Key words
warm-up exercise - range of motion - passive torque - passive stiffness - muscle force
Introduction
Static stretching (SS) is commonly performed to improve flexibility and as a
component of warm-up exercises in the belief that it will reduce the risk of injury
[26]
[31]. Many studies have shown that SS improves
flexibility as measured by tests of range of motion (ROM), passive torque (PT), and
passive stiffness [7]
[16]
[18]. However, recent review articles reported
that prolonged (>30–60 s) SS can have detrimental effects on
muscle performance [3]
[13]
[25]. Therefore, it may not be advisable to
engage in prolonged SS prior to high-level or competitive athletic or training
activities [3]. In contrast to SS, dynamic
stretching (DS) has recently been recommended as a component of warm-up exercises
conducted prior to engaging in athletic activities. Many recent studies have shown
that DS improves muscle power, jump height, and sprint time, and DS has been found
to have a more beneficial effect on performance than SS [14]
[22]
[32].
In terms of muscle performance, the available data appear to indicate that DS is more
suitable for warming up compared with SS. However, which stretching method is more
effective with respect to improving or maintaining flexibility remains unclear [2]. For example, Behm et al. [4] reported no difference between the effects
of SS versus DS on the outcome of the sit and reach test. However, Paradisis et al.
[21] reported that the effect of SS on the
outcome of the sit and reach test was greater than that of DS in adolescent boys and
girls. In contrast, Amiri-Khorasani and Kellis [1] reported that the effect of DS on flexibility was greater than that of
SS. Previous studies have shown that lower levels of muscle flexibility [30] and higher levels of stiffness [28] are associated with a higher risk of muscle
injury. Thus, stretching prior to engaging in athletic activity has multiple
benefits, including reducing the risk of injury and improving athletic performance.
However, although both static and dynamic stretches are performed as components of
warm-up exercises, few studies have directly compared the acute effects of static
and dynamic stretching in terms of flexibility parameters (e. g. range of
motion, passive torque at onset pain, and passive stiffness) and muscle force. A
comparison of the effects of these two types of stretching on flexibility and muscle
force could indicate which stretching methods are most suitable for warming up
before engaging in athletic activity.
In this study, we sought to compare the effects of SS and DS on ROM, PT at the onset
of pain, passive stiffness, and isometric muscle force. We hypothesized that DS
would be more effective than SS in terms of enhancing muscle performance. Moreover,
we hypothesized that the effects of DS on flexibility parameters would be equal to
or greater than the effects of SS under the same stretching conditions.
Materials and Methods
Study design
We conducted a randomized crossover trial. The participants completed measurement
sessions on two separate days, one for each stretching type. Specifically, they
completed either SS or DS of the right hamstrings for a 300 s period.
The order of stretching type was randomized. We obtained the ROM of passive knee
extension, PT at the onset of pain, passive stiffness, and maximum voluntary
isometric knee flexion force immediately before and after stretching. All
participants attended a familiarization session before the first testing day.
All measurements were taken at the same time of day (±1 h).
Participants
Sixteen healthy young men voluntarily participated in this study
(mean±standard deviation (SD): aged 22.2±1.2 y, height
170.7±6.2 cm, body mass 64.0±11.5 kg, body mass
index 21.9±3.3 kg/m2). All participants were
informed regarding the study purpose and protocol and provided written informed
consent. The study was approved by the Human Research Ethics Committee of our
institution (approval number: 14–23). Moreover, this study was performed
in accordance with the ethical standards of the International Journal of Sports
Medicine [8]. The inclusion criteria were
healthy males aged approximately 22 years. The exclusion criteria were lower
extremity joint contractures, history of surgical operation on the back or lower
extremities, neurological disorders, current regiment of hormones or
muscle-affecting drugs, ability to completely extend the right knee from a
sitting position as described below (i. e., exceptional flexibility),
engagement in competitive sports, regular resistance, aerobics, or flexibility
training. The participants were asked to refrain from vigorous physical activity
during the experimental period.
Procedures
Static stretching
For SS, each participant assumed a standing upright position and placed his
right heel (with an extended leg) on a platform 50 cm high. The
participant then reached forward with their arms toward the extended leg
while maintaining a proper lordotic curve [4] ([Fig. 1a]). SS was
performed at a tolerable intensity without pain [11]
[16]
[17]. Ten 30-s sets of SS were
performed with a 20-s rest period between each set.
Fig. 1 Positioning for a static stretching and
b dynamic stretching of the right hamstring. c
Positioning for the measurements of all dependent variables.
Dynamic stretching
For DS, each participant assumed a standing upright position beside parallel
bars and held a parallel bar with his left hand for stability. To stretch
the hamstrings, the participants intentionally contracted the right hip
flexors with the knee extended and flexed their right hip joint so that
their right leg swung up to the anterior aspect of their body [10]
[32] ([Fig. 1b]). The participants performed
this dynamic movement every 2 s. Each exercise was performed 5 times
slowly to practice, and then 10 times as quickly as possible without
bouncing [10]
[32]. Ten 30-s sets of DS (15
repetitions of the DS movement in each set) were performed with a 20-s rest
period between each set.
Dependent variables
We first measured the torque–angle relationship (ROM, PT at pain
onset, and passive stiffness) and then measured the isometric muscle force
immediately before and after stretching. All dependent variables were
obtained using an isokinetic dynamometer (PrimusRS; BTE Technologies,
Hanover, MD, USA). The torque and angle signals from the dynamometer were
subjected to analog-to-digital conversion (PL3508 PowerLab 8/35;
ADInstruments, Sydney, Australia) and stored in a personal computer.
Range of motion, passive torque at the onset of pain, and passive
stiffness
Measurements were taken with the participant in a sitting position with his
hip joint flexed ([Fig. 1c]) [11]
[16]
[17]. Each participant was seated on a
chair with the seat tilted maximally and a wedge-shaped cushion inserted
between the trunk and the backrest. The participant’s chest, pelvis,
and right thigh were stabilized with Velcro straps. The knee joint was
aligned with the axis of rotation of the dynamometer, and the lever arm
attachment was placed just proximal to the malleolus medialis. In this
position, the average angles of hip and knee flexion were
107.6±2.3° and 111.2±1.8°, respectively.
With the participant sitting in the chair ([Fig. 1c]), his knee was extended passively at
5°/s to the point of maximum knee extension just before the
onset of pain. Torque was recorded continuously during passive knee
extension [11]
[16]
[17]. ROM (in °) was defined
as the maximum knee extension angle from the initial position (0°),
and PT at the onset of pain (in Nm) was defined as the torque at the onset
of pain [11]
[17]. Passive stiffness (in
Nm/°) was defined as the slope of the regression line
calculated from the torque–angle relationship using the least
squares method [11]
[16]
[17]. Stiffness was calculated using
the same knee extension angle range before and after stretching, and the
calculated knee extension angle range was defined as the angle from the
50% maximum knee extension angle to the pre-stretching maximum knee
extension angle.
Isometric muscle force
Isometric muscle force (in Nm) was measured in the same position as that used
to measure the torque–angle relationship [11]
[16]
[17] ([Fig. 1c]). The participants were
instructed to sit with their arms crossed in front of their chest, and to
generate maximum knee flexion force for 3 s. They did this three
times with a 45-s rest period between trials [17]. Peak torque was obtained from each
contraction, and the average of the three trials was used for further
analysis.
Test–retest reliability
We confirmed test–retest reliability values for all dependent
variables by calculating intra-class correlation coefficients (ICCs) and
coefficients of variation (CVs). Prior to the data collection in the present
study, we conducted a pilot study to examine the test–retest
reliability for all dependent variables. The participants were 12 men. The
two tests were performed on two separate days and at the same time of the
day (±1 h). We calculated ICC and CV, and the results of
these assessments showed that reliability was acceptable for all measures
(ROM: 0.903 (ICC), 2.5% (CV); PT at the onset of pain: 0.934,
3.4%; passive stiffness: 0.910, 5.7%; isometric muscle
force: 0.915, 4.2%).
Statistical analyses
We determined the number of participants by conducting a sample size
estimation using data from the literature [17] and G*Power software (v 3.0.10; Franz Faul, Kiel
University, Kiel, Germany). The effect size of 300 s of static
stretching on knee extension ROM, which was calculated from previous data
[17], was 0.91599. On the basis of
the effect size, α level of 0.05, and a power (1-β) of 0.80,
the minimum number of participants was estimated to be 12. To strengthen the
power of the study, we recruited 16 participants.
We assessed the normality of the data using the Shapiro–Wilk test.
This test showed that the ROM and isometric muscle force were normally
distributed, but the other parameters were not. Thus, we applied
non-parametric tests to all absolute values and relative changes (in
%). We performed the Wilcoxon signed-rank test to identify
significant differences between the two stretching methods at each time
point as well as the relative change, or the difference from the
pre-stretching value. Instead of Cohen’s d, we used the r effect
size (ES) to calculate the ES for the change from pre- to post-stretching
and the between–post-stretching comparison (absolute value and
relative change respectively). The r ES was calculated by dividing the
Wilcoxon Z score by the square root of the sample size
(r=Z/√N) [29].
This ES was interpreted as follows: small effect, ≥ 0.1; medium
effect, ≥ 0.3; and large effect, ≥ 0.5 [29]. Analyses were performed using IBM
SPSS statistics version 21.0 (IBM Corp., Armonk, NY, USA), and significance
was set at p<0.05. All results are expressed as mean±SD.
Results
Range of motion
ROM increased significantly after both SS and DS (p<0.01) ([Table 1]). The ES values for the pre- to
post-stretching change were large (SS: 0.88, DS: 0.88). However, we observed no
significant differences between SS and DS for pre-stretching values,
post-stretching values, or relative change. Moreover, the ES values for the
between–post-stretching comparison reflected a medium ES (absolute
value: 0.45, relative change: 0.35).
Table 1 Effects of stretching on changes in dependent
variables.
|
Dependent variable
|
Stretching method
|
Pre
|
Post
|
Relative change (%)
|
Intra-condition difference
|
|
ROM (°)
|
Static stretching
|
87.1±6.6
|
101.5±6.5
|
116.7±3.4
|
p<0.01
|
|
Dynamic stretching
|
86.9±8.1
|
99.8±6.6
|
115.2±5.9
|
p<0.01
|
|
Inter-condition difference
|
p=0.96
|
p=0.07
|
p=0.16
|
-
|
|
PT at the onset of pain (Nm)
|
Static stretching
|
32.3±7.2
|
38.9±9.0
|
120.4±6.5
|
p<0.01
|
|
Dynamic stretching
|
32.3±6.2
|
37.7±6.0
|
117.7±12.3
|
p<0.01
|
|
Inter-condition difference
|
p=0.41
|
p=0.35
|
p=0.11
|
-
|
|
Passive stiffness (Nm/°)
|
Static stretching
|
0.433±0.123
|
0.385±0.112
|
89.0±7.9
|
p<0.01
|
|
Dynamic stretching
|
0.430±0.097
|
0.379±0.088
|
88.2±5.9
|
p<0.01
|
|
Inter-condition difference
|
p=0.92
|
p=1.00
|
p=0.38
|
-
|
|
Isometric muscle force (Nm)
|
Static stretching
|
69.2±15.8
|
59.7±15.6
|
85.7±8.6
|
p<0.01
|
|
Dynamic stretching
|
69.9±13.5
|
60.3±14.3
|
85.8±8.2
|
p<0.01
|
|
Inter-condition difference
|
p=0.23
|
p=0.80
|
p=0.96
|
-
|
ROM, range of motion; PT, passive torque.
Passive torque at the onset of pain
PT at the onset of pain increased significantly after both SS and DS
(p<0.01) ([Table 1]). The ES
values for the pre- to post-stretching change reflected a large ES (SS: 0.88,
DS: 0.88). However, we observed no significant differences between SS and DS for
pre-stretching values, post-stretching values, or relative change. Moreover, the
ES values for the between–post-stretching comparison reflected a small
or medium ES (absolute value: 0.23, relative change: 0.40).
Passive stiffness
Passive stiffness decreased significantly after both SS and DS (p<0.01)
([Table 1]). The ES values for the
pre- to post-stretching change reflected a large ES (SS: 0.87, DS: 0.87).
However, we observed no significant differences between SS and DS for
pre-stretching values, post-stretching values, or relative change. Moreover, the
ES values for the between–post-stretching comparison reflected a
negligible or small ES (absolute value: 0.00, relative change: 0.22).
Isometric muscle force
Isometric muscle force decreased significantly after both SS and DS
(p<0.01) ([Table 1]). The ES
values for the pre- to post-stretching change reflected a large ES (SS: 0.88,
DS: 0.88). However, we observed no significant differences between SS and DS for
pre-stretching values, post-stretching values, or relative change. Moreover, the
ES values for the between–post-stretching comparison reflected a
negligible ES (absolute value: 0.07, relative change: 0.01).
Discussion
In this study, we compared the effects of SS and DS on ROM, PT at the onset of pain,
passive stiffness, and isometric muscle force. We had hypothesized that DS would be
more effective than SS in terms of enhancing muscle performance and that the effects
of DS on flexibility parameters would be equal to or greater than the effects of SS
under the same stretching conditions. However, contrary to expectations, we found
that SS and DS did not differ in terms of their affects on ROM, PT at the onset of
pain, passive stiffness, or isometric muscle force.
We found that 300 s of SS increased ROM and PT at the onset of pain and
decreased passive stiffness and isometric muscle force. These changes were similar
to those previously reported after long periods (≥180 s) of SS [11]
[16]
[17]. Therefore, these effects were as
expected. However, contrary to our expectations, a total of 300 s of DS
significantly decreased isometric muscle force in a way that was similar to that
elicited by SS. Many previous studies have reported that DS improved performance
parameters [14]
[22]
[32]. In contrast, some review articles have
stated that SS decreased the maximum muscle force and performance [3]
[13]
[25]. A decrease in muscle force and
performance after SS might be caused by a reduction in a neural drive, such as a
central drive [27], as well as a reduction in
peripheral electromyographic activity [6]
[12]. Further, this decrease could be due to a
reduction in peripheral force-generating capacity, such as that caused by
musculotendinous stiffness, and associated changes in the muscle
length–tension relationship [6]
[23]. Previous studies have considered how
muscle stretching might theoretically reduce the force transfer efficiency from the
contractile component to the skeleton alongside stretch-induced reductions in muscle
stiffness, although this possibility has not been assessed directly in humans [2]. Our data indicate that the effects of
static and dynamic stretching on these variables might be similar. Specifically, our
findings suggest that the decrement in isometric muscle force after DS was partly
caused by the decrement in passive stiffness, as is the case following SS. However,
we did not measure changes in neurophysiological factors using electromyography or
other techniques. We also did not measure changes in the performance of muscle
contractions such as the rate of force development (contraction speed). Further
studies are needed to examine the detailed factors that might lead to decreased
isometric muscle force after stretching and the effects of stretching on
musculoskeletal biomechanics.
For the SS in the present study, participants performed 10 sets that were 30-s long
and were separated by a 20-s rest period. They were asked to stretch at an intensity
that was below the threshold for pain. For the DS in the present study, participants
performed 10 sets that were 30-s long and were separated by a 20-s rest period. Each
30-s set contained 15 repetitions of the DS movement, which involved contracting
antagonist muscle groups. The participants were asked to conduct the movements at
an
intensity that was below the pain threshold. Thus, in both the SS and DS conditions,
we asked participants to engage in stretching for a total of 300 s per day.
We selected this duration because previous studies showed that 300 s of
static stretching at a tolerable intensity and without pain significantly increased
ROM and PT, and decreased passive stiffness and isometric muscle force after
stretching [16]
[17]. Additionally, the use of these
parameters in the present study enabled us to compare the present data with those
from these previous studies.
Yamaguchi and Ishii [33] suggested that
explosive performance might be impaired as the volume of DS increases. Therefore,
the DS protocol in the present study might have induced a loss of force rather than
improving muscle performance because the total number of DS repetitions was
excessive (150 repetitions in total). However, another review reported that greater
improvements in peak force and power were observed when longer durations of DS, such
as 90 s, were performed [3]. These
inconsistent results might have been caused by variability among studies, which is
supported by another systematic review article that suggested that it was difficult
to demonstrate a dose–response relationship with respect to DS [2]. Herda et al. [9] reported that 4 30-s sets of DS in which
agonist muscle groups were contracted significantly decreased isometric muscle
force. These findings suggest that the characteristics of the DS protocol, such as
the number of repetitions, intensity, and type of contraction influence the effects
of the protocol on muscle force and performance. Therefore, further studies are
required to investigate how differences in DS protocols might affect muscle force
and performance.
In terms of flexibility, this study revealed that both SS and DS increased ROM and
PT
at the onset of pain and that they both decreased passive stiffness. Moreover, the
changes in these dependent variables were not different between SS and DS. Mizuno
et
al. [18] reported that an increase in ROM
immediately after SS was attributable to an increase in PT at the onset of pain and
a decrease in passive stiffness. These findings suggest that, as with SS, the
increase in ROM after DS was caused by the changes in PT at the onset of pain and
passive stiffness. Nakamura et al. [20]
revealed that a total of 300 s of SS decreased muscle–tendon unit
stiffness and muscle stiffness, and that the decrease in muscle–tendon unit
stiffness was due to the decrease in muscle stiffness. Therefore, the decrease in
passive muscle–tendon stiffness observed after SS in this study might have
been caused by the decrease in muscle stiffness. In contrast to the data for SS,
another recent study revealed that 4 30-s sets of DS in which antagonist muscle
groups were contracted did not affect passive muscle–tendon unit stiffness
[19], whereas 4 30-s sets of DS in which
agonist muscles were contracted decreased passive stiffness [9]. Moreover, Samukawa et al. [24] observed proximal displacement of the
muscle–tendon junction of the medial gastrocnemius, but no change in the
pennation angle or fascicle length after 5 30-s sets of DS in which antagonist
muscle groups were contracted. Given their findings, the researchers suggested that
DS primarily affects the tendinous tissues. Taken together, these previous studies
indicate that SS and DS might affect passive muscle–tendon unit stiffness in
different ways. Although many studies have examined the effect of SS on passive
stiffness, further studies are required to investigate the impact of DS.
We employed the same measurement parameters used in previous studies to investigate
PT at the onset of pain according to the pain threshold or stretch tolerance [7]
[15]. Previous authors have proposed that the
SS-induced increase in stretch tolerance is caused by a reduction in pain and
discomfort perception accompanied by changes in neural and psychological factors,
although the detailed mechanisms are unknown [5]. As with SS, DS has been found to significantly increase PT at the
onset of pain, and our data were consistent with those of previous studies [19]. Therefore, the increase in PT at the onset
of pain after DS might be caused by the same mechanism as that after SS.
Contrary to our expectations, we did not find any differences in the effects of SS
and DS on any of the dependent variables. Moreover, the ES values for the
between–post-stretching comparison were negligible or small for measurements
with the exception of ROM and PT at the onset of pain. Specifically, the
between–post-stretching comparison of ROM and PT at the onset of pain showed
a medium ES, although these were not statistically significant. These results
indicate that the amount of stretching stimulation does not differ between SS and
DS. Our stretching protocol might have contributed to these results because the
300-s stretching duration used in this study was relatively longer than that
commonly performed. Therefore, the effects of SS and DS may differ when a shorter
stretching duration is employed. Further studies are required to determine whether
specific SS and DS protocols have a differential influence on flexibility and muscle
performance.
Previous studies have reported that reduced muscle flexibility [30] and increased stiffness [28] are associated with a greater risk of
muscle injury. Therefore, we speculate that a total of 300 s of active
static and dynamic stretching may reduce the risk of injury in healthy individuals
during sports activities. Indeed, the present results show that both static and
dynamic stretching significantly increased ROM and PT at the onset of pain and
significantly decreased passive stiffness, indicating that they are both useful
preventative measures against injury when preparing to engage in athletic
activity.
The main limitation of this study was that we assessed only the effects of a longer
duration of stretching (a total of 300 s for each stretching type). We
suspect that it would be difficult to perform 300 s of stretching per muscle
as part of a regular stretching program. Therefore, future studies should compare
the effects of static and dynamic stretching with shorter durations of stretching
that are more commonly performed (20–60 s). Another limitation was
that we collected only isometric muscle force as a measure of muscle performance and
not other measures, such as electromyography and muscle contraction speed (rate of
force development).
In summary, in the present study, we found that both SS and DS significantly
increased ROM and PT at the onset of pain and significantly decreased passive
stiffness and isometric muscle force. Interestingly, SS and DS did not differ in
terms of the magnitude of change for all measurements. These results suggest that
a
total of 300 s of SS or DS increases flexibility and decreases isometric
muscle force, and that the effects of stretching do not differ between the two
stretching methods.
