Group Title: Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology 2009, 1:24
Title: Lower trunk kinematics and muscle activity during different types of tennis serves
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Title: Lower trunk kinematics and muscle activity during different types of tennis serves
Series Title: Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology 2009, 1:24
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Creator: Chow JW
Park SA
Tillman MD
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Sports Medicine, Arthroscopy, O

Rehabilitation, Therapy & Technology BioMed Central


Lower trunk kinematics and muscle activity during different types
of tennis serves
John W Chow*tl, Soo-An Parkt2 and Mark D Tillmant3

Address: 'Center for Neuroscience and Neurological Recovery, Methodist Rehabilitation Center, Jackson, Mississippi, USA, 2Department of
Orthopedic Surgery, Asan Medical Center, University of Ulsan, Seoul, South Korea and 3Department of Applied Physiology and Kinesiology,
University of Florida, Gainesville, Florida, USA
Email: John W Chow*; Soo-An Park; Mark D Tillman
* Corresponding author tEqual contributors

Published: 13 October 2009 Received: 19 June 2009
Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology 2009, 1:24 doi: 10.1 186/1758-2555-1-24 Accepted: 13 October 2009
This article is available from: 1/1/24
2009 Chow et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Background: To better understand the underlying mechanisms involved in trunk motion during
a tennis serve, this study aimed to examine the (I) relative motion of the middle and lower trunk
and (2) lower trunk muscle activity during three different types of tennis serves flat, topspin, and
Methods: Tennis serves performed by I I advanced (AV) and 8 advanced intermediate (Al) male
tennis players were videorecorded with markers placed on the back of the subject used to estimate
the anatomical joint (AJ) angles between the middle and lower trunk for four trunk motions
(extension, left lateral flexion, and left and right twisting). Surface electromyographic (EMG)
techniques were used to monitor the left and right rectus abdominis (LRA and RRA), external
oblique (LEO and REO), internal oblique (LIO and RIO), and erector spinae (LES and RES). The
maximal AJ angles for different trunk motions during a serve and the average EMG levels for
different muscles during different phases (ascending and descending windup, acceleration, and
follow-through) of a tennis serve were evaluated.
Results: The repeated measures Skill x Serve Type x Trunk Motion ANOVA for maximal AJ angle
indicated no significant main effects for serve type or skill level. However, the AV group had
significantly smaller extension (p = 0.018) and greater left lateral flexion (p = 0.038) angles than the
Al group. The repeated measures Skill x Serve Type x Phase MANOVA revealed significant phase
main effects in all muscles (p < 0.001) and the average EMG of the AV group for LRA was
significantly higher than that of the Al group (p = 0.008). All muscles showed their highest EMG
values during the acceleration phase. LRA and LEO muscles also exhibited high activations during
the descending windup phase, and RES muscle was very active during the follow-through phase.
Conclusion: Subjects in the Al group may be more susceptible to back injury than the AV group
because of the significantly greater trunk hyperextension, and relatively large lumbar spinal loads
are expected during the acceleration phase because of the hyperextension posture and profound
front-back and bilateral co-activations in lower trunk muscles.

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Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24

Low back injuries are common among competitive tennis
players [1-7]. General agreement exists that mechanical
stress to the spine is related to the development of degen-
erative disc disease in the lumbar region [8]. Among dif-
ferent tennis strokes, the serve may place more stress on
the lumbar spine than the other strokes because repetitive
trunk hyperextension is generally thought to be the pre-
disposing mechanism of spondylolysis [8-10]. Tennis
players may be at an increased risk of lumbar disc pathol-
ogy from rotational and hyperextension shearing effects
[2]. The three types of serves that are widely used in tennis
are the flat (minimum spin), topspin, and slice (sidespin)
serves. In general, the flat serve is associated with a more
forceful action and produces the fastest ball speed among
the three types of serves. The spin involved in the topspin
and slice serves permit the server to hit with greater accu-
racy. The racquet movement pattern and ball contact loca-
tion relative to the body are different among these serves

Because the spine is a complicated structure composed of
many segments, joints, discs and various supporting mus-
cles to protect the spinal cord and support the trunk
mobility, it is very difficult to pinpoint the anatomic struc-
tures that cause low back pain. Identifying the biome-
chanical pathophysiologic factors associated with lumbar
spinal loads may help to explain and prevent low back
pathology. In national and world class tennis players who
had structural disorders in their lumbar spines, Saal [12]
reported a 3-to-1 ratio of disc to postelement syndromes.
In addition, for the young tennis players, posterior ele-
ment pain spondylolysis with or without spondylolisthe-
sis compose the injury subset most frequently. Alyas et al.
[13] found pars injuries and facet joint arthroses, predom-
inately in the lower lumbar spine, to be relatively com-
mon in elite adolescent tennis players.

Motions of the trunk during occupational tasks have been
identified as potential risk factors for developing low back
disorders (LBD) in manual workers [14]. High values of
combined trunk velocities (e.g., simultaneous lateral flex-
ion and twisting velocities) were found to occur more
often in high LBD risk jobs than in low LBD risk jobs.
Dynamic strength of the trunk and structural loading are
considered the two major contributing factors to the rela-
tionship between trunk dynamics and LBD. Structural
loading factors include biomechanical factors that con-
tribute to loading on the spinal structures such as intra-
abdominal pressure, muscle activity, and the imposed
trunk moment, and the actual loads on the structures of
the spine.

Loading on the spinal structures can be very high during a
tennis serve. This is especially true when the racquet

moves behind the body and the vertebral column is later-
ally flexed and hyperextended. Acceleration of the racquet
before ball impact is accompanied by a rapid reversal of
the rotation of the lumbar spine from hyperextension to
flexion and right twist to left twist for a right-hander. This
cork-screwing motion transfers the force of its torque to
the spinal segments [15].

Body segmental and racquet kinematics of the tennis serve
have been investigated extensively [16-18]. However,
lumbar spine kinematics during the tennis serve have not
been reported. Because low back injury is one of the most
prevalent musculoskeletal diseases in tennis, a better
understanding of the underlying mechanisms involved in
trunk motion during a tennis serve is needed.

Information that identifies the muscles involved in stroke
production is important for coaches and physical trainers
[19]. Activity of the trunk muscles can be used to speculate
on the stress on the lumbar intervertebral joints during
dynamic tasks [20-23] and it has shown that the force gen-
eration and muscle recruitment activities associated with
twisting change significantly as a function of the torso
posture [24]. Prior EMG analyses of the tennis serve have
focused on the muscles in the hitting arm, shoulder
region, and lower extremity [25-31]. Very limited data on
the activity of the lower trunk muscles during the tennis
serve are available. Anderson [32] reported that both the
left and right external obliques were very active (greater
than 50% of the muscle's peak level of activity) during the
force production phase of the tennis serve. In a prelimi-
nary study, Chow et al. [33] examined the muscle activa-
tion patterns of eight lower trunk muscles during flat,
topspin, and slice serves in five male highly skilled tennis
players. They found no major differences in muscle activa-
tion pattern across different serve types, and bilateral dif-
ferences in muscle activation were more pronounced in
rectus abdominis and external oblique than in internal
oblique and lumbar erector spinae muscles. An apprecia-
ble amount of abdominal/low back and bilateral co-acti-
vation was observed during certain phases of the serve.

The purpose of this study was to evaluate the middle and
lower trunk kinematics and selected lower trunk muscle
activity during three different types of tennis serves (flat,
topspin, and slice) in skilled male players. Because players
struck the ball at a much more backward and to the left
(for a right-hander) location on their second as compared
to their first serve [11], players may need to arch backward
and laterally flex more when executing a topspin serve.
Therefore, it was hypothesized that greater spinal range of
motion and lower trunk muscle activity would be found
in the topspin serve when compared to the other serve

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Eleven advanced (AV) (United States Tennis Association
National Tennis Rating Program (NTRP) 5.5, age 25.3 +
4.1 years, height 180.3 + 5.2 cm, mass 80 8 kg) and eight
advanced intermediate (AI) (4.5 5.0, 23.4 6.5 years,
180.0 9.5 cm, 78 7 kg) male tennis players served as
the subjects. NTRP ratings range from 1.0 (beginner) to
7.0 (world class professional). All subjects were right-
handed and were in good physical condition and free of
injury at the time of participation. The NTRP ratings were
self-reported ratings. All subjects signed informed consent
documents before their participation.

Experimental Setup
Four gen-locked video cameras (60 Hz) were stationed
behind and to the left of the baseline of an indoor tennis
court (Figure 1). One of the cameras was used to capture
the whole body and racquet movements. The other three
cameras captured the locations of reflective markers
located on the back of the subject. A Peak event synchro-
nization unit was used to synchronize the video and elec-
tromyographic recordings (Peak Performance
Technologies, Inc., Inglewood, CO). For the purpose of
spatial reference, a Peak calibration frame (1.5 m x 1.4 m
x 1.3 m object space, 16 control points) was videotaped at
the beginning of each data collection session. The frame
was positioned at the baseline where the trunk of the sub-
ject was located during testing.

Data Collection
Each collection session started with EMG trials followed
by kinematics trials. Separate trials for EMG and kine-
matic data were needed because EMG electrodes inter-
fered with reflective marker placement on the lower back.

EMG Trials
After jogging on a treadmill for five minutes as a warm-up,
surface electrodes were attached to selected muscles of the
lower trunk including the left and right rectus abdominis
(3 cm lateral to the umbilicus), external obliques (approx-
imately 15 cm lateral to the umbilicus), internal oblique
(below the external oblique electrodes and just superior to
the inguinal ligament), and lumbar erector spinae (3 cm
lateral to L3 spinous process) [22]. The skin surfaces
where the electrodes were located were cleansed with alco-
hol and shaved when necessary. Electrodes were placed
over the bellies of each muscle parallel to the muscle's line
of action with a center-to-center distance of 2.5 cm. Using
a MESPEC 4000 telemetry system (Mega Electronics Ltd.,
Kuopio, Finland), the EMG signals were preamplified
with a gain of 500 and band pass filtered at 8-1500 Hz
(CMRR > 130 dB) close to the electrodes and telemetric-
ally transmitted to a central receiver (gain = 1, Butterworth
filter, 8-500 Hz band pass). The amplified EMG signals


Figure I
Overhead view of the experimental setup.

were sampled at 900 Hz (12-bit analog-to-digital conver-
sion) using the Peak Motus system.

To obtain maximum EMG levels, two maximal isometric
contractions were performed before the experimental tri-
als the bent-knee sit-up with the trunk inclined at
approximately 300 to the horizontal and the trunk hyper-
extension performed in the prone position on a treatment
table. In both maximal contractions, the feet were con-
strained and the resistance was applied manually at the
shoulders. One trial was performed for each maximal con-
traction and each trial lasted for about 5 s.

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Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24

After the isometric trials, the EMG transmitter was secured
to the left hip of the subject using elastic bands. The sub-
ject was then asked to perform three different types of
serves flat, topspin, and slice. In all trials, the subject
served with efforts comparable to his first serves during
competition. They were asked to target their serves at the
corner near the center line (flat and topspin) and sideline
(slice). Seven trials were performed for each serve type and
the serve type was presented in a random order. At the end
of each trial, the subject was asked to rate his own per-
formance based on the pace of the ball and landing loca-
tion using a 5-point scale (5 = excellent, 0 = poor). For
each trial, the EMG signals were collected for 5 s.

Kinematic Trials
After the EMG trials, electrodes were removed and eight
reflective markers (each 1 cm in diameter) were placed on
the back of the subject. The marker locations were right
and left tips of the 11th rib, T9 and T12 spinous processes,
right and left posterior superior iliac spines (PSISs), and
L3 and L5 spinous processes (Figure 2). The location of
the spine level was estimated using the techniques pro-
posed by Tully and Stillman [34] and Schache et al. [35].
These markers were used to estimate the 3-dimensional
(3D) orientations of the spinal regions directly above and
below the lumbar spinal segments. These two regions
were considered the middle and lower trunk for the pur-
pose of this study. As a result, the change in relative

11th ribs

T9 Z Y


Posterior Superior
Iliac Spine (PSIS)

Figure 2
Reflective marker locations and coordinate systems
for the middle and lower trunk.

motion between the middle and lower trunk was treated
as motion in the lumbar spinal segments.

To establish the neutral orientation for the markers
attached to each subject, the subject was asked to adopt a
self-selected comfortable standing posture with arms
folded in front of the torso while marker locations were
recorded. The subject was then asked to perform seven tri-
als for each type of serve and the order for the type of serve
was assigned randomly. Again, the subjects served with
efforts comparable to their first serves during competition
and rated their own performance at the end of each trial.

Data Reduction
For each subject, the two highest rated EMG and kinemat-
ics trials for each type of serve were analyzed. For each trial
being analyzed, four critical instants were identified from
the video recordings of whole body and racquet move-
ments: (a) beginning of the windup, the instant when the
racquet passed in front of the legs, (b) end of the ascend-
ing windup, the instant when the racquet reached the
highest position during the windup, (c) end of the win-
dup, the instant when the racquet reached the lowest posi-
tion behind the trunk, and (d) ball impact. For the
purpose of this study, the serve was divided into four
phases (Figure 3):

* Ascending windup [instants (a) to (b)]

* Descending windup [instants (b) to (c)]

* Acceleration [instants (c) to (d)]

* Follow-through [0.1 s duration after instant (d)]

These phases are of interest because of the distinct move-
ment and functional characteristics of the body and rac-
quet during these phases.

EMG Data
The raw EMG signals were filtered using a recursive digital
filter (Matlab Elliptic filter, 10-450 Hz band pass) and
full-wave rectified. The maximum isometric trial data
were smoothed using a moving average of 2 s and the larg-
est average EMG value recorded for each muscle was con-
sidered the maximum EMG level. The experimental trial
data were smoothed using a moving average of 50 ms
before normalizing to the respective maximum EMG lev-
els. An average normalized EMG value was computed for
each muscle in each phase for each trial analyzed.

Kinematic Data
For each standing or serving trial analyzed, coordinate
data were extracted from the video pictures (automatic
tracking) using a video-based motion analysis system

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Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24

d c

Figure 3
Critical instants of a serve -- (a) beginning of the windup, (b) end of the ascending windup, (c) end of the windup, and (d)
ball impact. Adapted from Chow et al. [33].

(Peak Motus Motion Measurement System). The three-
dimensional (3D) coordinates the eight reflective markers
located on the back of the trunk were transformed from
the Peak reference frame to local reference frames embed-
ded in the middle and lower trunk (Figure 2). Considering
the middle and lower trunk as adjacent segments of a
joint, the anatomical joint (AJ) angles between the two
segments is the relative orientation of the two segment-
embedded local reference frames (see Additional file 1,
[36]). The three components of the AJ angles represent the
rotations about the medio-lateral axis (flexion/extension
angle), antero-posterior axis (lateral flexion angle), and
longitudinal axis (twisting angle). For each subject, the AJ
angles obtained during serving trials were expressed as the
angular deviation from the AJ angles recorded at standing
posture (i.e., the AJ angles at standing posture are all 0).
The dependent variables for trunk motion were the maxi-
mal extension, left lateral flexion, and left and right twist-
ing angles during a serve. For each subject, average
normalized EMG values and maximum AJ angles over two
trials of the same serve type were used for subsequent

Data Analysis
For each serve type, phase and skill level combination,
mean and standard deviation were computed for each var-
iable of interest. For each muscle, the EMG parameters
were compared using a 2 x 3 x 4 (Skill x Serve type x
Phase) multivariate analysis of variance (MANOVA) with
repeated measures on the last two factors. Separate univar-
iate tests were performed for follow up testing when
appropriate, and Bonferroni's procedure was used to
adjust the overall type I error rate. To determine if signifi-
cant variations existed among skill groups, serve types and
trunk motions in the maximal AJ angle, a 2 x 3 x 4 (Skill
x Serve type x Trunk motion) ANOVA with repeated
measures on the last two factors was performed. An a-pri-
ori alpha level of 0.05 and an a-priori beta level of 0.20
were used in this study.

Muscle Activation
As expected, a significant main effect for the phase was
found in the average EMG level in each of the muscles
monitored (p < 0.001). In addition, no significant main
effect for the serve type or inter-factor interaction was

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Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24

found in any of the muscles (p > 0.695). In general, the
lower trunk muscles become active toward the end of the
ascending windup phase (Figure 4). For most muscles
tested, the largest average EMG levels were observed in
either the descending windup or acceleration phases.
When comparing overall muscle activation during a ten-
nis serve between the two skill groups, the subjects in the
AV group generally exhibited greater muscle activation
than the subjects in the AI group (Figure 5).

Rectus Abdominis
A significant main effect for the skill was found in the LRA
activity (p = .008). The AV group showed significantly
higher LRA activation than the AI group. Regardless of the
serve type, the activation patterns of the LRA and RRA are
quite similar activations in the descending wind-up and
acceleration phases are greater than the ascending windup
and follow-through phases (Table 1).

300 -

250 -

200 -


100 -

External Oblique
Different patterns of activation were observed in the two
EO muscles (Table 1). In general, the subjects in the AV
group exhibited greater LEO activity than the subjects in
the AI group and the difference was a statistical trend (p =

Internal Oblique
Among the muscles examined, the IO muscles have the
largest overall EMG levels (Figure 4). Different patterns of
activation were observed in the two IO muscles (Table 1).
The LIO was generally more active than the RIO through-
out a serve except in the follow-through phase. Very high
activation levels (average EMG levels greater than 100%
max) were found in the descending windup and accelera-
tion phases in the LIO.







Figure 4
Average EMG levels of different muscles during different phases.

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Figure 5
Average EMG levels of different muscles for the two skill groups.

Erector Spinae
Moderate activity was observed in all phases except the
ascending windup for both ES muscles and the descend-
ing windup phase for the RES (Table 1). Different patterns
of activation were observed in the two ES muscles (Figure
4). Instead of a relatively constant average EMG level after
the ascending wind-up phase observed in the LES, the
activity of RES increased steadily from ascending wind-up
to follow-through phase. Although not statistically signif-
icant (p = 0.069), the AV group showed greater RES acti-
vation than the AI group (Table 1).

Maximum Aj Angles
The repeated measures ANOVA performed on maximum
AJ angles revealed a significant main effect for trunk

motion (p < 0.001). This simply means that the middle-
lower trunk ROM is not the same in different principle
planes (Table 2). In addition, a significant interaction was
found between trunk motion and skill factors (p < 0.001)
(Figure 6). The AV group exhibited greater maximum AJ
angles in all trunk motions measured except the exten-
sion. No significant main effect for serve type was

The ANOVA also revealed significant differences between
the two skill levels for the extension and left lateral flexion
(Table 2). For the extension motion, the AV group exhib-
ited a significantly smaller maximum AJ angle than the AI
group (p = 0.018) while the opposite was observed in the
left lateral flexion (p = 0.038).

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200 -













Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24

Table I: Average (SD) EMG (%MAX) during different phases for different serve types and muscles for subjects of different skill levels.

Flat Topspin Slice


Left RA* AV 25

Al 12

Right RA AV 20

Al 8

Left EO AV 44

Al 30

Right EO AV 25

Al 18

Left 10 AV 51

Al 42

Right 10 AV 30

Al 25

Left ES AV 21

Al 14

Right ES AV 17

Al 9

Abbreviations: AWU ascending windup, DWU descending windup, ACC acceleration, FT follow-through, RA rectus abdominis, EO -
external oblique, 10 internal oblique, ES erector spinae, AV advanced, Al advanced-intermediate.
* Significant difference between AV and Al (p < 0.05).

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62 92 28 28 59 79 25 24 70 87 26
(34) (89) (28) (44) (40) (71) (24) (41) (59) (90) (16)

50 47 21 10 55 64 36 15 43 41 II
(39) (22) (21) (7) (48) (35) (44) (13) (33) (28) (6)

37 56 20 20 24 47 15 21 33 53 26
(43) (30) (11) (35) (16) (34) (7) (40) (28) (34) (23)

40 59 23 10 36 65 23 18 35 60 26
(35) (34) (14) (6) (32) (50) (14) (26) (26) (43) (19)

121 110 5 53 100 109 52 42 115 101 46
(86) (80) (40) (61) (68) (78) (35) (49) (100) (57) (24)

104 67 43 25 86 89 69 32 96 67 33
(95) (56) (35) (23) (60) (65) (95) (29) (84) (50) (29)

54 93 47 26 55 79 37 20 59 100 42
(32) (70) (36) (31) (44) (61) (27) (21) (36) (80) (23)

58 88 38 17 51 93 48 18 48 72 39
(16) (60) (21) (10) (14) (61) (35) (7) (14) (53) (34)

183 118 59 59 150 III 52 40 168 136 73
(144) (61) (38) (61) (94) (61) (36) (27) (127) (91) (68)

151 91 55 31 143 138 109 48 146 108 43
(113) (49) (26) (24) (97) (84) (123) (35) (119) (64) (9)

69 116 88 29 81 100 66 24 82 105 76
(38) (76) (92) (29) (60) (63) (60) (21) (58) (63) (76)

97 100 72 26 96 93 81 28 102 89 63
(74) (63) (50) (27) (87) (64) (51) (25) (101) (55) (54)

51 42 41 19 47 35 37 17 48 44 42
(28) (27) (19) (20) (29) (20) (17) (18) (28) (25) (23)

38 45 40 12 31 44 52 13 38 48 33
(24) (24) (31) (II) (23) (20) (63) (II) (25) (22) (19)

34 46 72 17 38 45 67 15 31 56 99
(34) (31) (65) (22) (37) (34) (66) (17) (39) (55) (142)

12 46 59 II 13 58 59 13 17 46 63
(5) (28) (21) (6) (8) (30) (22) (9) (10) (16) (25)

Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24




Figure 6
Interactions between skill level and trunk motion in maximum anatomical joint angle.

When executing a tennis serve, vigorous movement of the
trunk helps to generate as much angular momentum as
possible and transfer it to the racquet [37]. Dynamic sta-
bility of the spine is essential to prevent low back dysfunc-
tion [38,39] and is associated with sufficient strength and
endurance of the trunk stabilizing muscles and appropri-
ate activation sequencing of the trunk muscles [40]. Spi-
nal stability is also increased with either an increased
coactivation of antagonistic counteracting trunk muscles
or an increased intra-abdominal pressure during the static
condition [38,41]. The primary focus of this study was to
examine the role of lower trunk muscles in providing
dynamic stability of the lumbar spine during a tennis
serve and to speculate on the lumbar spine loads during a
tennis serve using lower trunk muscle activation and kin-
ematics data.

Muscle Activation
The hypothesis that greater activation levels in lower trunk
muscles would be found in topspin serves when com-
pared to the other serve types was not supported by our
results. In general, the activation patterns of different
muscles during a tennis serve are comparable to those
reported by Chow et al. [33]. However, Chow et al. [33]
used players with skill levels similar to the subjects in the
AV group of the present study. For the purpose of this dis-
cussion, an average EMG level of less than 10% max is
considered low. Muscle EMG values of approximately
50% and 100% max are considered moderate and high,

Rectus Abdominis
The RA muscles are active during trunk flexion motions
like curl-up, sit-up or leg raising exercises [42]. Due to its
vertical (longitudinal) alignment, the RA has minimal

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Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24

Table 2: Mean (SD) maximum anatomical joint angles in

Top-spin Slice

muscles acted as prime movers of twisting to the right
occurred during the descending windup and early acceler-
ation phases.

Motion* AV Al Al AV Al Internal Oblique
Ipsilateral 10 is the agonist to the contralateral EO for the
Extension# 19.3 27.5 19.3 31.9 20.0 26.9 axial rotation of the trunk [46]. Similar to the EO muscles,
(11.9) (10.9) (10.6) (17.2) (10.1) (7.6) an appreciable co-contraction of contralateral IO is com-
mon during trunk twists. Interestingly, bilateral difference
Left lateral Flexion# 16.0 12.3 15.5 10.9 15.4 12.4 in muscle activation is more pronounced in IO than in the
(4.1) (5.2) (5.4) (4.8) (5.3) (8.4) EO [51]. One explanation is that the function of EO is

Left Twisting 7.9 6.8 6.8 5.4 5.5 4.3 more complicated than just acting as a prime axial rotator
(4.3) (3.8) (4.1) (0.96 (3.8) (3.1) of the lower trunk [48,52]. Like the EO muscles, bilateral
co-contraction of IO observed during a tennis serve helps
Right Twisting 5.6 4.2 6.1 4.1 1 1.2 4.1 to provide a stabilizing force to the lumbar spine.
(4.6) (1.8) (6.9) (0.6) (12.7) (3.1)

* Significant difference among different motions (p < 0.01).
#Significant difference between AV (advanced) and Al (advanced
intermediate) groups (p < 0.05).

contribution to the production of torque in the transverse
plane [43]. High activation level of anterior supporting
muscles of the lower trunk including RA may lead to unfa-
vorable forces on the spine [44]. It has been suggested that
an average normalized EMG value to MVIC (maximal vol-
untary isometric contraction) below 30% is not consid-
ered very stressful to the spine structures [45]. During the
descending windup and acceleration phases of a tennis
serve, the EMG level of both RA muscles increased to 40%
or higher (Table 1). The AV group showed a higher level
of activation during the acceleration phase than the AI
group. With the center of gravity of the upper body
located behind the lumbar spine, the RA activity (co-con-
traction) in a hyperextended posture during the descend-
ing windup and acceleration phases can drastically
increase the loads on the lumbar spine and lead to harm-
ful stress to the lumbar spine structures.

External Oblique
The EO muscle is one of the anterior supporting muscles
of the lower trunk that is also active during trunk flexion
exercises [42]. However, it has been well recognized that
the contralateral EO muscle is one of the main movers for
axial rotation of the trunk [46,47]. An appreciable antag-
onistic activity of the ipsilateral EO during axial rotation
was also reported in the literature [48]. In the present
study, the co-contraction of bilateral EO muscles helps to
stabilize the lumbar spine during the tennis serve. This
type of co-contraction helps to increase the compressive
load and lead to the torsional stiffness of the lumbar spine
segments [49,50]. High activation of LRA and LEO for the
right-handers in the present study indicated that these

Erector Spinae
The lumbar ES lies lateral to the multifidus muscle and
forms the prominent dorsolateral contour of the back
muscles in the lumbar region. The lumbar ES consists of
two muscles the longisimus dorsi and iliocostalis. The
lines of action of these two muscles are mostly vertical (or
longitudinal) and a bilateral ES contraction can act as a
posterior sagittal extensor. However, when contracting
unilaterally, these muscles can act as lateral flexors of the
lumbar vertebrae [53]. During axial rotation, the ipsilat-
eral ES is more active than the contralateral ES [54]. It also
has been suggested that, during axial rotation, back mus-
cles maintain the spinal posture and stabilize the lumbar
spine [22,43].

In the present study, we found different patterns of muscle
activation during a tennis serve for the two ES muscles.
The LES is quite active throughout a tennis serve except
during the ascending windup phase while sequentially
increased activity from ascending windup to follow-
through phases was observed in the RES (Table 1). It is
obvious that, for the right-handed subjects in this study,
the LES assisted the lateral flexion to the left after the
ascending windup phase. To stabilize the trunk during an
unbalanced posture in the follow-through phase, the RES
becomes highly active during the follow-through phase.
The bilateral ES co-contraction is more pronounced in the
AV group. This is probably related to the greater left lateral
flexion found in the AV subjects (Figure 6). In addition to
bilateral co-contractions, front/back co-contractions exist
throughout a tennis serve and are especially high toward
to end of a serve. This may suggest that the lumbar spine
is subject to large compression loads during the follow-
through phase.

Lower Trunk Motion
In-vivo techniques have been employed to measure spine
kinematics during various physical activities [55-57].
However, it is not feasible to use these techniques in the

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Serve type

Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24

present study because of the large ROM associated with a
tennis serve. Alternatively, we used markers placed on the
lower back to estimate lumbar spinal motion during a ten-
nis serve. The major limitation of our procedures is the
skin movement relative to the spine during lower trunk
motions. Despite this limitation, the marker locations
allow for reasonable estimation of relative motion
between the middle and lower trunk during a tennis serve.

During the extension motion, the vertebral bodies
undergo posterior sagittal rotation and a small posterior
translation. A downward movement of the inferior articu-
lar processes and the spinous process is also involved
which limited by bony impaction between spinous proc-
esses [58]. This type of impaction is accentuated when the
joint is subjected to the action of the back muscles [59].
The maximal extension AJ angles obtained in the present
study fall within the extension ROM values reported in the
literature [56,60]. It should be emphasized that it may not
be adequate to compare the AJ angles in this study with
the ROM values reported by other investigators because of
the differences in measuring techniques.

The reason why the AI group had significantly greater
maximum extension AJ angles than the AV group is diffi-
cult to understand. One explanation is that, instead of
relying on lumbar hyperextension like the AI subjects did,
the subjects in the AV group relied more on the hyperex-
tension of the upper trunk (i.e., thoracic spine) to achieve
the overall trunk hyperextension needed for an execution
of a tennis serve.

Left Lateral Flexion
The lateral flexion of the lumbar spine involves a complex
and variable combination of lateral bending and rotatory
movements of the inter-body joints and diverse move-
ments of the zygapophysial (facet) joints. When com-
pared to lateral bending ROM values for lumbar spinal
motion segments measured by X-rays [60,61], the maxi-
mal left lateral flexion AJ angles are close to the summed
value of each motion segment in the lumbar spine. The
significantly greater maximal left lateral flexion AJ angle
exhibited by the AV group indicates that highly-skilled
right-handed players can reach for a greater height during
a tennis serve because of the greater left lateral flexion. The
significantly greater lateral flexion AJ angle corresponds to
the significantly greater LRA activity found in the AV
group. This implies that highly-skilled players are sub-
jected to greater asymmetric loads on their lumbar spines
due to the greater lateral flexion.

Axial Rotation
Axial rotation of the lumbar spine involves twisting of the
intervertebral discs and impaction of zygapophysial

joints. During axial rotation of an intervertebral joint, all
the fibers of the annulus fibrosus which are inclined to the
direction of rotation will be strained. The other half will
be relaxed. Based on the observation that an elongation of
collagen beyond about 4% of its resting length can lead to
injury of the fiber, it has been estimated that the maxi-
mum range of rotation of an intervertebral disc without
injury is about 3 [62]. The twisting ROM for each lumbar
spinal motion segment ranges from 0 2 [61]. The max-
imal twisting AJ angles found in the present study are
slightly greater than the ROM values reported in the liter-
ature. The maximal twisting AJ angles are considered
small compared to the amount of shoulder movement
during a tennis serve. This clearly indicates that the axial
rotation of the trunk during a tennis serve is mostly from
the twisting of the upper trunk.

Both bilateral and abdominal/back co-contractions
among lower trunk muscles are unavoidable during trunk
movements because these muscles function as units to
maintain the balance between mobility and stability of
the spinal column. As a result, the lumbar spine is sub-
jected to a large amount of compressive and torsional
stress during athletic movements due to the co-contrac-
tion. Although the importance of torsional stress in the
etiology of disc degeneration and prolapse is inconclusive
[63,64], the link between high compressive load and low
back injury and pain is well documented [65,66]. The acti-
vation patterns of the lower trunk muscles clearly demon-
strate a high degree of co-contraction during a tennis
serve, especially in the descending windup and accelera-
tion phases. In addition to the compressive load, the
hyperextension and lateral flexion of the trunk during var-
ious phases of a tennis serve may cause shear loads on the
lumbar spine. Consequently, stresses upon the various
anatomical structures may result in spinal injury and back

Practical Implications
The risk of spinal injury can be high for tennis players of
different skill levels. It is well known that physical activity
increases the amount of bone mineral in the skeleton
[67,68]. Granhed et al. [69] found that intensive weight
lifting would increase the bone mineral content in the
lumbar vertebrae to an extent that the spine can tolerate
extraordinary loads. Unlike most competitive tennis play-
ers, recreational players usually do not spend much time
on training or supervised strength and conditioning pro-
grams. As a result, their vertebrae are not as strong as the
trained individuals and are more susceptible to injury
when subjected to large lumbar spinal loads. Although
highly-skilled competitive players are likely to have
stronger vertebrae when compared to recreational players,
they are also susceptible to spinal injury due to other fac-

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Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology2009, 1:24

tors. Because competitive players complete a large
number of serves in practices and competitions, the accu-
mulative stress on the lumbar spine can be detrimental.

One of the risk factors that has been overlooked by sports
medicine practitioners is the possible link between the
"time of occurrence" and back injury. Adams et al. [70]
measured the range of lumbar flexion of human subjects
in the early morning and in the afternoon and the bend-
ing properties of cadaveric lumbar segments before and
after creep loadings that simulate a day's activity. They
concluded that lumbar discs and ligaments are at greater
risk of injury in the early morning compared with later in
the day. Although the focus of their study was on lumbar
flexion, it does have implications to lumbar spinal
motion in general. It seems reasonable to advise patients
with history of back disorders to avoid activity that will
put the lumbar spine in extreme range of motion such as
the tennis serve in the early morning. To gain more insight
into this issue, inclusion of the "time of occurrence" in
future epidemiological studies of acute low back injury is

The heavy involvement of lower trunk muscles in the ten-
nis serve reinforces the importance of abdominal and
lower back exercises in the strength and rehabilitation
programs designed for tennis players. Because most lower
trunk muscles undergo eccentric contractions during
selected phases of the serve, it is recommended that eccen-
tric training is included in the conditioning programs. The
strengthening of the lower trunk muscles not only will
enhance performance, but the tennis players will also ben-
efit in preventing low back injury and pain.

Recommendations for future studies
1. Future studies should examine if there are differences in
activation patterns of the lower trunk muscles during the
tennis serve among players of different skill levels includ-
ing beginners.

2. In addition to lower trunk muscles, other back muscles
such as multifidus and thoracic erector spinae muscles can
be examined.

3. The use of EMG techniques to identify the muscle acti-
vation characteristics that are associated with low back
pain [71] can be explored in tennis players.

4. Several epidemiological studies on low back disorders
among young competitive tennis players have been con-
ducted. However, such data are not available for non-
competitive players of different ages. Future effort should
examine the incidence of low back injuries in recreational


RA: rectus abdominis (LRA: left RA; RRA: right RA); EO:
external oblique (LEO: left EO; REO: right EO); IO: inter-
nal oblique (LIO: left IO; RIO: right IO); ES: erector spinae
(LES: left ES; RES: right ES).


AJ angle: anatomical joint angle; ANOVA: analysis of var-
iance; MANOVA: multi-variate analysis of variance; EMG:
electromyography or "electromyographic"; ROM: range of

Competing interests
The authors declare that they have no competing interests.

Authors' contributions
JWC contributed to conception and design, acquisition of
data, analysis and interpretation of data, and prepared the
manuscript with the assistance of the other authors. Both
SAP and MDT contributed to conception and design,
acquisition of data, and revised the manuscript critically
for important intellectual content.

Additional material

Additional file 1
Appendix. Computation of Anatomical Joint Angles.
Click here for file

This research project was funded in part by the United States Tennis Asso-
ciation (USTA). The authors wish to thank Kim Fournier, Guy Grover,
Greg Gutierrez, Chris Hasler, Ryan Mizell, Dana Otzel, joceyln Plesa, and
Dileep Ravi for their assistance in different aspects of this project.

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