Scand J Med Sci Sports 2014: 24: 950–957
doi: 10.1111/sms.12111
© 2013 John Wiley & Sons A/S.
Published by John Wiley & Sons Ltd
Effect of a 5-week static stretching program on hardness of the
gastrocnemius muscle
R. Akagi1,2, H. Takahashi2
College of Systems Engineering and Science, Shibaura Institute of Technology, Saitama, Japan, 2Department of Sports Sciences,
Japan Institute of Sports Sciences, Tokyo, Japan
Corresponding author: Ryota Akagi, College of Systems Engineering and Science, Shibaura Institute of Technology, 307 Fukasaku,
Minuma-ku, Saitama-shi, Saitama 337-8570, Japan. Tel: +81 48 720 6020, Fax: +81 48 720 6011, E-mail:
rakagi12@sic.shibaura-it.ac.jp
1
Accepted for publication 16 July 2013
This study investigated the effects of a static stretching
(SS) program on muscle hardnesses of the gastrocnemius
medialis (MG) and gastrocnemius lateralis (LG). Nineteen young men participated in this study. Either the
right or left leg was randomly selected to conduct three
bouts of 2-min SS of the plantar flexors 6 days a week for
5 weeks in each subject (the SS group), and the other leg
was assigned to a control group. Before (pretest) and
after (posttest) conducting the SS program, MG and LG
hardnesses were measured using shear wave ultrasound
elastography. The SS program was found to decrease
muscle hardnesses, but not to change the ratio of MG
hardness to LG hardness. There were no significant differences between the relative changes in the MG and LG
hardnesses from pretest to posttest in both the SS and
control groups. Significant correlations between the
muscle hardness ratios at pretest and posttest were
found in both groups. The results of this study suggest
that the current SS program is useful for improving
muscle condition in the plantar flexors, and that its longterm effects on the MG and LG hardnesses are of the
same degree.
Muscle hardness is a mechanical property that represents
transverse muscle stiffness (Murayama et al., 2000) and
is distinguished from musculo-tendinous unit (MTU)
stiffness along the longitudinal axis of a muscle
(Gennisson et al., 2005). Muscle hardness can indicate
muscle condition objectively (Morisada et al., 2006;
Yanagisawa et al., 2011) because muscles become harder
under a variety of conditions including those involving
cramps, spasms, and damage (Fischer, 1987; Murayama
et al., 2000). Hence, it is important to decrease muscle
hardness in order to improve muscle condition and/or
maintain good muscle condition.
Static stretching (SS) has been reported to be useful
for decreasing muscle hardness (Akagi & Takahashi,
2013). In our previous study (Akagi & Takahashi, 2013),
three bouts of 2-min SS of the plantar flexors (the gastrocnemius muscle and the soleus muscle), where
cramping commonly occurs (Ross & Thomas, 1995),
decreased the hardness of the gastrocnemius muscle and
MTU stiffness of the plantar flexors, and increased the
joint range of motion (ROM) of dorsiflexion. When the
SS duration is comparatively long, for example, 5–6 min
(Morse et al., 2008; Nakamura et al., 2011; Akagi &
Takahashi, 2013), the ROM increases due to decreased
MTU stiffness. This decrease in MTU stiffness is
affected by a decrease in muscle stiffness (Morse et al.,
2008; Kay & Blazevich, 2009; Nakamura et al., 2011). It
has been suggested that the intramuscular connective
tissue consists of parallel elastic components (i.e., the
endomysium, perimysium, and epimysium) causing
passive tension (Gajdosik, 2001). Furthermore, Purslow
(1989) reported that the connective tissue, particularly
the perimysium, is a major extracellular contributor to
passive stiffness. Thus, the SS-induced decrease in
muscle stiffness is considered to be influenced by
changes in properties of the intramuscular connective
tissue causing passive tension (Kubo et al., 2002;
Nakamura et al., 2011, 2012), likely resulting in the
SS-induced decrease in muscle hardness. It is natural to
think that these temporary phenomena are also observed
after routine SS. From the viewpoints of improving
muscle condition, maintaining good muscle condition,
and/or preventing muscle injury, clarification of the
effect of a routine SS program is more important than of
the acute effect of SS. Furthermore, the previous study
(Akagi & Takahashi, 2013) found that three bouts of
2-min SS of the plantar flexors did not change the joint
torque of the plantar flexors and affected muscle
hardnesses of the gastrocnemius medialis (MG) and gastrocnemius lateralis (LG) to the same degree despite the
existence of the original difference between them.
Routine SS is also expected to show these effects.
This study quantified the MG and LG hardnesses
before and after three bouts of 2-min SS of the plantar
950
Static stretching program and muscle hardness
flexors 6 days a week for 5 weeks using shear wave
ultrasound elastography (Nordez & Hug, 2010;
Shinohara et al., 2010; Akagi & Takahashi, 2013) in
addition to ROM of dorsiflexion and MTU stiffness and
joint torque of the plantar flexors. As mentioned above,
we hypothesized that the aforementioned acute effects of
three bouts of 2-min SS of the plantar flexors on each
parameter would remain after the 5-week SS program.
The purpose of this study was to determine the long-term
effect of SS of the plantar flexors on hardness of the
gastrocnemius muscle by examining both MG and LG.
During the measurements, each subject was instructed to lie in
a prone position on a reclining seat attached to a dynamometer
(Biodex System 4, Biodex Medical Systems Inc., Shirley, New
York, USA). The ankle was secured to a footplate attached to the
dynamometer by an inelastic belt, with the hip and knee joints
fully extended, so the ankle joint was aligned with the axis of the
dynamometer. The setup of ankle joint angle during each measurement is described below.
A set of measurements was completed within 20 min. The temperature of the experimental room was kept constant at around
25 °C throughout the measurements. The measurements of each
parameter except the lower leg circumference and the muscle
thickness of the posterior lower leg were carried out in a similar
manner to those of our previous study (Akagi & Takahashi, 2013).
Materials and methods
Subjects
Limb length and circumference, and muscle thickness
Nineteen young men [age, 23.7 ± 2.3 years; body height,
173.7 ± 4.7 cm; body mass, 72.4 ± 14.4 kg; mean ± standard
deviation (SD)] with no orthopedic abnormalities in their lower
legs participated in the present study. Six subjects were sedentary,
and the others reported engaging in 1–8 h/week of recreational
sports. None of the subjects were competitive athletes or were
engaged in systematic resistance training and stretching programs.
Either the right or left leg was randomly selected to perform SS in
each subject (the SS group; n = 19) and the other leg was assigned
to a control group (n = 19) in order to minimize between-group
variability due to personal factors such as exercise and activity
patterns (Folpp et al., 2006; Ben & Harvey, 2010). The study
protocol was approved by the Ethics Committee of the Japan
Institute of Sports Sciences and all experimental procedures were
performed in accordance with the Declaration of Helsinki. Written
informed consent was obtained from all subjects.
Lower leg circumference and muscle thickness of the posterior
lower leg were determined in the proximal 30% of the lower leg
length where the maximal cross-sectional area in the lower leg is
observed (Kanehisa et al., 1994). Subjects stood unsupported
during the measurements of the lower leg length to the nearest
0.5 cm with a steel tape, the lower leg circumference to the nearest
0.1 cm with a cloth tape, and the muscle thickness of the posterior
lower leg to the nearest 0.01 cm with a B-mode ultrasonic
apparatus (Aixplorer, SuperSonic Imagine, Aix-en-Provence,
France). In the muscle thickness measurement, a convex probe
(SuperCurved 6-1, SuperSonic Imagine) was prepared with watersoluble transmission gel and applied on the posterior skin surface.
The muscle thickness was determined as the distance from the
adipose tissue-muscle interface to the muscle-bone interface. Each
measurement was performed once.
Passive ROM
Experimental protocol
At the pretest, (1) lower leg length (the distance from the popliteal
crease to the lateral malleolus), (2) lower leg circumference,
(3) muscle thickness of the posterior lower leg, (4) passive ROM
of dorsiflexion, (5) hardnesses of MG and LG, (6) MTU stiffness
of the plantar flexors, and (7) joint torque developed during isometric maximal voluntary plantar flexion were measured in that
order. The testing order of right or left leg was randomized for
each subject. Lower leg circumference and muscle thickness of the
posterior lower leg were determined in order to confirm that the SS
program does not change the muscle size of the plantar flexors.
Afterwards, each subject was instructed to perform SS 6 days a
week for 5 weeks using a stretching board (H-7295, Toei Light
Co., Ltd, Soka-shi, Saitama, Japan). The method for performing
SS is described in our previous study (Akagi & Takahashi, 2013).
Subjects were instructed to stand erect with one foot on the stretching board during SS, with arms supported on the wall anterior to
the body and the other foot on the edge of the stretching board in
order not to lose their balance. Two minutes of SS was performed
three times with a 1-min interval between sets. During this interval, subjects got off the stretching board, sat on a chair, and
relaxed. For each subject, the ankle joint angle during SS was 3°
lower than the ROM. For subjects with ROM of more than 35°,
however, the ankle joint angle during SS was set as 32° which was
the maximum angle of the stretching board. It was confirmed that
the subjects could perform the SS without suffering discomfort or
pain.
In accordance with previous studies (Youdas et al., 2003; Folpp
et al., 2006; Kokkonen et al., 2007; Nakamura et al., 2012), the
posttest was conducted 1–3 days after the last SS session to
exclude influence of the intervention. The same parameters as for
the pretest were measured at the posttest.
It has been indicated that active ROM may be decreased because
of pain or weakness, and thus, passive ROM better estimates
actual joint motion (Soucie et al., 2011). Hence, passive ROM was
measured in this study. In order to determine the passive ROM of
dorsiflexion, the footplate of the dynamometer was moved manually by an examiner, starting at 0° and increasing to the dorsiflexion angle at which subjects felt discomfort or pain. This
dorsiflexion angle was measured three times, and the integral
mean value of the three measurements was defined as passive
ROM. The coefficient of variation (CV) for the three values was
3.1 ± 1.7% with an intraclass correlation coefficient type 1,3
[ICC(1,3)] of 0.984 (P < 0.001).
Muscle hardness
Hardnesses of MG and LG were measured at the proximal 30% of
the lower leg length, and at 30° of plantar flexion using shear wave
ultrasound elastography images obtained by an ultrasonic apparatus (Aixplorer, SuperSonic Imagine). At this angle, passive torque
around the ankle has been shown to be near zero (Kawakami et al.,
1998; Rienera & Edrichb, 1999), and thus, the muscle hardnesses
were expected to reflect the hardnesses of MG and LG themselves.
For each of these muscles, a linear probe (SuperLinear 15-4,
SuperSonic Imagine) was transversely placed on the muscle as the
highest muscle thickness in the mediolateral direction was
observed near the transverse center of the image (Fig. 1). Watersoluble transmission gel was applied to the contact surface.
As shown in Fig. 1, shear wave ultrasound elastography generated color-coded images with a scale from blue (soft) to red (hard)
depending on the magnitude of Young’s modulus. Of the stored
images in the ultrasonic apparatus at 11 Hz, a single image, on
which a stable color distribution was observed for a certain time,
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Joint torque
Joint torque of the plantar flexors was measured using the dynamometer as an index of muscle strength. The subjects performed
maximal voluntary contraction of isometric plantar flexion at 0° of
ankle joint angle with the hip and knee joints fully extended for
3 s. Joint torque was expected to be highest near the set angle
(Fukunaga et al., 1996). The joint torque data were digitally
recorded at a 100 Hz sampling frequency. The joint torque measurements were performed twice with at least a 2-min interval. If
the difference between two values of joint torque was more than
10% of the higher value, joint torque was measured once more. Of
the two or three joint torque measurements, the highest values
were adopted.
Day-to-day reproducibility of the measurements
Day-to-day reproducibility of the measurements of ROM of dorsiflexion, the MG and LG hardnesses, and MTU stiffness and joint
torque of the plantar flexors was verified in our previous study
(Akagi & Takahashi, 2013). As a result of performing the same
procedures on another day for two subjects, the CVs of the two
measured values were 2.5 ± 2.6% in ROM, 2.2 ± 1.6% in the MG
hardness, 3.5 ± 2.9% in the LG hardness, 8.4 ± 3.7% in MTU
stiffness, and 8.9 ± 3.7% in joint torque.
Statistical analyses
Fig. 1. Typical images of shear wave ultrasound elastography
for the gastrocnemius medialis (a) and lateralis (b).
was selected to determine the muscle hardness. In the image, a
10-mm-square region of interest (ROI) was set near the center part
where the muscle was thickest. In addition, a 5-mm-diameter
circle was set near the center of the ROI. In doing so, the Young’s
modulus within the circle was automatically calculated and was
taken as the muscle hardness.
The hardness measurements for MG and LG were performed
five times each, in random order. Of the five measured values, the
group of three measurements showing the lowest CV among the
10 possible groups was adopted and their mean value was used for
further analysis. The CVs for the three values adopted were
3.9 ± 2.2% in MG and 3.7 ± 2.1% in LG with ICCs(1,3) of 0.988
in MG and 0.982 in LG, respectively (P < 0.001).
MTU stiffness
Passive plantar flexion torque was measured using the dynamometer while the footplate of the dynamometer was moved at a
constant velocity of 5°/s by motor control from 30° of plantar
flexion to 25° of dorsiflexion, which was achieved by all subjects
without pain. The slope of the portion of the passive torque-angle
curve from 15° to 25° was defined as MTU stiffness (Kubo et al.,
2002; Nakamura et al., 2011). The MTU stiffness measurement
was performed twice, and the mean value of the two measurements
was adopted. The CV for the two values was 1.4 ± 2.1% with an
intraclass correlation coefficient type 1,2 [ICC(1,2)] of 0.998
(P < 0.001).
952
Significance of difference at the pretest between the SS and control
groups was assessed using the Student’s paired t-test for each
variable other than muscle hardnesses, and using two-way analysis
of variance [ANOVA; experimental group (SS group and control
group) × muscle group (MG and LG)] with repeated measures for
muscle hardnesses. After significant differences in all of the
parameters between the SS and control groups were not found at
pretest, the following analyses were performed.
Two-way ANOVA [test time (pretest and posttest) × experimental group (SS group and control group)] with repeated measures followed by the Bonferroni multiple comparison test was
used to investigate effects of the SS program on each variable
except for muscle hardnesses. Considering the experimental
design in this study, the presence or absence of effects of the SS
program was judged by the presence or absence of a significant
interaction between test time and experimental group. Regarding
examination of an effect of the SS program on the MG and LG
hardnesses, three-way ANOVA [test time (pretest and posttest) ×
experimental group (SS group and control group) × muscle group
(MG and LG)] with repeated measures followed by the Bonferroni
multiple comparison test was used. When a significant interaction
between test time and experimental group was found but those
among three factors, between test time and muscle group, and
between experimental group and muscle group were not found, the
effects of the SS program on muscle hardnesses were judged to be
present. In addition, the presence or absence of a significant main
effect of muscle group was investigated. When the main effect was
significant, relative changes in the MG and LG hardnesses from
pretest to posttest and the ratios of the MG hardness to the LG
hardness at pretest and posttest were calculated. To examine differences between these parameters in the SS and control groups,
two-way ANOVA [experimental group (SS group and control
group) × muscle group (MG and LG) for relative changes in the
MG and LG hardnesses; test time (pretest and posttest) × experimental group (SS group and control group) for ratios of the MG
hardness to the LG hardness] with repeated measures was used. In
both groups, Pearson’s product-moment correlation coefficients
between the muscle hardness ratios at pretest and posttest were
also calculated.
Static stretching program and muscle hardness
Table 1. Descriptive data on variables in the control and static stretching (SS) groups at the pretest (n = 19)
Lower leg length (cm)
Lower leg circumference (cm)
Muscle thickness (cm)
ROM (°)
Muscle hardness (kPa)
MG
LG
MTU stiffness (Nm/°)
Joint torque (Nm)
Control group
SS group
P-value
Effect size
39.8 ± 1.7
38.0 ± 3.4
7.6 ± 0.7
35 ± 4
39.7 ± 2.0
37.9 ± 3.5
7.6 ± 0.8
35 ± 5
Difference between experimental groups: 0.841
Difference between experimental groups: 0.755
Difference between experimental groups: 0.409
Difference between experimental groups: 0.876
r = 0.048
r = 0.075
r = 0.196
r = 0.038
27.0 ± 5.9
32.0 ± 6.3
1.14 ± 0.45
116 ± 21
27.3 ± 7.3
32.4 ± 5.8
1.20 ± 0.49
118 ± 25
Interaction (experimental group × muscle group): 0.454
Main effect (experimental group): 0.370
Difference between experimental groups: 0.331
Difference between experimental groups: 0.643
ηp2 = 0.031
ηp2 = 0.045
r = 0.230
r = 0.111
Values are mean ± standard deviation.
ROM, range of motion; MG, gastrocnemius medialis; LG, gastrocnemius lateralis; MTU, musculo-tendinous unit.
Descriptive data are presented as mean ± SD. Statistical significance was set at P < 0.05. When the results of the Student’s paired
t-test, two-way ANOVA, and three-way ANOVA are presented, r
or ηp2 are shown as indices of effect size with the P-value.
Results
Differences in each parameter between the SS and
control groups at pretest
Table 1 shows the descriptive data on each parameter in
the SS and control groups. There were no significant
differences in the parameters except muscle hardnesses
between the SS and control groups at pretest. For muscle
hardnesses, an interaction between test time and experimental group and main effects of test time and experimental group were not significant.
Changes in each parameter with the SS program
There were no significant interactions between test time
and experimental group and no significant main effects of
test time and experimental group for lower leg length,
lower leg circumference, muscle thickness of the posterior
lower leg, and joint torque of the plantar flexors (Table 2).
A significant interaction between test time and experimental group was found in passive ROM of dorsiflexion and
MTU stiffness of the plantar flexors (Table 2). A significant difference in passive ROM of dorsiflexion between
pretest and posttest was found in the SS group, but was not
found in the control group (Table 2). In both the SS and
control groups, MTU stiffness of the plantar flexors at
pretest was not significantly different from that at posttest
(Table 2). For muscle hardnesses, three-way ANOVA
revealed a significant interaction between test time and
experimental group, but the other interactions were not
significant (Table 2). There was a significant difference in
muscle hardnesses between pretest and posttest in the SS
group, whereas a corresponding difference in the control
group was not found (Table 2). Moreover, a significant
main effect of muscle group was found (Table 2).
Relative changes in hardnesses of MG and LG with
the SS program
Relative changes in the MG and LG hardnesses from
pretest to posttest were −9.5 ± 10.6% and −9.4 ± 7.7% in
the SS group, and 3.8 ± 9.8% and 3.6 ± 9.6% in the
control group, respectively. A significant interaction
between experimental group and muscle group was not
found (P = 0.959, ηp2 < 0.001). A significant main effect
of experimental group on relative changes in the MG and
LG hardnesses was found (P < 0.001, ηp2 = 0.642) but
there was no significant main effect of muscle group on
them (P = 0.974, ηp2 < 0.001).
Ratios of the MG hardness to the LG hardness at
pretest and posttest
Ratios of the MG hardness to the LG hardness at pretest
and posttest were 0.812 ± 0.122 and 0.812 ± 0.129 in the
SS group, and 0.855 ± 0.170 and 0.853 ± 0.141 in the
control group. There were no significant main effects of
test time (P = 0.906, ηp2 = 0.001) and experimental
group (P = 0.296, ηp2 = 0.060) on the muscle hardness
ratios without a significant interaction between test time
and experimental group (P = 0.971, ηp2 < 0.001). Correlation coefficients between the muscle hardness ratios at
pretest and posttest in the SS and control groups were
both significant (Fig. 2).
Discussion
There were significant interactions between test time and
experimental group for passive ROM of dorsiflexion and
MTU stiffness of the plantar flexors, indicating that three
bouts of 2-min SS of the plantar flexors 6 days a week
for 5 weeks induced a significant increase in passive
ROM and a decrease in MTU stiffness (Table 2). These
results are in line with previous findings (Guissard &
Duchateau, 2004; Marshall et al., 2011; Nakamura et al.,
2012), but not consistent with several other studies
(Folpp et al., 2006; Gajdosik et al., 2007; Ben & Harvey,
2010) reporting that the increased ROM after long-term
SS programs was dependent not on decreased MTU stiffness but on the increased stretch tolerance because the
MTU stiffness did not change with the SS programs.
There is a discrepancy between these studies in longterm SS programs (e.g., SS duration per session, frequency per week, and period of the program) and in the
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Akagi & Takahashi
Table 2. Descriptive data on variables in the control (CON) and static stretching (SS) groups at pretest and posttest (n = 19)
Lower leg length (cm)
Lower leg circumference (cm)
Muscle thickness (cm)
ROM (°)
Muscle hardness (kPa)
MG
LG
MTU stiffness (Nm/°)
Joint torque (Nm)
CON
SS
CON
SS
CON
SS
CON
SS
CON
SS
CON
SS
CON
SS
CON
SS
Pretest
Posttest
P-value
39.8 ± 1.7
39.7 ± 2.0
39.8 ± 1.8
39.8 ± 1.9
Interaction
Test time × experimental group: 1.000
Main effect
Test time: 0.083
Experimental group: 0.816
Interaction
Test time × experimental group: 0.193
Main effect
Test time: 0.861
Experimental group: 0.944
Interaction
Test time × experimental group: 0.658
Main effect
Test time: 0.443
Experimental group: 0.616
Interaction
Test time × experimental group: <0.001
Multiple comparison
Pre-test vs. post-test in CON: 0.573
Pre-test vs. post-test in SS: <0.001
38.0 ± 3.4
37.9 ± 3.5
7.6 ± 0.7
7.6 ± 0.8
35 ± 4
35 ± 5
27.0 ± 5.9
27.3 ± 7.3
32.0 ± 6.3
32.4 ± 5.8
1.14 ± 0.45
1.20 ± 0.49
116 ± 21
118 ± 25
37.9 ± 3.4
38.0 ± 3.5
7.6 ± 0.7
7.6 ± 0.7
35 ± 5
41 ± 6
27.8 ± 5.4
24.4 ± 5.4
32.9 ± 6.2
30.0 ± 4.4
1.16 ± 0.45
1.12 ± 0.50
119 ± 17
121 ± 20
Interaction
Test time × experimental group × muscle group: 0.520
Test time × experimental group: <0.001
Test time × muscle group: 0.768
Experimental group × muscle group: 0.540
Main effect
Muscle group: <0.001
Multiple comparison
Pre-test vs. post-test in CON: 0.125
Pre-test vs. post-test in SS: <0.001
Interaction
Test time × experimental group: 0.031
Multiple comparison
Pre-test vs. post-test in CON: 0.797
Pre-test vs. post-test in SS: 0.142
Interaction
Test time × experimental group: 0.955
Main effect
Test time: 0.255
Experimental group: 0.572
Effect size
ηp2 < 0.001
ηp2 = 0.158
ηp2 = 0.003
ηp2 = 0.092
ηp2 = 0.002
ηp2 < 0.001
ηp2 = 0.011
ηp2 = 0.033
ηp2 = 0.014
ηp2 = 0.723
ηp2 = 0.018
ηp2 = 0.784
ηp2 = 0.023
ηp2 = 0.588
ηp2 = 0.005
ηp2 = 0.021
ηp2 = 0.706
ηp2 = 0.126
ηp2 = 0.505
ηp2 = 0.723
ηp2 = 0.018
ηp2 = 0.784
ηp2 < 0.001
ηp2 = 0.071
ηp2 = 0.018
Values are mean ± standard deviation.
ROM, range of motion; MG, gastrocnemius medialis; LG, gastrocnemius lateralis; MTU, musculo-tendinous unit.
examined muscles, and therefore, it is difficult to achieve
consensus on whether MTU stiffness changes with longterm SS programs or not. Given that a decrease in MTU
stiffness was found after the SS program in the current
study and that the one session of the same SS program
resulted in decreased MTU stiffness in our previous
study (Akagi & Takahashi, 2013), however, the
decreased MTU stiffness is likely to have contributed to
the increased ROM with the SS program in the current
study. In some studies (Kubo et al., 2002; Nakamura
et al., 2011), a decrease in MTU stiffness induced by
20-day or 4-week SS programs was influenced by a
decrease in muscle stiffness but not by a decrease in
tendon stiffness. Gajdosik (2001) has suggested that the
cytoskeleton of the sarcomere and the intramuscular
connective tissue consist of parallel elastic components
(i.e., the endomysium, perimysium, and epimysium)
causing passive tension. Furthermore, Purslow (1989)
954
has reported that the connective tissue, particularly the
perimysium, is a major extracellular contributor to
passive stiffness. Thus, a decrease in muscle stiffness is
considered to be affected by changes in properties of the
intramuscular connective tissue causing passive tension
(Kubo et al., 2002; Nakamura et al., 2011, 2012). These
phenomena can operate to decrease in muscle hardness.
For muscle hardnesses, only an interaction between
test time and experimental group was significant
(Table 2). That is, the MG and LG hardnesses decreased
significantly with the SS program as expected. Taking
this finding together with the previous one on an acute
effect of the same SS on the MG and LG hardnesses
(Akagi & Takahashi, 2013), it is suggested that the
current SS is effective in decreasing muscle hardness,
e.g., improving muscle condition.
There was no significant interaction between test time
and experimental group for joint torque of the plantar
Static stretching program and muscle hardness
Fig. 2. Relationships between the muscle hardness ratios (gastrocnemius medialis/lateralis) at pretest and posttest in the static
stretching (a) and control (b) groups.
flexors (Table 2). This shows that the SS program did not
change the joint torque of the plantar flexors, which is
consistent with previous in vivo studies (Kubo et al.,
2002; Guissard & Duchateau, 2004; Marshall et al.,
2011). Using immobilized mouse or rat soleus muscles,
short-duration stretching for several weeks was found to
prevent much of the muscle atrophy after immobilization
(Williams, 1990; Okita et al., 2001). Additionally,
Stauber et al. (1994) found that muscle hypertrophy was
induced by 4 weeks of stretching in rat soleus muscles.
Considering that muscle size is a major determinant of
muscle strength (Fukunaga et al., 2001; Akagi et al.,
2009), these observations suggest the possibility that a
routine SS program may result in an increase in muscle
strength in vivo. In the current study, however, interactions between test time and experimental group for lower
leg length, lower leg circumference, and muscle thickness of the posterior lower leg were not significant
(Table 2), indicating that muscle size of the plantar
flexors was not changed by the SS program. This should
contribute to the present result of joint torque of the
plantar flexors. A single session of the same SS program
also did not change joint torque (Akagi & Takahashi,
2013). Hence, it is suggested that three bouts of 2-min
SS and its routine execution do not influence muscle
strength in the plantar flexors.
A significant main effect of muscle group on muscle
hardness was found (Table 2), indicating that the MG
hardness was significantly lower than the LG hardness
regardless of test time and experimental group. Nevertheless, for relative changes in muscle hardnesses, no significant interaction between experimental group and muscle
group or no main effect of muscle group was seen. Moreover, there were no significant main effects of test time
and experimental group on ratios of the MG hardness to
the LG hardness without a significant interaction between
test time and experimental group, and significant correlations between the muscle hardness ratios at pretest
and posttest were found in the SS group similarly to
the control group (Fig. 2). These results suggest that the
original difference in the muscle hardness between MG
and LG of the plantar flexors does not change greatly with
the SS program, as was also seen for a single session of the
same SS program (Akagi & Takahashi, 2013). That is, the
MG and LG hardnesses of the plantar flexors should be
affected to the same degree by routine SS.
There are some limitations of this study. The first
limitation is that the ankle joint angle during SS for
subjects with ROM more than 35° was uniformly set as
32°. Therefore, although the acute effects of SS on each
variable for the subjects with ROM more than 35° was
not significantly different from those of the other subjects (Akagi & Takahashi, 2013), the long-term effects
might show different trends. To examine this possibility,
changes in passive ROM and MTU stiffness of the
plantar flexors and relative changes in the MG and LG
muscle hardnesses from pretest to posttest were calculated in the SS group, and any differences between those
with ROM more than 35° (n = 10) and the others (n = 9)
were tested by an unpaired t-test. As a result, there were
no significant differences in the variables between them.
Thus, the restriction of the set ankle angle during SS
should have only a small impact on the present results.
The second limitation is that a within-subjects design
was used in this study. The advantage of the design is to
minimize between-group variability due to personal
factors such as exercise and activity patterns (Folpp
et al., 2006; Ben & Harvey, 2010). The gastrocnemius
muscle is used in many daily activities such as walking
and postural stability (Ishikawa et al., 2005; Ushiyama
& Masani, 2011). If the use frequency of the muscle in
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Akagi & Takahashi
daily activities for the SS group is different from that for
the control group, there is a concern that this discrepancy
may strongly affect the value of muscle hardness
because determinants of muscle hardness are poorly
understood. Hence, the within-subjects design should be
useful to achieve the objective of this study. On the other
hand, the within-subjects design may have a disadvantage that it does not account for the possibility of an
effect of SS on the contralateral leg (Ben & Harvey,
2010). When performing hamstring stretching, it is possible that stretch applied to one leg may increase passive
hip flexion in the untreated contralateral leg and therefore that both legs would improve with the SS program
(Ben & Harvey, 2010). When performing the SS of the
plantar flexors used in this study, however, it is hard to
think that the contralateral plantar flexors is stretched.
Correspondingly, the effect of SS on the contralateral leg
is expected to be negligible in this study. The last limitation is that the soleus, which is one of the plantar
flexors, was not investigated in this study. As our previous study (Akagi & Takahashi, 2013) described, a lot of
variation is seen in the measurement values of the soleus
hardness compared with those of MG and LG hardnesses
because the soleus is located deeper than MG and LG.
Hence, we did not determine the muscle hardness of the
soleus, and evaluation of the effect of the SS program on
the soleus hardness remains an issue. Given that the SS
program induces decreases in the muscle hardnesses of
both MG and LG, however, it is strongly suggested that
the soleus hardness should also be decreased by the SS
program. Clarification of this point requires future
research to more strongly support the findings obtained
in this study.
Perspectives
Three bouts of 2-min SS of the plantar flexors 6 days a
week for 5 weeks induced an increase in passive ROM of
dorsiflexion and decrease in muscle hardnesses of the MG
and LG and MTU stiffness of the plantar flexors, but did
not change the joint torque. These results indicate that the
SS program is effective for preventing muscle injury and
improving muscle condition in the plantar flexors.
Although the MG hardness was originally lower than the
LG hardness, there was no significant difference in relative change in muscle hardness from pretest to posttest
between MG and LG. Furthermore, the SS program did
not change the ratio of MG hardness to LG hardness, and
a significant correlation between the muscle hardness
ratios at pretest and posttest was found in the SS group.
Thus, it is suggested that the original difference between
the MG and LG hardnesses remains after conducting the
current SS program, and that the long-term effects of SS
on the MG and LG hardnesses are of the same degree.
Key words: shear wave ultrasound elastography,
Young’s modulus, passive range of motion, musculotendinous unit stiffness, joint torque
Acknowledgements
This study was supported by a Grant-in-Aid for Young Scientists
(B) (No. 24700689).
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