maximum knee flexion (MKF) obtained by the athletes during the serve preparation phase:
Greater Knee Flexion group (GKF, n = 16) and Smaller Knee Flexion group (SKF, n = 16).
Due to the unavailability of a clear definition in the literature about high and small values
of knee flexion during the serve, this division method was used to guarantee that the two
groups had distinct knee flexion values.
2.2. Procedures
An interview to evaluate the volunteer's eligibility to participate in this study was
conducted and followed by body height, weight, and knee passive range of motion (ROM)
measurements. These assessments were performed by the same examiner. The participants
Sensors 2021, 21, x FOR PEER REVIEW 3 of 11
Sensors 2021, 21, 5254 An interview to evaluate the volunteer's eligibility to participate in this study3 owf a10s
conducted and followed by body height, weight, and knee passive range of motion (ROM)
measurements. These assessments were performed by the same examiner. The partici-
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(The Mathworks, Natick, MA, USA). The system was calibrated following the N-pose plus
walk process, as recommended by the manufacturer [23]. The right-handed participants
were asked to serve from the deuce courtside (i.e., the right side of the court) aiming at
the target area bordering the "T" (middle) of the service boxes (Figure 2) for 5 min, or
Sensors 2021, 21, 5254 The software MVN Analyze was used to collect and export kinematic data, which
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Matlab (The Mathworks, Natick, MA, USA). The system was calibrated following t4hoef 1N0 -
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aiming at the target area bordering the "T" (middle) of the service boxes (Figure 2) for 5
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which contain the biomechanical model and the motion files, were exported from this
software and imported into Visual3D software for analysis. Using the built-in functions of
Visual3D pipelines, the front (i.e., lead leg) knee flexion angle was calculated based on the
shank's position and orientation relative to the thigh around Y-axis, using the Y-X-Z Cardan
sequence, i.e., flexion, abduction, rotation [28]. Similarly, the front knee extension velocity
Sensors 2021, 21, 5254 5 of 10
was also calculated. The front knee corresponded to the left knee of the right-handed
players and the right knee of the left-handed players.
The serve preparation phase was determined on the Visual3D from the racket's
maximum anterior position to the maximum knee flexion [27]. Racket�ball impact was
identified as being the highest racket linear velocity in the anterior direction [29]. The
maximum knee extension velocity was calculated between the serve events MKF and
racket�ball impact. These two serve events were also visually inspected and confirmed in
Visual3D and in the recorded videos. Pre-impact racket resultant velocity was calculated in
the Matlab as the norm of racket's linear velocity in the three planes of motion, just before
(1 frame) the racket�ball impact [11,30,31]. Racket impact height (i.e., the racket's vertical
distance to the ground) was also obtained and expressed as a percentage of the standing
height of the participant (normalized racket height). The mean of five valid tennis serve
trials from each participant was used for analysis.
2.4. Reliability
A pilot study with ten subjects was conducted to investigate the study's measures and
procedure reliability. Data were collected in two different sessions, one week apart. The
test�retest reliability (ICC2,5) for maximum knee flexion during the preparation phase of
serve, maximum knee extension velocity, pre-impact racket resultant velocity, and racket
impact height were 0.93, 0.94, 0.89, and 0.88, respectively.
2.5. Statistical Analysis
Descriptive statistics were used for participants' characterization. Normality was
tested using Shapiro�Wilk's test. A chi-squared test was used to compare groups for ITN
and sex. Independent samples t-tests were used to compare groups for the maximum knee
extension velocity, pre-impact racket velocity, and normalized racket height at impact. The
mean differences (MD) between groups and Cohen's d effect sizes (ES) [32] were calculated.
All analyses were performed in SPSS software (IBM Corp., Armonk, NY, USA) with a
significance level of = 0.05.
3. Results
3.1. Participants
There were thirteen right-handed and three left-handed, and fourteen right-handed
and two left-handed participants in the SKF and GKF groups, respectively. No difference
between SKF and GKF groups was found for the descriptive variables, except for the serve
maximum knee flexion that was used to divide groups and was found to be 19.08 greater
in the GKF (Table 1). Results of the chi-squared test also showed no difference between
groups for the level of play (ITN) (p = 0.494) and sex (p = 0.194).
Table 1. Descriptive data of the participants.
Descriptive Data SKF (n = 16) GKF (n = 16) p-Value
Body height (m) a 1.66 � 0.08 1.67 � 0.06 0.942
Body mass (kg) a 54.75 � 6.25 56.08 � 6.69 0.567
13.81 � 1.05 14.25 � 1.24 0.305
Age (years) b 6.50 � 2.42 7.00 � 2.10 0.537
Tennis playing experience (years) a 8.75 � 1.44 9.25 � 1.24 0.361
4.38 � 0.72 4.63 � 0.62 0.361
Weekly tennis training (h) b 55.64 � 8.66 74.72 � 5.88 <0.001 *
Weekly conditioning training (h) b
Serve maximum knee flexion () a
Results are reported as mean and standard deviation. * p < 0.001. SKF: Smaller Knee Flexion group. GKF: Greater Knee Flexion group.
a Variables normally distributed. p-values from the independent samples t-test are reported above. b Variables not normally distributed.
p-values from the Mann�Whitney test are reported above.
Serve maximum knee flexion (�) a 55.64 � 8.66 74.72 � 5.88 <0.001 *
Results are reported as mean and standard deviation. * p < 0.001. SKF: Smaller Knee Flexion
group. GKF: Greater Knee Flexion group. a Variables normally distributed. p-values from the inde-
pendent samples t-test are reported above. b Variables not normally distributed. p-values from the
Sensors 2021, 21, 5254 Mann�Whitney test are reported above. 6 of 10
3.2. Tennis Serve Performance and Knee Extension Velocity
3.2. TTenhneispSree-rivme pPaercftorrmacaknectevaenldocKitnyeewEaxste3n.3si3onkmVe/lhochitiygher and the maximum knee exten-
bsssbicieeoorttninwwpTvetveieeheevlnnleeoocpggcdiirrrtatoeoyyt-uuaiwwmpposasapfssffaoko1c1rnr3t3et0r0the.ha.3e3cea00knnneo�g/to/rlsrvsmemehhaaliaiolnggilczdhihizeeteeydarrdnwirignnaraucaGGsklcakeK3Krte.F3Ftvi3mettihmhklpoaamapcnncia/ttSSychhtKKiehhFnFii.eg.gtiThThhghtheehe(retTsrra(eaaenTgbwadwiltbeatatlahs2sel)enn.2pomT)ol.aisamsntTxtaeaieitmmtiasisseeruttierimcsiceaepaslrklridefnedosiseifrefefnffetoehetrrerxeeedtntndehcncieeene--
dFeigsucrriep3ti.ve data of knee angle and angular velocity in the sagittal plane are presented in
FTiagbulere23. C. omparative table of the tennis serve performance and knee extension velocity.
Table 2. ComparTateivnentiasblSeeorfvtehe tennis serve performance and knee extension velocity. 16) p ES
Performance
Tennis Serve Descriptive SKF (n = 16) GKF (n =
Performance
SeKxNDFtMMMiemR:MveoneeeSasCCCepraaasmaccmlnnniIIIaroxko999aic555���icapelnmt%%%lilttetihiSSSvyrzruvDDDeeeeKem(ilsdkgonumheckrleitatnt/acyF(hen%kle)e(te�)xt/314iso11101S2)n999251K...2.1861.g.1F21016r23o(-�--n�u�MMMp=14123.29635.1eeeG005.7.136CCCaaa...6K64430)nnnIII515999F555���:%%%GSSSrDDDea541te32721G2r365224KK....44584.4.134Fn121045111109e2(--�-99925e�n1�.2...F861.12511=1.1l0126132915.2e1.40.673x13---�6...3�64177i�)o14212473n35296.50.7.0g136...r46304o155u415p217232.6235200p40.4....:58400.4.24.pp102101435-2496v---��**a�1521l1u53291.e.6470.31s3....14776E724S00010:..00...C1920134o304285h6.E4**e((S2llnaa6'rrsgg((100lldeeaa...))149rrgg324865ee))
Racket resultant
velocity (km/h)
Normalized racket
impact height (%)
Maximum knee
extension velocity (/s)
SKF: Smaller Knee Flexion grouepf.fGecKtFs:izGer.eaStDer: KstnaenedFalerxdiodnegvrioautipo.np.: Cp-Iv95a%lu: e9s5.%ESc:oCnofhidene'nscdeeifnfetcetrsvizael.. *SDp:<st0a.n0d5a. rd deviation. CI95%:
95% confidence interval. * p < 0.05.
Figure 3. Knee motion in the sagittal plane. Top: knee angle. Bottom: knee angular velocity.
SFlioingleiudsr: elmin3ee.asKn: nmveeaealmuneosvt.aioSlunheaisnd. otShwhesa:sdasogtwaitnstda: lasprtdalannddeea.vrTdiaotdpioe:nvk.inaBteileounae.n: ggBlrleue.aeBt:eogrttrkoenmatee:erkfnkleenxeeieaonnflgeguxrlioaorunpvger(loGoucKiptFy().G. SRKoelFidd).:
Rsmeda:llsemr kanlleeer fklnexeieoflnegxrioounpg(rSoKuFp).(STKhFe)s.eTrhvee csyercvleewcyacsletimwea�sntoimrme�anliozremd ableiztwedeebnettwheeeevnetnhtes e"vmeanxtsi-
"mmuamximknueme fklnexeieoflne"xainond""arancdk"ert�abckaellt�imbaplal cimt"p. act".
4. Discussion
This study aimed to investigate the influence of the knee flexion magnitude during
the serve preparation phase on knee extension velocity and the tennis serve performance of
junior competitive players. To improve the ecological validity of the results, this study was
performed on a tennis court using wireless inertial sensors. We found that tennis players
with greater knee flexion during the preparation phase of their serve had 32% higher knee
extension velocity and 16% higher pre-impact racket velocity than players with less knee
flexion. The greater knee angular velocity may have contributed to the greater racket
Sensors 2021, 21, 5254 7 of 10
velocity found in this study. Nevertheless, the normalized racket impact height was not
significantly different between groups. These findings suggest that tennis coaches and
players should consider the magnitude of knee flexion when planning training to improve
serve performance.
The greater pre-impact racket velocity found in participants with greater knee flexion
may be explained by increases in knee extension velocity and possibly in mechanical energy
generation and transfer throughout the kinetic chain. Increased knee flexion during the
preparation phase of a serve typically leads to increases in the range of knee extension
during the lower limb drive phase (propulsion) of the serve [12]. Displacement throughout
a greater joint range of motion seems to be related to greater joint velocity. As found in the
present study, Anderson and Sidaway [33], who analysed the soccer kick, also found that
players who flexed their knee more during the preparation phase had greater maximum
knee extension velocity during the acceleration phase. This association between knee
flexion angle and extension velocity may be explained by the fact that covering a greater
joint range of motion would give the individuals more time to apply acceleration and
increase joint velocity. Complementarily, another possible explanation relies on the stretch-
shortening cycle (SSC) function. As the knee flexes during the serve preparation phase,
the quadriceps contract eccentrically [8], which may result in elastic energy storage [34,35].
This energy may be used, at least partially, to increase knee extension velocity during
serve. Due to the relationship between joint velocity and kinetic energy, it is expected
that increased knee extension velocity during a serve increases the energy generated by
lower limbs, which is typically transferred, through the kinetic chain, to the trunk, upper
limb, and finally to the racket. This mechanism would ultimately increase the serve
speed [36,37]. The pre-impact racket velocity is strongly correlated with the post-impact
ball speed [10]. Thus, pre-impact racket velocity is commonly reported as an indicator of
serve performance [7,30,31].
Our results corroborate with Sgro et al. [6], who also found that players with greater
knee flexion were also the ones with faster serves. However, they divided groups based on
the participants' level of play and the group with greater knee flexion and faster serve were
advanced tennis players, who were compared with lower level players. Therefore, their
results were influenced by the player's game level, while participants in our study were all
at the same level (intermediate) with no statistical difference between groups. Additionally,
in the current study, no differences were found between groups for other variables that
may influence serve performance--such as body height and mass, age, tennis experience
time, and weekly training volume--addressed in previous studies [3,38�40].
Contrary to our results, Elliott et al. [5] found no difference in professional players'
serve speed regardless of their knee flexion efficiency. However, they reported the angle
of knee flexion at maximum shoulder external rotation (i.e., after the preparation phase),
when the knee is typically already extending [36]. While greater knee flexion at maximum
shoulder external rotation could be due to a greater knee flexion during the preparation
phase [5], it is also reasonable to believe that the opposite could be true. Speculatively,
players with less knee flexion at maximum shoulder external rotation could be the ones who
flexed their knee more during the preparation phase of a serve and had a more effective
lower limb drive. Therefore, examining knee flexion during the preparation phase is a more
appropriate method to evaluate the contribution of knee flexion to serve performance.
In contrast to our hypothesis, the normalized racket impact height was not different
between players with different knee flexion magnitudes. Although the players with greater
knee flexion had a more effective lower limb drive and, therefore, a potential to reach
higher, their racket impact height was not greater, as we expected. Our result agrees with
Girard et al. [8], who did not find an association between vertical ground reaction force
and racket impact height during the serve of intermediate-level tennis players. Although
these authors did not measure knee flexion, the vertical ground reaction force is expected
to increase as knee flexion increases during the serve preparation phase [7]. A possible
explanation for these findings relies on a dependency of racket impact height on other
Sensors 2021, 21, 5254 8 of 10
factors, such as the ball toss height and shoulder mobility, which were not measured. As
the racket height at impact depends on the ball location, if the ball toss height is low, for
example, the player will not hit at higher locations. Similarly, shoulder mobility deficits
may limit the player's ability to hit the ball higher at serve impact.
The maximum knee flexion values found in the present study, using an inertial mea-
surement system, were within the range found in other studies that used optoelectronic
motion analysis [12,27] or videography systems [6]. However, the pre-impact racket resul-
tant velocity was lower than the reported in other studies [30,31]. This difference may be
explained by the differences in the sample's level of play and methods used. The participants
of Gillet et al. [30] and Rogowski et al.'s [31] studies were advanced tennis players (ITN 2 to
4), whereas the participants of our study were intermediate level. It is known that the level
of play impacts serve speed. Higher play levels are related to greater serve speed [6,39]. The
difference in racket velocity may also be explained by the different methods for measuring
the racket's velocity. Gillet et al. [30] and Rogowski et al. [31] reported the velocity at the
centre of the racket face. In contrast, in the current study, the inertial sensor to track racket
velocity was placed on the top of the grip (Figure 1). Mitchell et al. [29] found differences
of up to 70% when comparing racket velocity measured at the centre of the racket face
and around the middle portion of the grip. Our results seem to agree with the literature if
considering the location on the racket where the velocity was tracked.
The method used in the present study brings some limitations that should be discussed.
The inertial system's wireless update rate may be considered low (60 Hz), which was the
instrument's maximum frequency. However, each inertial sensor internally sampled
data at a high frequency (1000 Hz), which helped maintain acquisition accuracy during
dynamic motion. Another limitation was the fact that this study did not investigate
the effects of greater knee flexion during the preparation phase of the serve on upper
joint motions (trunk and dominant upper limb). Since these joint motions could also
be related to serve performance, more studies must explore these effects. Moreover, the
neuromusculoskeletal maturity (e.g., growth rate) of the studied adolescents, which affects
their physical capabilities and coordination, was not controlled. However, all participants
were competitive athletes with approximately seven years of tennis experience, and so
their high skill level may have helped to overcome the possible impact of physiological
changes (not measured) on their performance.
To the best of our knowledge, this is the first study to investigate the effects of lower
limb motion on serve performance on the tennis court (sport-specific setting) with an
inertial measuring system. This approach indicates that using these types of sensors
can provide more realistic analyses than those performed in a laboratory setting. Sports
scientists and professionals can rely on the method used and consider the results obtained
in their practice. However, further studies are necessary to investigate the effects of specific
methods to increase knee flexion during the preparation phase on serve performance.
5. Conclusions
The serve pre-impact racket velocity of junior tennis players of intermediate level with
greater knee flexion during the serve preparation phase was higher than for those with
less knee flexion. Additionally, greater knee extension velocity was found in this group,
indicating a more effective lower limb drive during the serve. However, the racket impact
height was not different between groups. The magnitude of knee flexion should be seen as
a contributor to the pre-impact racket velocity.
Author Contributions: Conceptualization, J.F.H., T.R.S., F.A.M., M.B., T.R.T.S. and S.T.F.; methodol-
ogy, J.F.H., T.R.S., M.B. and S.T.F.; software, J.F.H., F.A.M., M.B. and S.T.F.; validation, J.F.H. and S.T.F.;
formal analysis, J.F.H., T.R.S., F.A.M. and S.T.F.; investigation, J.F.H., T.R.T.S. and F.A.M.; resources,
J.F.H., F.A.M. and S.T.F., data curation, J.F.H.; writing--original draft preparation, J.F.H. and S.T.F.;
writing--review and editing, J.F.H., T.R.S., F.A.M., M.B., T.R.T.S. and S.T.F.; visualization, J.F.H.,
T.R.S., F.A.M., M.B., T.R.T.S. and S.T.F.; supervision, T.R.S., M.B. and S.T.F.; project administration,
Sensors 2021, 21, 5254 9 of 10
J.F.H., T.R.S. and S.T.F.; funding acquisition, T.R.S. and S.T.F. All authors have read and agreed to the
published version of the manuscript.
Funding: This research was funded by the Brazilian Funding Agencies: Coordena��o de Aperfei�oa-
mento de Pessoal de N�vel Superior--CAPES, finance code 001 and process number 88887.364812/2019-
00, Conselho Nacional de Desenvolvimento Cient�fico e Tecnol�gico--CNPQ, and Funda��o de
Amparo � Pesquisa do Estado de Minas Gerais-FAPEMIG.
Institutional Review Board Statement: The study was conducted according to the guidelines of the
Declaration of Helsinki and approved by the Institutional Ethics Committee of the Federal University
of Minas Gerais (UFMG) (protocol code 93692218.2.0000.5149 and date of approval: 9 August 2018).
Informed Consent Statement: Informed consent was obtained from all subjects involved in the
study. Written informed consent has been obtained from the patient(s) to publish this paper.
Acknowledgments: We thank the Servi�o Social da Ind�stria (SESI)/Federa��o das Ind�strias do
Estado de Minas Gerais (FIEMG) and the manager of the Innovation Center in Ergonomics, Carla
A. Gon�alves Sirqueira, for the availability of the inertial measuring system used in this study. We
also thank the tennis players for their voluntary participation, their coaches for the support, and the
Minas T�nis Clube and FlySports for their cooperation during this study.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
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