Download Effects of ballistic stretching training on the properties of human

J Appl Physiol 117: 29 –35, 2014.
First published May 8, 2014; doi:10.1152/japplphysiol.00195.2014.
Effects of ballistic stretching training on the properties of human muscle and
tendon structures
Andreas Konrad and Markus Tilp
University of Graz, Institute of Sports Science, Graz, Austria
Submitted 26 February 2014; accepted in final form 5 May 2014
stiffness; ultrasound; passive resistive torque; maximum voluntary
contraction; range of motion
stretching methods are static, ballistic,
and proprioceptive neuromuscular facilitation (PNF) stretching
(7, 26, 33, 45). All methods are used for both acute (a single
stretching session) and short-term (repeated stretching training
for 3– 8 wk) stretching and are able to increase the range of
motion (RoM) (28, 30, 31, 36). Various authors have reported
that repeated static stretching does not affect the torque-angle
curve (9, 12, 27, 40) or standardized torque (2, 3, 8, 22) in the
pre- and postintervention. Others, however, have identified
decreased PRT, and therefore changes in the torque-angle
curve, after a short-term static stretching regime (11, 21, 30,
36). Furthermore, there is some evidence that static stretching
does not alter maximal isometric torque (MVC, 21) or tendon
stiffness (defined as force-length relationship during an isometric ramp contraction with maximal voluntary effort; 21, 30)
following a 3- to 6-wk training period. Although there have
been several studies on the effects of short-term static stretching on structural parameters (21, 30, 36), similar studies on
PNF (20, 31) or ballistic (30) stretching training are scarce.
During a ballistic stretch, the joint is placed, similar to a static
stretch, at the maximum RoM, where a dynamic “ballistic”
movement is applied on the stretched structure at the end of the
RoM. To the best of the authors’ knowledge, so far only
Mahieu and coworkers (30) have analyzed the effects of a
short-term ballistic stretching program on functional and structural muscle-tendon parameters. Mahieu et al. (30) reported an
increase in RoM, no changes in PRT, and a decrease in
Achilles tendon stiffness. However, several structural parameters that might affect and explain RoM changes, such as
muscle and tendon stiffness during passive movements (17,
20), as well as fascicle length and pennation angle (20, 34),
were not analyzed by these authors (30). Therefore, the objective of this study was to analyze the effects of a ballistic
stretching program on the functional and structural parameters
of the ankle plantar flexors. Since tendon, like muscle tissue,
undergoes substantial structural changes as a result of a number
of chronic processes, such as aging (37, 38), chronic use (5),
disuse (6, 39), exercise (41), and static or PNF stretching (17,
20), we also expected changes as a result of the ballistic
stretching training.
Due to the findings in the literature, we hypothesized to
observe a gain in RoM, a decrease in PRT, but no change in
MVC following a 6-wk ballistic stretching training program.
Moreover, we expected that the ballistic stretching training
would result in structural changes, i.e., more compliant muscle
or tendon tissue, as well as longer fascicle length and/or
smaller pennation angles.
THE THREE MOST COMMON
Address for reprint requests and other correspondence: M. Tilp, Institute of
Sports Science, Graz Univ., Mozartgasse 14, A-8010 Graz, Austria (e-mail:
[email protected]).
http://www.jappl.org
METHODS
Subjects
Thirty healthy male (mean ⫾ SD: 23.2 ⫾ 2.4 yr, 179.9 ⫾ 6.2 cm,
75.3 ⫾ 6.9 kg) and 18 healthy female (mean ⫾ SD: 22.1 ⫾ 2.7 yr,
170.1 ⫾ 4.1 cm, 61.9 ⫾ 5.9 kg) police cadets participated in this
study. Each subject was informed about the testing procedure but not
about our hypotheses, and they each gave written consent to participate in the study. Competitive athletes and participants with a history
of lower-leg injuries were excluded. The Ethical Committee of the
University of Graz approved the study.
Experimental Design
A total of 48 police cadets participated in the study, and they were
randomly assigned to a ballistic stretching group (N ⫽ 24) and a
control group (N ⫽ 24) by picking cards in a blind manner. All the
subjects were asked to maintain their normal physical activities during
the study. Teachers of the police school were informed about the study
and were asked to maintain the intensity and extent of physical
activities during their lessons (2 per wk). The ballistic stretching
group undertook a collective ballistic stretching training program, 5
times/wk for 6 wk in the morning, before education in the police
school started. Investigators controlled the stretching training at least
once a week by random and unannounced visits to ensure the accomplishment of the stretching training. Furthermore, subjects were asked
to keep a diary of their stretching performance by documenting every
single stretching session, which was collected at the end of the study.
8750-7587/14 Copyright © 2014 the American Physiological Society
29
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Konrad A, Tilp M. Effects of ballistic stretching training on the properties of human muscle and tendon structures. J Appl Physiol 117: 29–35,
2014. First published May 8, 2014; doi:10.1152/japplphysiol.00195.2014.—
The purpose of this study was to investigate the influence of a 6-wk
ballistic stretching training program on various parameters of the
human gastrocnemius medialis muscle and the Achilles tendon. It is
known that ballistic stretching is an appropriate means of increasing
the range of motion (RoM), but information in the literature about the
mechanical adaptation of the muscle-tendon unit (MTU) is scarce.
Therefore, in this study, a total of 48 volunteers were randomly
assigned into ballistic stretching and control groups. Before and
following the stretching intervention, we determined the maximum
dorsiflexion RoM with the corresponding fascicle length and pennation angle. Passive resistive torque (PRT) and maximum voluntary
contraction (MVC) were measured with a dynamometer. Muscletendon junction (MTJ) displacement allowed us to determine the
length changes in tendon and muscle, and hence to calculate stiffness.
Mean RoM increased significantly from 33.8 ⫾ 6.3° to 37.8 ⫾ 7.2°
only in the intervention group, but other functional (PRT, MVC) and
structural (fascicle length, pennation angle, muscle stiffness, tendon
stiffness) parameters were unaltered. Thus the increased RoM could
not be explained by structural changes in the MTU and was likely due
to increased stretch tolerance.
30
Ballistic Stretching Training Effects on Muscle and Tendon
In the week before and in the week after the 6-wk intervention, the
RoM, PRT, MVC, and several parameters of the muscle (passive
muscle stiffness, pennation angle, fascicle length) and tendon structure (active and passive tendon stiffness) of the gastrocnemius medialis (GM) were determined.
Measures
Fig. 1. Schematic representation of the study and subject flow. DO, drop-outs;
PQ, poor quality of the ultrasound videos; ROM, range of motion; PRT,
passive resistive torque; MVC, maximum voluntary contraction.
Konrad A et al.
measurement was repeated. The difference between the maximum
dorsiflexion and the position in rest (neutral position) was defined as
dorsiflexion RoM. RoM was intentionally not assessed with a dynamometer, to ensure natural behavior of the subjects.
Passive resistive torque (PRT) measurement. To investigate PRT,
an isokinetic dynamometer (CON-TREX MJ, CMV AG, Duebendorf,
Switzerland) with the standard setup adjusted for ankle joint movement was used. Subjects lay prone with their knee fully extended on
a bench and were secured with a strap on the upper body to exclude
any evasive movement. The foot was fixed barefooted with a strap to
the foot plate of the dynamometer. The ankle joint was carefully
aligned with the axis of the dynamometer and fixed to the plate to
avoid any heel displacement. The dynamometer moved the ankle joint
from an initial 10° plantar flexion to a dorsiflexion position, which
corresponded to 95% of the individual maximum dorsiflexion RoM
previously measured in the RoM measurement. During pilot measurements, we recognized a conditioning effect during the first two passive
movements, similar to the active conditioning reported by Maganaris
(25). Therefore, the ankle joint was moved passively for three cycles,
and measurements were taken during the third cycle, to avoid bias
related to the conditioning effect. Similar to the studies by Kubo et al.
(21) and Mahieu et al. (31), the velocity of the dynamometer was set
at 5°/s to exclude any reflexive muscle activity. Participants were
asked to relax during the measurements.
Maximum voluntary contraction (MVC) measurement. MVC measurements were performed with the dynamometer at a neutral ankle
position (90°). Participants were instructed to perform three isometric
MVCs of the plantar flexors for 5 s, with rest periods of at least 1 min
between the measurements, to avoid any fatigue. The attempt with the
highest MVC value was used for the further analysis.
Electromyography (EMG). Muscular activity was monitored by
EMG (myon 320, myon AG, Zurich, Switzerland) during PRT and
MVC measurements. Surface electrodes (Blue Sensor N, Ambu A/S,
Ballerup, Denmark) were placed on the muscle bellies of the GM and
the tibialis anterior. In the PRT measurements, the EMG (normalized
to plantar flexor MVC) was monitored post hoc to ensure that the
subject was relaxed, i.e., did not show any EMG activity. The sample
rate was 2,000 Hz. The EMG signals were high-pass filtered (10 Hz,
Butterworth), and root-mean-square (RMS, 50 ms window) values
were calculated.
Measurement of elongation of the muscle-tendon structures. A
real-time ultrasound apparatus (mylab 60, Esaote S.p.A., Genova,
Italy) with a 10 cm B-mode linear-array probe (LA 923, Esaote
S.p.A.) was used to obtain a longitudinal ultrasound image of the GM.
During the PRT and MVC measurement, the ultrasound probe was
placed on the distal end of the GM (Fig. 2), where the muscle is
connected to the Achilles tendon, i.e., the muscle-tendon junction
(MTJ, 18). The ultrasound probe was secured with a standard orthopedic stocking to prevent any displacement of the probe, which was
tested in pilot measurements. To determine the muscle elongation
during PRT measurement, the echoes of the MTJ in the ultrasound
videos were manually tracked (16, 18). To determine the tendon
elongation during MVC measurement, the echoes of a fascicle insertion at the deep aponeurosis near the MTJ were manually tracked (21).
During RoM measurement, the length of the GM fascicle and its
pennation angle with the deep aponeurosis were determined from the
ultrasound videos. The ultrasound probe was placed at 50% of the GM
muscle length (34). The fascicle length and the pennation angle were
measured at a neutral position of the ankle joint (90°) and at maximum
dorsiflexion.
The ultrasound images were recorded at 25 Hz, with an image
depth resolution of 74 mm. During PRT and MVC measurement, the
videos were synchronized with the rest of the data via the signals of
a function generator (Voltkraft, Hirschau, Germany). The videos were
cut and digitized in VirtualDub open-source software (version 1.6.19,
www.virtualdub.org) and were analyzed in ImageJ open-source software (version 1.44p, National Institutes of Health).
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To ensure a high scientific standard, all measurements were undertaken by the same investigator. In addition to a written introduction,
subjects were personally informed about the procedure. Pre- and
posttraining tests were executed at the same time of day, and the
temperature in the laboratory was kept constant at ⬃20.5°C. To avoid
any bias, measurements were performed without warm up and in the
following order: 1) range of motion (10-min break); 2) passive
resistive torque (1-min break); 3) maximum voluntary contraction
(see Fig. 1).
Range of motion (RoM) measurement. Dorsiflexion RoM was
measured with an electronic goniometer (Biovision, Wehrheim, Germany) fixed to the ankle joint with Leukotape (BSN Medical, Vibraye, France). Participants were first instructed to stay upright in a
neutral position, with the ankle joint angle at 90°. They were then
asked to step back with one leg and bring the ankle joint to maximum
dorsiflexion, keeping their heel on the ground. The knee of the tested
leg had to remain fully extended, and the knee of the opposite leg
flexed. Both feet were kept in a parallel position, and hands could be
placed on a wall to ensure balance. Special attention was paid to the
appropriate position of the stretched leg during the measurement, to
avoid any pronation of the foot. If some pronation was observed, the
•
Ballistic Stretching Training Effects on Muscle and Tendon
•
Konrad A et al.
31
Ballistic Stretching Program
Subjects of the ballistic stretching group were asked to perform a
ballistic stretching training program of their plantar flexor muscles,
while the control group did not receive any intervention. The stretching was done 5 times/wk for a 6-wk period. Each subject was
informed about the stretching procedure. Subjects were instructed to
undertake the stretching of the plantar flexors in a standing wall push
position, and to stretch until a point of discomfort was reached.
Furthermore, they had to move up and down with the front knee at a
frequency of 1 Hz. A stretching intervention consisted of a 30-s
ballistic stretch of the lower leg. This procedure was repeated four
times during each stretching session, alternating both legs, with no
rest in between, resulting in a total stretch period of 120 s for each
muscle. This procedure was chosen because Ryan et al. (43) reported
that 4 ⫻ 30 s of static stretching decreases muscle-tendon unit
stiffness.
Statistical Analyses
Each video was measured by two investigators, and the mean
values of both measurements were used for further analysis of the
muscle-tendon structure. With the exception of the principal investigator, the investigators were not informed of the hypotheses of the
study or the group allocation and subject names. During the analysis
of the PRT measurement, every fifth frame, and for MVC measurement every second frame, were measured by the investigators, corresponding to a time resolution of 0.2 and 0.08 s, respectively.
Similar to the approach used by other authors (18, 24, 34, 35), the
cadaveric regression model of Grieve et al. (10) was used to obtain the
length changes of the MTU of the GM during passive movements.
The difference between the MTU length change and the elongation of
the muscle was defined as the tendon elongation.
Calculation of Muscle/Tendon Force, Passive Muscle/Tendon
Stiffness, Active Tendon Stiffness, and Muscle-Tendon Stiffness
The muscle force of the GM was estimated by multiplying the
measured torque by the relative contribution of the physiological
cross-sectional area (18%) of the GM within the plantar flexor
muscles (21, 30, 31), and dividing by the moment arm of the triceps
surae muscle (MA), which was measured individually with a measuring tape as the distance between the malleolus lateralis and the
Achilles tendon at rest (neutral position). The mean value of the
moment arm was 4.41 cm and the range was 3.0 – 6.0 cm.
Active tendon stiffness was calculated by linear regression between
the difference of active force (0 –100% MVC) and the passive force at
neutral ankle position (19) and the related tendon length changes
during the MVC measurements. Passive tendon stiffness, muscle
stiffness, and muscle-tendon stiffness were calculated by linear regression between the passive force (⬃0 –30% MVC) produced from
the neutral ankle position (90°) to maximum dorsiflexion, and the
related tendon length, muscle length, and joint angle changes, respectively. Please note that the term “passive tendon stiffness” was used
for the force-length relationship during PRT measurement and therefore differs from previous studies (30, 31). To distinguish between the
force-length relationships from passive measurements performed in
our study, we have defined this parameter as “active tendon stiffness”
throughout the text. The quality of the linear regressions was assessed
with Pearson correlation coefficients.
SPSS (version 20.0, SPSS, Chicago, IL) was used for all the
statistical analyses. To determine the inter-rater reliability of the
muscle-tendon displacement measurements, an intraclass correlation
coefficient (ICC) was used. A Kolmogorov-Smirnov test was used to
verify the normal distribution of all the parameters. To prove the
homogeneity between the baseline characteristic of both groups,
t-tests were performed. Tendon, muscle, and muscle-tendon stiffness
calculations were controlled with Pearson correlation coefficients. To
assess the reliability of our methods, paired t-tests were performed to
test if the mean values of the pre- and postmeasurements of the control
group were equal. Subsequently, we performed paired t-tests to test
the effect of the stretching protocol in the intervention group. An
alpha level of P ⫽ 0.05 was defined for the statistical significance of
all the tests.
RESULTS
Data Exclusion and Measurement Quality
There was no significant difference between the groups,
except in the “(passive) muscle stiffness” parameter. Due to
subject drop-out and the poor quality of the ultrasound videos,
three (six) subjects of the RoM measurement, four (nine)
subjects of the PRT measurement, and five (four) subjects of
the MVC measurement of the ballistic stretching (control)
group, respectively, had to be excluded from the study (Fig. 1).
The subject drop-outs were all due to injuries. In the ultrasound
videos with poor quality, fascicle insertion points at the deep
aponeurosis (MVC measurement) or the MTJ (PRT measurement) were not identifiable with the necessary precision. Dropouts and data exclusion did not change the homogeneity of the
groups.
The mean (range) ICC of the ultrasound video analysis of
both investigators was 0.99 (0.978 – 0.998), 0.96 (0.831–
0.999), and 0.96 (0.801–1.000) for the RoM, PRT, and MVC
measurements, respectively. Values above 0.80 were classified
as acceptable (48).
The mean values of the Pearson correlation coefficients of
the linear regression were 0.99, 0.96, 0.89, and 0.96, with
ranges of 0.88 – 0.99, 0.81– 0.99, 0.76 – 0.96, and 0.93– 0.98,
with all P ⬍ 0.05, for passive tendon stiffness, active tendon
stiffness, muscle stiffness, and muscle-tendon stiffness, respectively.
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Fig. 2. Images showing the displacement of the MTJ during a passive
movement from neutral position (A) of the ankle joint to maximum dorsiflexion (B).
32
Ballistic Stretching Training Effects on Muscle and Tendon
•
Konrad A et al.
Table 1. Results of maximum dorsiflexion RoM, as well as changes in fascicle length and pennation angle during RoM
measurement; PRT, passive tendon stiffness, muscle stiffness, and muscle-tendon stiffness during passive measurements; and
MVC torque and active tendon stiffness during MVC measurements
Ballistic
PRE
Range of motion, °
Fascicle length at rest, cm
Fascicle length in stretching position, cm
Pennation angle at rest, °
Pennation angle in stretching position, °
33.8 ⫾ 6.3
6.4 ⫾ 0.7
7.4 ⫾ 0.9
17.2 ⫾ 2.2
15.3 ⫾ 1.9
Passive resistive torque, N·m
Passive tendon stiffness, N/mm
Muscle stiffness, N/mm
Muscle-tendon stiffness, N·m/°
25.2 ⫾ 8.1
13.3 ⫾ 3.6
9.2 ⫾ 2.3
0.83 ⫾ 0.22
MVC torque, N·m
Active tendon stiffness, N/mm
85.9 ⫾ 38.3
18.6 ⫾ 3.4
Control
POST
N ⫽ 21
37.8 ⫾ 7.2*
6.3 ⫾ 0.9
7.4 ⫾ 0.7
17.6 ⫾ 2.1
15.3 ⫾ 1.8
N ⫽ 20
24.2 ⫾ 8.3
13.2 ⫾ 3.9
9.9 ⫾ 1.8
0.87 ⫾ 0.22
N ⫽ 19
89.9 ⫾ 32.2
19.8 ⫾ 5.3
P
PRE
0.00
0.24
0.97
0.35
0.97
32.1 ⫾ 7.7
6.1 ⫾ 0.8
7.4 ⫾ 0.9
17.8 ⫾ 1.9
15.4 ⫾ 1.8
0.24
0.9
0.25
0.26
22.2 ⫾ 7.5
13.9 ⫾ 3.7
6.8 ⫾ 2.1
0.78 ⫾ 0.22
0.24
0.26
92.7 ⫾ 29.3
20.2 ⫾ 5.8
POST
N ⫽ 18
31.8 ⫾ 7
6.2 ⫾ 0.9
7.4 ⫾ 0.9
18.2 ⫾ 2.3
15.6 ⫾ 1.6
N ⫽ 15
21.8 ⫾ 8.3
12.4 ⫾ 4.9
6.8 ⫾ 2.7
0.71 ⫾ 0.21
N ⫽ 20
90.1 ⫾ 33.2
19.3 ⫾ 5.1
P
0.61
0.50
0.86
0.45
0.76
0.68
0.25
0.94
0.12
0.63
0.40
Range of Motion (RoM) and the Related Structural Muscle
Parameters
Following the 6-wk stretching intervention, the ballistic
stretching group had a significantly increased dorsiflexion
RoM (P ⫽ 0.00, see Table 1). Fascicle length and pennation
angle did not change in the neutral or maximum dorsiflexion
position. No parameter changes were observed in the control
group.
Passive Resistive Torque (PRT) and Related Structural
Muscle-Tendon Parameters
There was no significant change in PRT at the same maximum ankle joint angle for the pre- and postsession data (Table
1). Figure 3 shows the relationship between ankle joint angle
and the corresponding PRT of the ballistic stretching group. No
significant differences were observed in any joint angle. Moreover, muscle-tendon stiffness, and passive tendon and muscle
stiffness did not change after the ballistic stretching interven-
Fig. 3. Relationship between passive resistive torque and ankle joint angle
before and after the ballistic stretching intervention (N ⫽ 20) (mean ⫾ SE).
tion. No parameter changes could be found in the control
group.
In Fig. 4, A and B, the elongation of muscle and tendon in
relationship to the PRT data is shown in steps of 5° from 0° to
25°. Moreover, Fig. 4, C and D, shows the elongation of the
tendon and muscle as a function of the ankle angle.
Maximum Voluntary Contraction (MVC) and Tendon
Stiffness
Plantar flexor MVC was the same after the short-term
stretching intervention. Tendon stiffness calculated from the
MVC measurements did not change in both the ballistic
stretching and control groups (see Table 1).
DISCUSSION
The functional parameters investigated in this study were
RoM, PRT, muscle-tendon stiffness, and MVC. Similar to
previous studies (30, 44), the maximum dorsiflexion RoM
increased significantly after the short-term ballistic stretching
training in the present study. The RoM increase of 4° was
similar to the results of Mahieu et al. (30), who reported a
slightly smaller ankle joint range of motion (3.2°) after a 6-wk
ballistic stretching training program.
Muscle-tendon stiffness and PRT did not change, which is in
accordance with the findings of Mahieu et al. (30), who are the
only other researchers to have studied the effects of ballistic
stretching training on these parameters. However, the findings
do not support our hypothesis of a decreased PRT as a result of
a ballistic stretching intervention. Following a single ballistic
stretching intervention, some authors (14) reported decreasing
PRT and muscle-tendon stiffness, while others (29) determined
unchanged muscle-tendon stiffness. Decreasing passive forces
were also reported in similar animal studies (46, 47). In
summary, the results of muscle-tendon stiffness after a single
stretching session are controversial. However, together with
the results of the short-term stretching studies, one could
speculate that passive forces and PRT rapidly return to the
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Values are means ⫾ SD. RoM, range of motion; PRT, passive resistive torque; MVC, maximum voluntary contraction. *Significant difference between PRE
and POST session data.
Ballistic Stretching Training Effects on Muscle and Tendon
•
Konrad A et al.
33
Fig. 4. Relationship between passive resistive torque and tendon (A) and muscle elongation (B) during passive movement before
and after the ballistic stretching intervention.
Elongation of the tendon (C) and the muscle
(D) during passive dorsiflexion, in relation to
ankle angle before and after the ballistic
stretching intervention (N ⫽ 20) (mean ⫾
SE). Data from the control group are omitted
because there is no statistical or visible difference between the measurements.
In their studies of short-term static stretching, both Kubo et
al. (21) and Mahieu et al. (30) reported unchanged tendon
stiffness. However, studies of short-term PNF stretching have
shown controversial results: Konrad et al. (20) detected decreasing tendon stiffness following a 6-wk PNF stretching
training program, while Mahieu et al. (30) reported no such
changes in the MTU. In summary, the current study and other
studies of static stretching (21, 30), which have reported
similar adaptations in tendon stiffness, indicate that the tendon
structure reacts the same to stretching, regardless of whether it
is a sustained or repetitive stimulus.
In addition to tendon stiffness during active conditions, the
current study investigated muscle-tendon stiffness and the
separate muscle and tendon stiffness during passive movements. As with the measurement of active tendon stiffness,
passive tendon stiffness was unaltered after the ballistic
stretching intervention in the current study. In their study of
ballistic stretching, Mahieu et al. (30) observed constant PRT
but decreased tendon stiffness (measured from MVC). They
explained this counterintuitive result with the hypothesis that
tendon stiffness from active state and PRT are not directly
related, due to the different forces associated with the different
tests. However, the current study recorded a similar behavior of
the tendon in both active and passive conditions, and both in
accordance with PRT, which all remained unchanged by the
ballistic stretching intervention.
Furthermore, no changes in muscle stiffness or muscletendon stiffness were detected, indicating that the muscle belly
with the contractile elements and its connective tissues (endomysium, perimysium, and epimysium) were not affected by
the ballistic stretching training program. However, our methods do not allow us to exclude an adaptation in the muscle at
a cellular level.
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initial state, and there is no underlying chronic adaptation due
to stretching.
As with several other short-term studies of static stretching
(11, 21) and PNF stretching (15), an unchanged maximal
isometric torque was observed after the ballistic stretching
intervention. Thus it appears that ballistic stretching has no
detrimental effect on MVC.
In addition to the functional parameters, several structural
parameters (muscle stiffness, active and passive tendon stiffness, fascicle length, pennation angle) which might explain the
increasing RoM were investigated in this study. To the best of
our knowledge, so far only Mahieu et al. (30) have investigated
the effect of a 6-wk ballistic stretching training program on
active tendon stiffness. In the current study, active tendon
stiffness was unaltered after a 6-wk ballistic stretching training
program, while Mahieu and colleagues (30) reported decreased
active tendon stiffness. Based on the assumptions of McNair et
al. (32), Mahieu et al. (30) speculate that as a result of the
cycling motion, polysaccharides and water are redistributed
within the collagen framework, which leads to a decrease in
stiffness. Although the current study and the study of Mahieu
et al. (30) both investigated ballistic stretching methods, differences in stretch intensity and amplitude could be one reason
for the different results. Mahieu et al. (30) noted that their
subjects were instructed to move up and down at a pace of one
movement per second with the front knee. Although we instructed our subjects in the same way, we cannot be certain that
we achieved the same stretch intensity and amplitude. This
might have caused the different results. Other possible explanations for the controversial results in tendon stiffness with
similar methods could be the warm-up routine, or the fact that
the subjects in the study of Mahieu et al. (30) wore shoes,
compared with the subjects being barefooted in the current
study.
34
Ballistic Stretching Training Effects on Muscle and Tendon
Konrad A et al.
Conclusions
This study has shown that a 6-wk ballistic stretching training
program of the lower-leg muscle increases dorsiflexion RoM
but has no effect on muscle and tendon tissue. Therefore,
altered tolerance to stretching seems to be a plausible explanation for the gains in RoM. However, further studies, including other structural parameters such as muscle hardness which
might give an additional explanation for the gain in RoM,
should be undertaken. Furthermore, future studies should include follow-up measurements to assess a possible decrease in
the RoM following the end of the stretching training.
GRANTS
This study was supported by a grant (Project P23786-B19) from the
Austrian Science Fund FWF.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
AUTHOR CONTRIBUTIONS
Author contributions: A.K. and M.T. conception and design of research;
A.K. performed experiments; A.K. and M.T. analyzed data; A.K. and M.T.
interpreted results of experiments; A.K. prepared figures; A.K. and M.T.
drafted manuscript; A.K. and M.T. edited and revised manuscript; A.K. and
M.T. approved final version of manuscript.
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Similarly, fascicle length and pennation angle during RoM
measurements did not change in the intervention group. Due to
the findings on static stretching in animal studies (4), Mahieu
et al. (30) hypothesized that changes in RoM might be a result
of the increasing number of sarcomeres in series. However,
since fascicle length remained unaltered in the present study
following the 6 wk of ballistic stretching, we cannot support
this hypothesis. Unchanged fascicle lengths were also reported
by Nakamura et al. (36) following a 4-wk static stretching
training program. Therefore, it seems improbable that static
stretching in humans induces similar effects in the muscle
structure to those seen in the stretching interventions done in
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In conclusion, the present study showed an increase in RoM
but no significant adaptation in measured structural parameters
which could explain the RoM gains. The lack of changes in
structural parameters in the lower-leg MTU following a shortterm ballistic stretching training program does not support our
hypotheses. Hence, a probable explanation for the results of the
current study could be that the increased RoM following
stretching is due to an altered perception of stretch and pain, or
stretch tolerance (12, 13, 26), rather than altered muscular or
tendon structures. However, the present study might not have
investigated all the structural parameters that affect RoM. A
novel study by Akagi and Takahashi (1) investigated the
so-called hardness (transverse muscle stiffness) in the gastrocnemius muscle following a 5-wk static stretching training
program. The results of this study revealed a decrease in the
hardness of the gastrocnemius medialis and lateralis; in other
words, a structural adaptation in the muscle. This is in contrast
to other studies (21, 30) of short-term static stretching, in
which no changes in the structural parameters of the MTU
were reported.
A possible bias could have occurred due to the specific
subject group of police cadets in this study. Police cadets have
to pass a physical exam to enter the police school and have
regular lessons of physical activity. However, the exam is not
as demanding as an entry exam for sports science at the
University, and lessons of physical education are only 2 h/wk.
Therefore, the physical state of police cadets is probably
slightly above average. Thus we neither expect significantly
different muscle-tendon properties in our subject group compared with the general public nor any influence of the general
training program on the tested parameters. This was confirmed
by the lack of changes also in the control group.
There are some limitations to the current study. First, the
persons taking the measurements were not all blind to the
intervention. Therefore, a bias in the results cannot be completely excluded, although the interrater reliability was excellent (mean ICC: 0.96 – 0.99). Second, the method of measuring
the moment arm of the ankle joint in vivo was quite simple.
However, values obtained in the study were very similar to
others using magnetic resonance imaging (MRI) data (42) or
ultrasound (23). Third, it is very difficult to standardize amplitude, velocity, and force of the ballistic stretches. An alternative approach would have been to use the dynamometer in
the training process. However, in human studies of stretching,
this is very complex and has, to date, only been used by Kubo
et al. (21) for a small training group (eight subjects) and a short
duration (3 wk). Furthermore, such a procedure does not
represent the typical stretching training used in practice.
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