Journal of Applied Biomechanics, 2011, 27, 336-344
© 2011 Human Kinetics, Inc.
Morphological and Mechanical Properties of Muscle and
Tendon in Highly Trained Sprinters
Keitaro Kubo,1 Toshihiro Ikebukuro,2 Hideaki Yata,3 Minoru Tomita,2 and Masaji Okada2
1University
of Tokyo; 2Kokushikan University, Tokyo; 3Wako University, Tokyo
The purpose of this study was to investigate muscle and tendon properties in highly trained sprinters and their
relations to running performance. Fifteen sprinters and 15 untrained subjects participated in this study. Muscle
thickness and tendon stiffness of knee extensors and plantar flexors were measured. Sprinter muscle thickness
was significantly greater than that of the untrained subjects for plantar flexors, but not for knee extensors (except
for the medial side). Sprinter tendon stiffness was significantly lower than that of the untrained subjects for
knee extensors, but not for plantar flexors. The best official record of a 100-m race was significantly correlated
to the muscle thickness of the medial side for knee extensors. In conclusion, the tendon structures of highly
trained sprinters are more compliant than those of untrained subjects for knee extensors, but not for plantar
flexors. Furthermore, a thicker medial side of knee extensors was associated with greater sprinting performance.
Keywords: knee extensor, plantar flexor, tendon elongation, muscle strength
Sprint running induces stretch–shortening cycles in
the muscle–tendon complex of the lower limbs. Recent
studies have demonstrated using ultrasonography that
the tendon properties are related to the performances
during stretch–shortening cycle exercises (Bojsen-Moller
et al., 2005; Kubo et al., 1999, 2000a, 2007a; Stafilidis
& Arampatzis, 2007). In particular, we reported that the
tendon properties of knee extensors were more compliant
in sprinters compared with untrained subjects (Kubo et
al., 2000a). Furthermore, Stafilidis and Arampatzis (2007)
also demonstrated that the maximal tendon elongation in
knee extensors was greater in excellent sprinters (best
official record of a 100-m race; 11.04 ± 0.17 s) than that
in inferior sprinters (best official record of a 100-m race;
11.64 ± 0.23 s). However, the average of best official
record of a 100-m race in these previous studies (Kubo et
al., 2000a; Stafilidis & Arampatzis, 2007) was about 11.0
s. Considering the finding of Stafilidis and Arampatzis
(2007), it is likely that the tendon properties of sprinters
are dependent on the level of athletic competition. No
studies have investigated the tendon properties of faster
sprinters compared with the above-quoted previous studies; that is, the average of best official record of a 100-m
race was faster than 11.0 s.
Keitaro Kubo (Corresponding Author) is with the Department
of Life Science, University of Tokyo, Meguro, Tokyo, Japan.
Toshihiro Ikebukuro, Minoru Tomita, and Masaji Okada are
with the Faculty of Physical Education, Kokushikan University,
Tokyo, Japan. Hideaki Yata is with the Sports Science Laboratory, Wako University, Machida, Tokyo, Japan.
336
It is well known that muscle function and size in
knee extensors and plantar flexors play important roles
during locomotion, that is, walking and running. Indeed,
some previous studies showed that the force-generating
capability of knee extensors played a major role in sprint
running (Alexander, 1989; Bret et al., 2002). Some
previous researchers reported the muscle size (mass,
cross-sectional area, thickness) of knee extensors and
plantar flexors in sprinters (e.g., Korhonen et al., 2009;
Kumagai et al., 2000). However, there were a few studies investigating the muscle size in sprinters compared
with untrained subjects (Abe et al., 2000; Maughan et
al., 1983). For example, Maughan et al. (1983) reported
that the muscle cross-sectional area of knee extensors was
greater in the sprinters than in the control subjects but this
difference was not significant. In these studies, however,
the muscle cross-sectional area and thickness were measured for limited slice of computerized tomography and
ultrasonography. This implied that the training-induced
changes in muscle size might be overlooked because
muscle hypertrophy did not occur equally throughout the
entire length of the muscle (Kubo et al., 2008; Narici et
al., 1996). In fact, our recent study showed that significant
increases of muscle thickness at proximal and medial
sites in knee extensors were observed after 6 months
of walking training, although the muscle thickness at
middle site did not change (Kubo et al., 2008). Therefore,
it is possible that the muscle thickness at these sites (at
proximal and medial sites in knee extensors) is greater in
sprinters than in untrained subjects, since both walking
and sprinting are generally classified as “locomotion.”
In the current study, we aimed to investigate the
morphological and mechanical properties of muscle and
Muscle and Tendon Properties in Sprinters 337
tendon in knee extensors and plantar flexors of highly
trained sprinters (the average of best official record of
a 100-m race was faster than 11.0 s) and their relations
to running performance. According to the above-quoted
previous studies (Kubo et al., 2000a; Kubo et al., 2008;
Stafilidis & Arampatzis, 2007), we hypothesized that the
tendon structures in knee extensors were more compliant
in highly trained sprinters than in inferior sprinters and
untrained subjects, and that peculiar differences in muscle
thickness among synergists of knee extensors and/or
plantar flexors were observed for highly trained sprinters.
Methods
Subjects
Fifteen well-trained male 100-m sprinters participated in
this study. All the subjects of sprinters had participated
in competitive meets at the regional or intercollegiate
level within the preceding year, and their best official
record of a 100-m race within 1 year of these tests ranged
from 10:58 s to 10:98 s (10.79 ± 0.13 s). The duration of
training experience in sprint running for sprinters ranged
from 4.2 to 9.8 years (6.2 ± 2.8 years). They performed
sprint training at least six times per week. In a preliminary
study, 15 untrained subjects whose age, body height,
and limb length were similar to those of sprinters were
selected as a control group. The physical characteristics
of the subjects are summarized in Table 1. All untrained
subjects were either sedentary or mildly active but none
had been involved in any type of regular exercise program for at least 1 year before the test. This study was
approved by the office of the Faculty of Physical Education, Kokushikan University, and was consistent with its
requirements for human experimentation. The subjects
were fully informed of the procedures to be used as well
as the purpose of this study. Written informed consent
was obtained from all the subjects.
Muscle Strength and Neural Activation
Maximal voluntary isometric contraction (MVC) was
measured by means of specially designed dynamometers
(Applied Office, Tokyo, Japan) for knee extension and
plantar flexion, respectively. All measurements were
performed on the right lower limb. During each task, subjects exerted isometric torque from zero (relax) to MVC
within 5 s. During the knee extension task, subjects sat
in an adjustable chair with support for the back and the
hip joint flexed at an angle of 80° (full extension = 0°)
to standardize the measurements and localize the action
to the appropriate muscle group. During torque measurements, the hips and back were held tightly in the seat
using adjustable lap belts. The axis of the knee joint was
aligned with the axis of the lever arm of the dynamometer.
The right ankle was firmly attached to the lever arm of
the dynamometer with a strap and fixed with a knee joint
flexed at an angle of 90° (full extension = 0°). During the
plantar flexion task, subjects lay prone on a test bench
and the waist and shoulders were secured by adjustable
lap belts and held in position. The ankle joint was set at
90° with the knee joint at full extension, and the foot was
securely strapped to a footplate connected to the lever arm
of the dynamometer. Before the test, subjects performed
a standardized warm-up and submaximal contractions to
become accustomed to the test procedure. Each task was
repeated two or three times per subject with at least 3 min
between trials. The highest value among these trials was
recorded as the muscle strength for each.
When the voluntary torque peaked, evoked twitch
contractions were imposed by supramaximal electrical
stimulations. The stimulating lead electrodes were placed
on the skin over the femoral nerve at the inguinal region
and the midbelly of the quadriceps femoris muscle for
knee extensors and on the skin of the right popliteal
fossa and oriented longitudinal to the estimated path of
the tibial nerve with the anode distal for plantar flexors,
respectively. A high-voltage stimulator (SEN-3301,
having a specially modified isolator SS-1963, NihonKoden, Japan) generated rectangular pulses (triple stimuli
with a 500-µs duration for one stimulus and an interstimulus interval of 10 ms). Maximal twitch contractions were
evoked in the resting muscle by progressively increasing
the stimulation intensity until increases failed to elevate
twitch torque further. The stimulus intensity that elicited
peak twitch torque was used throughout the duration of
the measurements. The difference between peak twitch
torque and MVC (twitch torque) was measured. Shortly
(within 1–2 s) after MVC, the same stimulation was
given to the muscle at rest (control twitch torque). The
measured values shown below are the means of two
trials. The neural activation level (%) of the muscles was
calculated as [1 – (twitch torque during MVC / control
twitch torque)] × 100, as previously reported (Becker &
Awiszus, 2001).
Elongation and Stiffness of Tendon
Structures
Elongations of the tendon structures in knee extensors and
plantar flexors were also assessed during isometric knee
extension and plantar flexion. An ultrasonic apparatus
Table 1 Mean (SD) age and physical characteristics of the subjects
Age (years)
Height (cm)
Body mass (kg)
Thigh length (cm)
Lower leg length (cm)
Sprinters
20.8 (1.0)
174.5 (4.3)
67.6 (5.3)
39.8 (1.2)
39.8 (1.5)
Untrained Subjects
20.7 (1.8)
173.1 (4.2)
70.2 (7.7)
39.7 (1.2)
39.8 (1.4)
338 Kubo et al.
(SSD-2000, Aloka, Tokyo, Japan) with an electronic
linear array probe (7.5-MHz wave frequency with 80-mm
scanning length; UST 5047–5, Aloka) was used to
obtain longitudinal ultrasonic images of vastus lateralis
and medial gastrocnemius muscles by the procedures
described previously (Kubo et al., 2005). The ultrasonic
images were recorded on videotape at 30 Hz, and synchronized with recordings of a clock timer for subsequent
analyses. The point at which one fascicle was attached to
the aponeurosis was visualized on the ultrasonic images.
This point moved proximally during isometric torque
development up to a maximum (see Figure 1 in Kubo et
al., 2005). The displacement of this point is considered to
indicate the lengthening of the deep aponeurosis and the
distal tendon. To correct the measurements taken for the
tendon and aponeurosis elongation, additional measurements were taken under passive conditions. To monitor
joint angular rotation an electrical goniometer (Penny &
Giles, Biomechanics Ltd., Gwent, UK) was placed on
the lateral aspect of each joint. The displacement of each
site caused by rotating the knee and ankle from 110° to
70° was digitized in sonographs taken. Thus, for each
subject the displacement of each site obtained from the
ultrasound images could be corrected for that attributed to
joint rotation alone. The elongation of tendon structures
was calculated as the differences of measured displacement of the analyzed point between active (isometric
contraction) and passive conditions. In the current study,
only values corrected for angular rotation are reported.
The measured torque (TQ) during isometric knee
extension and plantar flexion were converted to muscle
force (Fm) by the following equations (Kubo et al. 2007b,
2009):
where k is the relative contribution of the physiological
cross-sectional area in each of vastus lateralis muscle
within the knee extensors (22%; Narici et al., 1992) and
medial gastrocnemius muscle within the plantar flexors
(18%; Fukunaga et al., 1996), and MA is the moment arm
length in each of quadriceps femoris muscles at 90° (43
mm; Smidt, 1973) and triceps surae muscle at 90° (50
mm; Rugg et al., 1990). In the current study, the Fm and
the tendon elongation values above 50% of MVC were
fitted to a linear regression equation, the slope of which
was adopted as stiffness.
The repeatability of the tendon stiffness measurements was investigated on two separate days in our
previous studies with eight young males (Kubo et al.,
2001, 2002). The intraclass correlation coefficient for
the test–retest of the stiffness was 0.89 for knee extensors
and 0.89 for plantar flexors. The coefficient of variance
of the stiffness was 6.3% for knee extensors and 5.6%
for plantar flexors.
measured with an ultrasonic apparatus (SSD-900, Aloka,
Japan). The subjects remained in a supine position for the
measurements of knee extensors and a prone position for
the measurements of plantar flexors with legs straight and
the muscles relaxed. The anthropometric locations of the
measurement sites were first precisely determined and
marked by experienced technicians before the ultrasonic
measurement. A transducer with a 7.5-MHz scanning head
was coated with water-soluble transmission gel, which
provided acoustic contact without depressing the dermal
surface. One ultrasonic image of each site was obtained,
and the thickness of each muscle was analyzed in the
middle side of each image. The thickness of each site was
measured to the nearest 0.1 mm. The anatomical sites for
the measurements are noted below and presented in Figure
1. For knee extensors, the mean values of muscle thickness
at central (three sites), lateral (three sites), and medial
sides (two sites) were adopted as their representative of
each part. For plantar flexors, the mean values of muscle
thickness of medial gastrocnemius muscle (three sites),
lateral gastrocnemius muscle (three sites), and soleus
muscle (six sites) were adopted as their representative
of each part. The repeatability of the muscle thickness
measurements was investigated on seven separate days in
one subject. For knee extensors, the coefficient of variance
was 2.1% for central side, 2.6% for lateral side, and 2.4%
for medial side. For plantar flexors, the coefficient of variance was 1.9% for medial gastrocnemius muscle, 2.2%
for lateral gastrocnemius muscle, and 2.5% for soleus
muscle. Furthermore, in a preliminary study (n = 20),
we found that the mean thickness values of each muscle
in knee extensors and plantar flexors were significantly
correlated to the muscle volume measured by magnetic
resonance imaging (all p < .05): rectus femoris, r = .581;
vastus lateralis, r = .666; vastus intermedius, r = .455;
vastus medialis, r = .465; medial gastrocnemius, r = .762;
lateral gastrocnemius, r = .653; and soleus, r = .558.
Knee extensors were measured at eight sites: on the
anterior central (rectus femoris and vastus intermedius),
lateral (vastus lateralis and vastus intermedius), and
medial (vastus medialis and vastus intermedius) surfaces
30% (proximal; excluded medial site), 50% (middle), and
70% (distal) between the lateral condyle of the femur and
the greater trochanter.
Plantar flexors were measured at six sites: on the posterior medial (medial gastrocnemius, soleus, and flexor
digitorum) and lateral (lateral gastrocnemius, soleus, and
tibialis posterior) surfaces 20% (proximal), 30% (middle)
and 40% (distal) between the lateral malleolus of fibula
and the lateral condyle of the tibia.
In addition, the thicknesses of the patella and Achilles tendon were measured at 50% of patella tendon length
and at 30 mm proximal from the calcaneus, respectively.
During the measurement of tendon thickness, the knee
and ankle joint angles were at 90°.
Muscle and Tendon Thickness
Statistics
The muscle thickness for knee extensors (eight anatomical sites) and plantar flexors (six anatomical sites) were
Descriptive data are represented as the means ± SD. Oneway ANOVA was used for the comparison between the
Fm = k × TQ × MA–1
Muscle and Tendon Properties in Sprinters 339
Figure 1 — Thick bars represent the locations of sonographic scanning sites for knee extensors (eight sites) and plantar flexors
(six sites), respectively.
two groups. A linear regression analysis was performed
on the relationships among the measured variables of
muscles and tendons, and the best official record of a
100-m race for sprinters. The level of significance was
set at p < .05.
Results
Table 2 shows the measured variables of muscles for the
two groups. For knee extensors, the thickness of medial
side was significantly greater in sprinters than in untrained
subjects (p = .020), although there were no differences
in the muscle thickness of other parts for knee extensors.
For plantar flexors, the thickness values of all measured
muscles (medial gastrocnemius, lateral gastrocnemius,
and soleus) were significantly greater in sprinters than in
controls for plantar flexors (all p < .05). The MVC tended
to be higher in sprinters than in untrained subjects for
plantar flexors (p = .073), but not for knee extensors (p =
.686). For both knee extensors and plantar flexors, there
were no differences in neural activation levels between
sprinters and untrained subjects.
Table 3 shows the measured variables of tendons for
the two groups. For both tendons, there was no difference
in tendon thickness between sprinters and untrained subjects. The L values at force production levels beyond 400
N were significantly greater in sprinters than in untrained
subjects for knee extensors, but not for plantar flexors
(Figure 2). Similarly, the maximal elongation of tendon
structures of sprinters was significantly greater than that
of untrained subjects for knee extensors (p < .001), but
not for plantar flexors (p = .176). The stiffness of tendon
structures of sprinters was significantly lower than that
of untrained subjects for knee extensors (p = .030), but
not for plantar flexors (p = .437).
For untrained subjects, the stiffness of tendon structures was significantly correlated to the MVC in both
knee extensors (r = .613, p < .05) and plantar flexors (r
= .726, p < .01) (Figure 3). For sprinters, by contrast,
the stiffness of tendon structures was not correlated to
the MVC in both knee extensors (r = .195, p > .05) and
plantar flexors (r = .244, p > .05) (Figure 4).
For sprinters, the best official record of a 100-m race
was significantly correlated to the muscle thickness of
the medial side for knee extensors (r = –0.616, p < .05)
(Figure 5). However, no significant correlations were
found between the best official record of a 100-m race
and the other measured variables of muscles and tendons.
Discussion
According to the previous findings (Kubo et al., 2000a;
Stafilidis & Arampatzis, 2007), the tendon structures in
knee extensors were more compliant in excellent sprinters
(the best official record of a 100-m race was about 11.0
s) than that in inferior sprinters and untrained subjects.
The present result using highly trained sprinters (the
best official record in a 100-m race was from 10.58 to
Table 2 Mean (SD) measured variables of muscle for sprinters and untrained subjects
Sprinters
Untrained Subjects
231.6 (45.1)
223.8 (55.1)
Knee Extensors
MVC (N⋅m)
Muscle thickness at three central sites (mm)
44.4 (4.5)
42.0 (4.7)
Muscle thickness at three lateral sites (mm)
43.2 (3.3)
44.2 (5.1)
Muscle thickness at two medial sites (mm)
42.3 (4.5) *
38.6 (3.8)
Neural activation level (%)
92.7 (5.1)
92.2 (5.6)
MVC (N⋅m)
146.7 (18.3)
130.8 (25.8)
Muscle thickness of MG at three medial sites (mm)
25.5 (3.1) **
22.4 (2.6)
Plantar Flexors
Muscle thickness of LG at three lateral sites (mm)
21.6 (2.4) **
18.5 (3.6)
Muscle thickness of SOL at six sites (mm)
20.5 (2.6) *
18.1 (2.1)
92.1 (4.1)
90.3 (10.1)
Neural activation level (%)
08_Kubo_jab_2010_0122_fig3
*Significantly different from untrained subjects (*p < 0.05, **p < 0.01, ***p < 0.001).
Stiffness of tendon structures
in knee extensors (N · mm -1)
140
A
120
100
80
r = 0.613
(p<0.05)
60
40
100
150
200
250
300
350
MVC in knee extensors (Nm)
Stiffness of tendon structures
in plantar flexors (N · mm -1)
80
B
70
60
r = 0.726
(p<0.01)
50
40
30
20
10
50
75
100
125
150
175
MVC in plantar flexors (Nm)
Figure 2 — The muscle force (Fm): elongation of tendon
structures (L) in knee extensors (A) and plantar flexors (B) for
sprinter (closed) and untrained subject (open). *Significantly
different from untrained subjects at p < .05.
340
Figure 3 — The relationships between MVC and stiffness of
tendon structures in knee extensors (A) and plantar flexors (B)
for the untrained subject group.
Figure 4 — The relationships between MVC and stiffness of
tendon structures in knee extensors (A) and plantar flexors (B)
for the sprinter group.
Figure 5 — The relationships between the best official record
of a 100-m race and maximal elongation (A) and stiffness (B)
of tendon structures in knee extensors, muscle thickness at the
medial side of knee extensors (C) for the sprinter group
Table 3 Mean (SD) measured variables of tendon for sprinters and untrained subjects
Sprinters
Untrained Subjects
3.15 (0.47)
3.27 (0.50)
Knee Extensors
Tendon thickness (mm)
Maximal elongation of tendon structures (mm)
29.3 (4.7) ***
22.7 (3.8)
Stiffness of tendon structures (N⋅mm–1)
70.5 (11.7) **
90.0 (21.0)
4.28 (0.54)
4.34 (0.63)
18.1 (2.9)
16.0 (3.6)
44.0 (12.4)
39.7 (15.9)
Plantar Flexors
Tendon thickness (mm)
Maximal elongation of tendon structures (mm)
Stiffness of tendon structures
(N⋅mm–1)
*Significantly different from untrained subjects (**p < 0.01, ***p < 0.001).
341
342 Kubo et al.
10.98) agreed with these previous findings. Considering
these findings, it is possible that the more compliant
tendon structures in knee extensors are advantageous to
the sprinters in improving performance. Some previous
studies using ultrasonography showed that the rapid
shortening of tendon structures at the first stage of concentric phase during stretch-shortening cycle exercises
was assumed to play a role in lowering the velocity of
muscle fibers to near-isometric conditions (Kawakami et
al., 2002; Kubo et al., 2000b). Therefore, the more compliant tendon structures in sprinters would help to develop
the higher force at the stance phase in which the foot
contacts the ground. Furthermore, Kubo et al. (2000a)
and Stafilidis and Arampatzis (2007) found the significant
relations between the tendon properties in knee extensors
and the performance of 100m race. Unfortunately, we did
not find that the maximal tendon elongation or tendon
stiffness were significantly correlated to the best official
record of a 100-m race. The discrepancy between the
present and previous studies could be explained by the
range of the best official record of a 100-m race.
For plantar flexors, there were no differences in
the maximal elongation and stiffness of tendon structures between the two groups. This result agreed with
the previous findings (Kubo et al., 2000a; Stafilidis &
Arampatzis, 2007). However, Arampatzis et al. (2007)
reported that the elastic properties of tendon structures
in plantar flexors were stiffer in sprinters than in long
distance runners and untrained subjects. In the study of
Arampatzis et al. (2007), the MVC of plantar flexors was
significantly higher in sprinters than in endurance runners
and untrained subjects. In the current study, however,
there was no significant difference in the MVC of plantar
flexors between sprinter and untrained subject. As mentioned later, the MVC of plantar flexors was significantly
correlated to the stiffness of tendon structures. Considering these points, the discrepancy between the present
and previous studies could be explained by the difference
in MVC between the two groups. Similarly, we found
that the remarkable difference in the tendon properties
between the long-distance runners and untrained subjects
was found for knee extensors, but not for plantar flexors
(Kubo et al. 2010). Accordingly, it is likely that no difference in the tendon properties for plantar flexors can
be attributed to the lower plasticity compared with those
for knee extensors. Regardless, further investigations are
needed to clarify this point.
The tendon structures in lower limbs are frequently
subjected to considerable loads during running. For both
knee extensors and plantar flexors, therefore, the tendon
stiffness might be related to the muscle strength. According to the finding of Muraoka et al. (2005), the stiffness
of human Achilles tendon was correlated to the maximal
muscle force of plantar flexors. In untrained subject of
the current study, there were significant correlations
between MVC and stiffness of tendon structures for both
knee extensors and plantar flexors (Figure 3). For both
knee extensors and plantar flexors of sprinter, however,
the stiffness of tendon structures was not related to MVC
(Figure 4). Recent studies indicated that the influences
of training on tendon stiffness differ among the exercise
protocols used. In particular, we reported that the tendon
stiffness did not change after ballistic and plyometric
training regimens, although the tendon stiffness increased
markedly after isometric training of a longer duration
(Kubo et al., 2001, 2007b). Taking the present result into
account, we may say that the sprint training of sprinter
had a great impact on the muscle properties in lower limbs
but not on their tendon properties. To clarify this point,
we need to investigate the genetic effects on the tendon
properties for sprinters in the future study.
Another interesting finding of this study was that
the thickness of the medial side of knee extensors was
greater in sprinters than in untrained subjects, and thus
that was significantly correlated to the best official record
of a 100-m race. Some previous researchers demonstrated
that the hamstring and psoas major muscles played an
important role during sprint running (Hoshikawa et al.,
2006; Mero et al., 1992). However, there were a few studies investigating the muscle size in sprinters compared
with untrained subjects (Abe et al., 2000; Maughan et
al., 1983). In addition, the muscle size of lower limbs in
sprinters might not be evaluated correctly because the
muscle size was measured for only one slice of magnetic
resonance imaging and ultrasonic image. Maughan et al.
(1983) reported that the muscle cross-sectional area of
knee extensors was greater in the sprinters than in the
control subjects but this difference was not significant.
In the current study, the difference in the muscle thickness of only the medial side (mainly, the vastus medialis
muscle) of knee extensors was found between sprinter
and untrained subject. Toumi et al. (2007) reported that
the activation of vastus medialis muscle was higher during
landing of “single-leg” squat jump, whereas the vastus
medialis and vastus lateralis muscles were activated in
a coordinated manner during a squat jump using “both
legs.” Therefore, it seems reasonable to support the idea
that the vastus medialis muscle among the knee extensor
muscles is subjected to considerable loads during sprint
running, that is, the repeated landing using a single leg.
To calculate the muscle force, we used the ratio of
vastus lateralis muscle to the knee extensors and medial
gastrocnemius muscle to the plantar flexors in terms of
physiological cross-sectional area from the literature
(Fukunaga et al., 1996; Narici et al., 1992) as the contribution of the muscle for force development. For plantar
flexors, there were no significant differences in the ratios
of muscle thickness of “lateralis gastrocnemius / medialis
gastrocnemius” and “soleus / medialis gastrocnemius”
between the two groups (data not shown), although
the thicknesses of each muscle (medial gastrocnemius,
lateral gastrocnemius, and soleus) were significantly
greater in sprinters than in untrained subjects (Table 2).
Therefore, the relative contribution of medial gastrocnemius muscle for force production of plantar flexors was
similar between the two groups. On the other hand, the
muscle thickness of the medial side of knee extensors
was significantly greater in sprinters than in untrained
Muscle and Tendon Properties in Sprinters 343
subjects, although there were no differences in the muscle
thickness of other parts for knee extensors (Table 2).
Considering this point, it is likely that the actual tendon
elongation of vastus lateralis muscle for sprinter would
be underestimated, if the contribution of vastus lateralis
muscle for force production of knee extensors might
be lower in sprinters than in untrained subjects. Hence,
we considered that these points did not affect the main
result, which was that the tendon structures of sprinters
were more compliant than those of untrained subjects
for knee extensors.
Furthermore, we calculated the muscle force using
the moment arm length from the previous studies (Rugg
et al., 1990; Smidt, 1973). Unfortunately, we have no
definite information on the difference in moment arm
length between sprinters and untrained subjects. As far
as we know, there were few reports concerning the differences in moment arm length between the two groups
(Arampatzis et al., 2007; Lee & Piazza, 2009). Lee &
Piazza (2009) reported that the Achilles tendon moment
arm length of sprinters was 25% smaller than that of nonsprinters. However, Arampatzis et al. (2007) showed that
there was no difference in the ratio of tendon elongation
to joint rotation during passive rotation (corresponding to
the moment arm length) among the sprinters, endurance
runners, and untrained subjects. Furthermore, some previous studies indicated that the moment arm length changed
with changes in joint angle and exerted force level (e.g.,
Maganaris et al., 1998). Therefore, it would be difficult
to measure the accurate moment arm length at present.
In conclusion, the tendon structures of highly trained
sprinters are more compliant than those of untrained subjects for knee extensors, but not for plantar flexors. Furthermore, the thickness of medial side of knee extensors
(mainly, the vastus medialis) was significantly correlated
to the best official record of a 100-m race.
Acknowledgments
This study was supported by the Grant-in-Aid for Exploratory
Research (19650168 to K. Kubo) from Japan Society for Promotion of Science.
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