Comparison of the Effects of Long-Lasting Static Stretching and Hypertrophy Training on Maximal Strength, Muscle Thickness and Flexibility in the Plantar Flexors

Abstract

Maximal strength measured via maximal voluntary contraction is known as a key factor in competitive sports performanceas well as injury risk reduction and rehabilitation. Maximal strength and hypertrophy are commonly trained by performingresistance training programs. However, literature shows that long-term, long-lasting static stretching interventions can alsoproduce significant improvements in maximal voluntary contraction. The aim of this study is to compare increases in maximalvoluntary contraction, muscle thickness and flexibility after 6 weeks of stretch training and conventional hypertrophy training.Sixty-nine (69) active participants (f= 30, m = 39; age 27.4 ± 4.4 years, height 175.8 ± 2.1 cm, and weight 79.5 ± 5.9 kg) weredivided into three groups: IG1 stretched the plantar flexors continuously for one hour per day, IG2 performed hypertrophytraining for the plantar flexors (5 × 10–12 reps, three days per week), while CG did not undergo any intervention. Maximalvoluntary contraction, muscle thickness, pennation angle and flexibility were the dependent variables. The results of a seriesof two-way ANOVAs show significant interaction effects (p < 0.05) for maximal voluntary contraction (ƞ2 = 0.143–0.32,p < 0.006), muscle thickness (ƞ2 = 0.11–0.14, p < 0.021), pennation angle (ƞ2 = 0.002–0.08, p = 0.077–0.625) and flexibility(ƞ2 = 0.089–0.21, p < 0.046) for both the stretch and hypertrophy training group without significant differences (p= 0.37–0.99,d = 0.03–0.4) between both intervention groups. Thus, it can be hypothesized that mechanical tension plays a crucial role inimproving maximal voluntary contraction and muscle thickness irrespective whether long-lasting stretching or hypertrophytraining is used. Results show that for the calf muscle, the use of long-lasting stretching interventions can be deemed analternative to conventional resistance training if the aim is to increase maximal voluntary contraction, muscle thickness andflexibility. However, the practical application seems to be strongly limited as a weekly stretching duration of up to 7 h a week is opposed by 3 × 15 min of common resistance training

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European Journal of Applied Physiology (2023) 123:1773–1787
https://doi.org/10.1007/s00421-023-05184-6
ORIGINAL ARTICLE
Comparison oftheeffects oflong‑lasting static stretching
andhypertrophy training onmaximal strength, muscle thickness
andflexibility intheplantar flexors
KonstantinWarneke1 · KlausWirth2· MichaelKeiner3· LarsH.Lohmann4· MartinHillebrecht5· AnnaBrinkmann6·
TimWohlann1· StephanSchiemann1
Received: 24 October 2022 / Accepted: 16 March 2023 / Published online: 8 April 2023
© The Author(s) 2023
Abstract
Maximal strength measured via maximal voluntary contraction is known as a key factor in competitive sports performance
as well as injury risk reduction and rehabilitation. Maximal strength and hypertrophy are commonly trained by performing
resistance training programs. However, literature shows that long-term, long-lasting static stretching interventions can also
produce significant improvements in maximal voluntary contraction. The aim of this study is to compare increases in maximal
voluntary contraction, muscle thickness and flexibility after 6weeks of stretch training and conventional hypertrophy training.
Sixty-nine (69) active participants (f = 30, m = 39; age 27.4 ± 4.4years, height 175.8 ± 2.1cm, and weight 79.5 ± 5.9kg) were
divided into three groups: IG1 stretched the plantar flexors continuously for one hour per day, IG2 performed hypertrophy
training for the plantar flexors (5 × 10–12 reps, three days per week), while CG did not undergo any intervention. Maximal
voluntary contraction, muscle thickness, pennation angle and flexibility were the dependent variables. The results of a series
of two-way ANOVAs show significant interaction effects (p < 0.05) for maximal voluntary contraction (ƞ2 = 0.143–0.32,
p < 0.006), muscle thickness (ƞ2 = 0.11–0.14, p < 0.021), pennation angle (ƞ2 = 0.002–0.08, p = 0.077–0.625) and flexibility
(ƞ2 = 0.089–0.21, p < 0.046) for both the stretch and hypertrophy training group without significant differences (p = 0.37–0.99,
d = 0.03–0.4) between both intervention groups. Thus, it can be hypothesized that mechanical tension plays a crucial role in
improving maximal voluntary contraction and muscle thickness irrespective whether long-lasting stretching or hypertrophy
training is used. Results show that for the calf muscle, the use of long-lasting stretching interventions can be deemed an
alternative to conventional resistance training if the aim is to increase maximal voluntary contraction, muscle thickness and
flexibility. However, the practical application seems to be strongly limited as a weekly stretching duration of up to 7h a week
is opposed by 3 × 15min of common resistance training.
Keywords Maximum strength· Resistance training· Mechanical tension· Range of motion· Calf muscle
Abbreviations
MVC Maximal voluntary contraction
ROM Range of motion
MSt Maximal strength
mTOR Mammalian target of rapamycin
p70S6k Protein S6 kinase
IG1 Intervention group 1
IG2 Intervention group 2
CG Control group
Communicated by William J. Kraemer.
* Konstantin Warneke
Konstantin.Warneke@stud.leuphana.de
1 Institute forExercise, Sport andHealth, Leuphana
University, 21335Lüneburg, Germany
2 University ofApplied Sciences Wiener Neustadt,
WienerNeustadt, Austria
3 Department ofSport Science, German University ofHealth
andSport, 85737Ismaning, Germany
4 Institute ofSports Science, Carl von Ossietzky University
ofOldenburg, 26129Oldenburg, Germany
5 University Sports Center, Carl von Ossietzky University
ofOldenburg, 26129Oldenburg, Germany
6 Assistive Systems andMedical Device Technology, Carl
von Ossietzky University ofOldenburg, 26129Oldenburg,
Germany
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1774 European Journal of Applied Physiology (2023) 123:1773–1787
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ICC Intraclass correlation coefficient
CMD Calf muscle device
ORTH Goniometer of the orthosis
KtW Knee-to-wall test
M Mean
SD Standard deviation
ANOVA Analysis of variance
MTh Muscle thickness
LP Leg press
Introduction
While stretch training in humans is commonly used to
improve flexibility, a meta-analysis of animal studies
showed significant hypertrophic effects (Warneke etal.
2022b) with increases in muscle cross-sectional area
of up to 141.6% with d = 5.85 as well as an increase in
maximal strength of up to 95% with d = 12.34 following
chronic stretching for sixweeks. However, evidence for
stretch-mediated hypertrophy and strength increases in
humans is contradictory and scarce. On the one hand,
Nunes etal. (2020) reviewed current literature pointing
out that mostly used stretching durations in humans of
up to twomin per session seem not to be sufficient to
induce hypertrophy. This might be explained by large dif-
ferences regarding training durations (two min per ses-
sion vs. chronic 24h of stretch) as well as muscle protein
synthesis between animals and humans (Garibotto etal.
1997; Sayegh and Lajtha 1989). On the other hand, there
are conflicting results regarding stretch-induced maximal
strength increases in humans probably based on high het-
erogeneity between studies regarding the way in which
the stretch was induced combined with a lack in stating
stretching intensity. While some studies showed signifi-
cant increases in maximal strength in response to long-
term stretching interventions of up to 30min per training
session (Mizuno 2019; Yahata etal. 2021), others were
not able to induce significant changes in strength capacity
following stretching interventions (Nakamura etal. 2021;
Sato etal. 2020). All listed studies were performed includ-
ing participants with a low training status or even with
untrained participants. Since in animal model stretching
durations of up to 24h per day were used (Warneke etal.
2022b), a comparison to human studies performed previ-
ously seems not to be adequate. Thus, it could be assumed
that previous studies in humans may not have used suf-
ficient stretching volume (stretching duration × training
frequency per week) or intensity leading to inconsistent
significant increases (Nakamura etal. 2021; Nunes etal.
2020; Yahata etal. 2021). Based on this, Warneke etal.
(2022a, d) investigated the effects of long-lasting static
stretching interventions of up to twohours per day on
sevendays per week in the plantar flexors of physically
active humans to improve comparability to stretching
durations used in animal studies. The authors determined
significant maximal strength improvements of up to 22%
while—in a different study—significant stretch-mediated
hypertrophy of approximately 15.3% (d = 0.84) could
be induced by using long-lasting static stretch training
of onehour per day, sevendays a week (Warneke etal.
2022a, d). To date, increases in maximal strength and
muscle thickness are commonly associated with resist-
ance training routines (Ralston etal. 2017; Refalo etal.
2021; Schoenfeld etal. 2017). Different authors found
maximal strength increases of 11.9% (d = 0.47) up to
17.0 ± 8.75% (d = 1.0) (Green and Gabriel 2018) as well
as hypertrophic effects via magnetic resonance imaging of
up to 5.2 ± 2.7% (d = 0.3) in young, recreationally active
to moderately trained participants in the lower extremities
within sixweeks (Souza etal. 2014). To achieve improve-
ments in maximal strength, on the one hand, inducing
metabolic stress (Millender etal. 2021) via high training
volume and frequency (Grgic etal. 2018; Ralston etal.
2017) seems to be beneficial. On the other hand, intensity
regulated by mechanical loading seems to be of crucial
importance to achieve maximal strength increases and
hypertrophy (Krzysztofik etal. 2019; Schoenfeld etal.
2015). In resistance training, the morphological and
functional adaptations are accompanied by stimulation
of anabolic signaling pathways such as mTOR/p70S6k
(Lamas etal. 2010; Vissing etal. 2013). Interestingly,
Sasai etal. (2010) as well as Tatsumi (2010) showed the
activation of this pathway due to muscle stretching. Based
on very similar adaptations and underlying physiological
responses, the question arises whether long-lasting stretch
training could be used as an alternative to commonly used
resistance training to induce significant increases in maxi-
mal strengthand muscle thickness.
Consequently, the aim of the present study was to
investigate the effects of long-lasting stretching interven-
tions on maximal strength, muscle thickness and the pen-
nation angle and compare the effects with a commonly
used hypertrophy training program for the calf muscle.
Since enhanced flexibility can be assumed when perform-
ing stretch training (Medeiros etal. 2016) and literature
leads to the assumption that a resistance training using
full range of motion (ROM) could also lead to improve-
ments in flexibility (Afonso etal. 2021), the effects on
ROM of both training interventions will be investigated
as done by Warneke etal. (2022a, d). It was hypothesized
that both interventions, daily long-lasting stretching and a
commonly used resistance training to achieve hypertrophy,
would lead to significant increases in maximal strength,
hypertrophy and flexibility gains, independent of the
respective intervention group.
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1775European Journal of Applied Physiology (2023) 123:1773–1787
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Methods
To compare the effects of a one-hour daily stretch train-
ing with those of a commonly used hypertrophy training,
recreationally active participants were recruited from the
university sports program. They were divided into a stretch
training group (IG1) and a hypertrophy training group
(IG2) performing either a daily long-lasting stretch train-
ing or a resistance training routine which is commonly used
to induce hypertrophy in the plantar flexors. Therefore, a
pre–post-design with a six-week intervention period, incor-
porating two maximal strength tests with extended and
flexed knee joint for the plantar flexors, two flexibility tests
for the range of motionin dorsiflexion of the ankle joint
as well as a sonography assessment to examine changes in
muscle thickness and the pennation angle were performed.
Before testing, a warm-up routine consisting of five min-
utes of bodyweight ergometer cycling with 1 Watt/kg was
performed.
Participants
Ad hoc sample size calculation was performed using d = 0.7
for F-tests with repeated measures and within–between
interaction, based on previous studies (Warneke etal. 2022a)
pointing out a total sample size of at least 36 participants (12
per group). To increase the power of the investigation and
counteract potential dropouts 69 recreationally active and
non-competitive participants from sports study programs
and local sports clubs were recruited. Participants were clas-
sified as novice to recreationally active when they performed
either two or more training sessions per week in a gym or
a team sport in addition to their physical education classes
if they were physical education students or completing at
least three resistance training sessions continuously for the
previous six months. Therefore, participants had some train-
ing experience in resistance training with commonly used
intensity and volume to induce hypertrophy (5 × 10–12 rep-
etitions) as well as in team sports, such as soccer, basketball,
tennis or handball. Participants with an increased risk for
thromboses or serious injury in the lower extremities entail-
ing surgery and immobilization within the past year were
excluded from the study. Consequently, training status was
classified as moderately trained as no untrainedparticipants
as well as no elite sport athletes were included. The par-
ticipants were randomly allocated to the three groups (IG1,
IG2 and CG). If participants had skipped more than three
stretch training sessions or more than two resistance training
sessions, respectively, data would not have been considered
for further evaluation. This was, however, not the case. All
participants were instructed to continue performing their
previous training routines to avoid a decrease in performance
in any group by stopping training. Therefore, the stretching
and hypertrophy training intervention was accompanied by
either the university sports program or the training routine
in the gym the participants were used to. This was also the
case in the control group. Characteristics of the participants
are shown in Table1.
All participants were informed about the experimental
risks and provided written informed consent to participate
in the present study. Furthermore, approval for this study
was obtained from the university’s institutional review
board (Carl von Ossietzky University of Oldenburg, No.
121-2021). The study was performed in accordance with
the Helsinki Declaration.
Testing procedure
Figure1 illustrates the measuring procedure used in both the
pre- and post-test. The study was conducted from March to
August 2022. The post-test was performed at the same time
of day as the pre-test. All testing sessions were performed
between 9am and 5pm. Participants were instructed to eat
a meal latest two hours before testing.
Maximal strength testing
It can be assumed that there are differences in muscle inner-
vation of the triceps surae dependent on the knee joint angle
(Warneke etal. 2022c). Thus, the isometric maximal volun-
tary contraction was assessed using single-leg testing with
extended and flexed knee joint.
Table 1 Characteristics of
participants for overall sample
size and divided into IG1, IG2
and CG
IG1 stretching group, IG2 hypertrophy group, CG control group
Group NAge (in years) Height (in cm) Weight (in kg)
Total 69 (f = 30, m = 39) 27.4 ± 4.4 175.8 ± 2.1 79.45 ± 5.9
IG1 23 (f = 10, m = 14) 27.4 ± 3.1 176.2 ± 5.6 81.0 ± 6.2
IG2 23 (f = 9, m = 13) 26.3 ± 2.6 175.6 ± 4.9 79.3 ± 5.3
CG 23 (f = 11, m = 12) 27.9 ± 6.1 174.4 ± 6.3 79.1 ± 7.0
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1776 European Journal of Applied Physiology (2023) 123:1773–1787
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Maximal isometric strength testing withextended knee
joint
A 50 × 60 cm force plate with ± 5000N and a 13-bit
analog-to-digital converter attached to a 45° leg press was
used to measure the maximal isometric force production
with an extended knee joint. In the starting position (see
Fig.2) the ankle joint angle was set to be 90°. The partici-
pants were instructed to perform a maximal plantar flexion
in response to an acoustic signal and hold the maximal
voluntary contraction for three seconds. After each trial,
participants rested for one minute to avoid fatigue. Meas-
urements were conducted until no improvement in maxi-
mal strength was recorded with a minimum of three tri-
als. For isometric strength measurements, high reliability
(intraclass correlation coefficient = 0.99) can be assumed
(Warneke etal. 2022a).
Fig. 1 Flow chart of the testing
procedure used in pre- and
post-test
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1777European Journal of Applied Physiology (2023) 123:1773–1787
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Maximal isometric strength testing withflexed knee joint
A calf muscle testing device was used to assess maximal
isometric strength with a flexed knee joint. The maximal
strength was determined using a 10 × 10cm force plate with
force sensors “Kistler Element 9251” with a resolution of
1.25N, a pull-in frequency of 1000 Hertz and a measurement
range of ± 5000N. The vertical forces (Fz) were recorded via
a charge amplifier “Typ5009 Charge Amplifier” and a 13-bit
analog-to-digital converter NI6009 (see Fig.3). The partici-
pants were instructed to perform maximal plantar flexion for
three seconds in response to an acoustic signal. Testing was
performed until participants could not improve the achieved
maximal strength values with a minimum of three trials.
High reliability can be assumed using maximal isometric
strength testing (intraclass correlation coefficient = 0.99)
(Warneke etal. 2022d).
Determination ofskeletal muscle architecture
Muscle thickness and pennation angle were measured in the
lateral and medial gastrocnemius using two-dimensional
B-mode ultrasound with a linear transducer (12–13MHz,
Mindray Diagnostic Ultrasound System). The measurement
was conducted with the participant laying in a prone position
with fully extended legs and their feet hanging down at the
end of a table to ensure no contraction in the calf muscles.
The transducer was placed at 25% of the distance between
the most lateral point of the joint space of the knee and the
most lateral tip of the lateral malleolus (Perkisas etal. 2021).
By holding and rotating the transducer around the sagittal-
transverse axis, it was ensured that the superficial and deep
aponeuroses were as parallel as possible to optimize the
visibility of the fascicles as continuous striations from one
aponeurosis to the other (see Fig.4). The transducer was
positioned at the midpoint of each muscle belly perpendicu-
lar to the long axis of the participant’s leg (Sarto etal. 2021).
Both muscle thickness and pennation angle were obtained
by averaging three measurements across the proximal, cen-
tral and distal portions of the acquired ultrasound images
(Franchi etal. 2017; Sarto etal. 2021). Two investigators
performed the image processing independently using Mic-
roDicom (Sofia, Bulgaria). With the measurement device
stated above, the reliability can be classified as high with an
intraclass correlation of 0.88–0.95 (Warneke etal. 2022a).
Fig. 2 Leg press testing device for maximal isometric strength with
extended knee joint (MVC180)
Fig. 3 Calf muscle testing device equipped with force plates to meas-
ure maximal isometric strength with flexed knee joint (MVC90)
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1778 European Journal of Applied Physiology (2023) 123:1773–1787
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Range ofmotionmeasurement
Range of motion in the upper ankle joint was recorded via
the knee-to-wall test and the goniometer on the orthosis.
Range ofmotion testing viaknee‑to‑wall test
A sliding device was used for the knee-to-wall stretch. Par-
ticipants were instructed to place the foot on the attached
marker. The contralateral leg was held in the air and partici-
pants were allowed to stabilize the body with their hands
placed on a doorframe. The participants pushed the board
of the sliding device forward with their knee until the heel
of the standing leg started to lift off. Throughout the test, the
investigator pulled on a sheet of paper placed under the heel
of each participant. The measurement was stopped as soon
as the sheet could be removed. The distance achieved was
read off in cm from the attached measuring tape. Depending
on ankle range of motion, this measurement can be seen as
screening flexibility with a flexed knee joint. Three valid
trials were performed per leg and the furthest distance was
used for evaluation. Range of motion assessment with com-
parable methods can be classified as high with an intraclass
correlation of 0.99 (Warneke etal. 2022a).
Range ofmotion testing viagoniometer oftheorthosis
Range of motion in the ankle with an extended knee joint
was measured via goniometer of the orthosis. For this pur-
pose, the foot of the participant was placed on an object with
the same height as the chair. While the participants were
wearing the orthosis the foot was brought into a maximally
dorsiflexed position keeping the knee joint in an extended
position. The right angle between the lower leg and foot is
classified as neutral 0°. Each big indentation of the goniom-
eter corresponds to an increase in dorsiflexion of 5° and each
little indentation corresponds to an increase of 2.5°. Range
of motion assessments in the ankle joint using a goniometer
can be classified as high with an intraclass correlation coef-
ficient of 0.99 (Warneke etal. 2022a).
Intervention
Stretch training (IG1)
The stretching group (IG1) was instructed to perform
a onehour daily stretch training for the calf muscles for
sixweeks. To realize this long-lasting stretch training, a calf
muscle stretching orthosis was provided (see Fig.5). The
intervention was performed with the dominant leg which
was determined as the leg used when performing single-leg
jumps.
Fig. 4 Sonography to investigate muscle thickness and pennation
angle in the calf muscle
Fig. 5 Orthosis used for calf muscle stretching
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1779European Journal of Applied Physiology (2023) 123:1773–1787
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Subjects were instructed to wear the orthosis with an
extended knee joint. To improve consistency regarding the
used magnitude of stretch, the used ankle angle was quan-
tified by the goniometer of the orthosis. Thus, the stretch
could be replicated and better standardized within the six-
week training intervention. Participants were instructed to
reach a maximally dorsiflexed position with an individual
stretching pain of 7–8 on a visual analog scale of 1–10.
Participants were instructed to sit with their backs straight
against the backrest and place their intervened foot on a sup-
port object at the same height as their chair. All subjects
completed a stretching diary in which the daily stretching
duration as well as the angle of the goniometer were written
down to record the stretch duration and intensity (Fig.5).
Hypertrophy training (IG2)
IG2 was instructed to perform a resistance training routine
commonly used to achieve hypertrophy in the plantar flex-
ors. Participants performed calf muscle hypertrophy train-
ing with an extended knee joint on a 45° leg press with five
sets of 10–12 repetitions on three non-consecutive days per
week. Training sessions lasted about 15min. The inter-set
rest was 90s with the instruction to perform each set over
full range of motion until failure. If more than 12 repeti-
tions were accomplished, more weight was added. When a
participant was not able to manage ten repetitions, the load
was reduced. Participants had to complete a training diary
in which training day and load were documented.
Statistical analysis
The analysis was performed with SPSS 28 (IBM, Armonk,
New York, USA). Data is provided as mean (M) ± stand-
ard deviation (SD) for the pre–post values. The normal
distribution of data was checked via Shapiro–Wilk test.
Reliability was determined and is provided with intra-
class correlation coefficient, coefficient of variability and
95% confidence interval (CI) for aforementioned tests (see
Table2). 95% CI for intraclass correlation coefficients
and the coefficient of variability are interpreted consid-
ering the general guidelines by Koo and Li (2016): poor
reliability ≤ 0.5, moderate reliability = 0.5–0.75, good reli-
ability ≥ 0.75–0.9, excellent reliability ≥ 0.9. Reliability for
sonography was determined between best and second-best
value as the “within day” reliability (see Table2). Two
investigators evaluated the ultrasound images independently
from one another to ensure inter-rater reliability. Moreover,
Levene’s test for homogeneity in variance was performed.
A one-way ANOVA was used to rule out significant differ-
ences between groups in pre-test values. A series of two-way
ANOVAs with repeated measures was performed for data
analyses of the pre-post comparisons. To investigate the dif-
ferences in increases between the intervention groups and
the control group, the Scheffé test was used as post hoc test.
Effect sizes are presented as Eta squares (ƞ2) and categorized
as: small effect ƞ2 < 0.06, medium effect ƞ2 = 0.06–0.14, high
effect ƞ2 > 0.14 (Cohen 1988). Additionally, effect sizes are
reported with Cohen’s d (Cohen 1988) and categorized as:
small effects d < 0.5, medium effect d = 0.5–0.8, high effect
d > 0.8. The level of significance was set to p < 0.05. Pear-
son correlations were calculated for pre–post comparisons
in maximal strength and muscle thickness.
Results
Results of reliability are shown in Table2. Descriptive statistics
for maximal strength and flexibility are provided in Table3 and
descriptive statistics for muscle thickness and pennation angle
are listed in Table4. All data were normally distributed.
The evaluation of pre-test group differences showed
no significance between groups (F = 0.161–1.699,
p = 0.191–0.813).
Table3 shows the descriptive statistics of the maximal
strength and flexibility assessment in plantar flexion.
Maximal strength analysis
Figure6 illustrates changes in maximal strength using the
maximal strength measurement with extended and flexed
knee joint for the intervened leg.
Plantar flexor maximal voluntary contraction
withextended knee joint
Results for maximal strength measured using the maximum
voluntary contraction in the plantar flexors with extended
Table 2 Reliability for the pre-test values
MVC maximal voluntary contraction, KtW knee-to-wall test, ORT
range of motion measurement with orthosis, SONO measurement
of muscle thickness via sonography, Pa Pennation angle, 180 MVC
measured with extended knee joint, 90 MVC measured with flexed
knee joint, L lateral head of the gastrocnemius, M medial head of the
gastrocnemius
Parameter ICC (95%-CI) CV (95%-CI) in%
MVC180 0.984 (0.978–0.989) 1.72 (1.44–2.01)
MVC90 0.983 (0.976–0.988) 1.97 (1.66–2.33)
KtW 0.991 (0.984–0.995) 0.94 (0.35–1.59)
ORT 0.992 (0.981–0.995) 0.64 (0.22–1.19)
SONOL 0.876 (0.83–0.91) 5.21 (4.4–6.15)
SONOM 0.917 (0.885–0.94) 3.5 (2.96–4.07)
PaL 0.878 (0.833–0.912) 6.64 (5.64–7.74)
PaM 0.81 (0.743–0.861) 6.49 (5.2–7.98)
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1780 European Journal of Applied Physiology (2023) 123:1773–1787
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knee joint showed high, significant increases with a time
effect of ƞ2 = 0.572, p < 0.001 and a significant time × group
interaction (ƞ2 = 0.319, p < 0.001). Post hoc testing pointed
out no significant differences for increases from pre- to post-
test between the stretching group (IG1) and the hypertrophy
training group (IG2) (p = 0.387, d = 0.4) but differences in
favor of the intervention groups between the stretching group
(IG1) and the control group (CG) (p < 0.001, d = 1.17) as
well as between the hypertrophy training group (IG2) and
the control group (CG) (p < 0.001, d = 0.9). Therefore, no
change in the control group but significant increases in both
intervention groups were obtained.
Plantar flexor maximum voluntary contraction withflexed
knee joint
Results for maximal strength in the plantar flexors meas-
ured with flexed knee joint also showed a high, significant
increase with a time effect of ƞ2 = 0.282, p < 0.001 and a
significant time × group interaction (ƞ2 = 0.143, p = 0.006).
Furthermore, post hoc testing pointed out no significant
difference for the increases in maximal strength between
the stretching group (IG1) and the hypertrophy training
group (IG2)(p = 0.986, d = 0.05). There were differences
in favor of the intervention groups with moderate effect
Table 3 Descriptive statistics and results of two-way ANOVA for maximal strength and ROM
IG1 stretching group, IG2 hypertrophy training group, CG control group, MVC maximal voluntary contraction, KtW ROM Measurement via
knee-to-wall test, ORT range of motion measurement via goniometer of the orthosis, 180 MVC testing in extended knee joint, 90 MVC testing in
flexed knee joint
Parameter Pretest (M ± SD) in NPost-test (M ± SD) in NPre-Post-Diff. in % Time effect Time × group
IG1MVC180 1522.61 ± 310.25 1796.78 ± 368.08 + 18.00 p < 0.001 p < 0.001
IG2MVC180 1594 ± 321.78 1807.8 ± 361.11 + 13.36 F = 88.26 F = 15.49
CG 1557.05 ± 284.46 1585.57 ± 292.04 + 1.8 ƞ2 = 0.57 ƞ2 = 0.32
IG1MVC90 1314.7 ± 305.79 1440.61 ± 332.67 + 9.58 p < 0.001 p = 0.006
IG2MVC90 1371.8 ± 289.45 1508.44 ± 258.7 + 9.96 F = 25.908 F = 5.51
CG 1334.76 ± 235.36 1340.33 ± 205.81 + 0.42 ƞ2 = 0.28 ƞ2 = 0.14
IG1KtW 11.72 ± 2.52 12.98 ± 2.55 + 10.75 p < 0.001 p = 0.046
IG2KtW 12.26 ± 2.1 13.36 ± 2.31 + 8.97 F = 48.96 F = 3.24
CG 11.71 ± 12.17 12.17 ± 2.0 + 3.93 ƞ2 = 0.43 ƞ2 = 0.09
IG1ORT 8.35 ± 2.08 9.39 ± 1.41 + 12.46 p < 0.001 p < 0.001
IG2ORT 7.92 ± 1.637 8.64 ± 1.31 + 9.09 F = 39.37 F = 8.85
CG 8.17 ± 1.25 8.21 ± 1.03 + 0.49 ƞ2 = 0.37 ƞ2 = 0.21
Table 4 Descriptive statistics of muscle thickness and the pennation angle
IG1 stretching group, IG2 hypertrophy training group, CG control group, MThL muscle thickness in the lateral head of gastrocnemius, MThM
muscle thickness in the medial head of gastrocnemius, PaL pennation angle in the lateral head of the gastrocnemius, Pa M pennation angle in the
medial head of the gastrocnemius
Parameter Pretest (M ± SD) in NPost-test (M ± SD) in NPre-Post-Diff. in % Time effect Time × group
IG1MThL 14.53 ± 2.43 15.21 ± 2.11 + 4.68 p < 0.001 p = 0.021
IG2MThL 14.83 ± 2.91 16.09 ± 3.35 + 8.5 F = 15.51 F = 4.08
CG 14.33 ± 2.48 14.40 ± 2.32 + 0.49 ƞ2 = 0.19 ƞ2 = 0.11
IG1MThM 19.55 ± 2.59 21.06 ± 2.88 + 7.72 p < 0.001 p = 0.006
IG2MThM 19.25 ± 3.47 20.87 ± 3.09 + 8.42 F = 19.46 F = 5.48
CG 18.49 ± 3.13 18.41 ± 2.87 −0.43 ƞ2 = 0.23 ƞ2 = 0.14
IG1PaL 13.39 ± 2.33 13.49 ± 2.73 + 0.75 p = 0.549 p = 0.625
IG2PaL 14.14 ± 2.91 14.59 ± 2.28 + 3.18 F = 0.36 F = 0.47
CG 12.67 ± 2.86 12.55 ± 2.76 −0.95 ƞ2 = 0.01 ƞ2 = 0.02
IG1PaM 17.32 ± 4.07 19.46 ± 3.24 + 12.3 p < 0.001 p = 0.077
IG2PaM 16.92 ± 3.18 19.07 ± 3.04 + 12.71 F = 12.81 F = 2.66
CG 16.51 ± 3.92 16.62 ± 3.67 + 0.67 ƞ2 = 0.16 ƞ2 = 0.08
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1781European Journal of Applied Physiology (2023) 123:1773–1787
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sizes between the stretch training group (IG1) and the con-
trol group (CG) (p = 0.029, d = 0.6) as well as between the
hypertrophy training group (IG2) and the control group
(CG) (p = 0.013, d = 0.651). Therefore, the results show
significant increases in both intervention groups without
any significant change in the control group.
Range ofmotion analysis
Range ofmotion viaknee‑to‑wall stretch
Results of the knee-to-wall test demonstrated high, signifi-
cant increases with a time effect of ƞ2 = 0.426, p < 0.001
and a time × group interaction (ƞ2 = 0.169, p = 0.046). Post
hoc testing showed no significant differences between the
increases of the stretching (IG1) and hypertrophy training
group (IG2) with p = 0.882, d = 0.24, while there were mod-
erate magnitudes in effect sizes for differences in favor of
the intervention groups between the stretching group (IG1)
and the control group (CG) (p = 0.062, d = 0.53) as well as
between the hypertrophy training group (IG2) and the con-
trol group (CG) (p = 0.152, d = 0.42), showing increases in
all groupswithout a significant difference between groups..
Range ofmotion viagoniometer oftheorthosis
Furthermore, there was a high, significant increase in the
flexibility measured with the goniometer of the orthosis with
a time effect of ƞ2 = 0.374, p < 0.001 and a significant, high
time × group interaction (ƞ2 = 0.212, p < 0.001). Post hoc
testing determined no significant difference for the increases
between the stretching (IG1) and the hypertrophy training
group (IG2) (p = 0.378, d = 0.38). There were significant
differences in favor of the intervention groups between the
stretching group (IG1) and the control group (CG) (p < 0.001,
d = 0.9) and the hypertrophy training group (IG2) and the
control group (CG) (p = 0.022, d = 0.61), showing no signifi-
cant change in the control group, while there were significant
range of motion increases in both intervention groups.
Muscle thickness andpennation angle analyses
Table4 shows the descriptive statistics for muscle thick-
ness and the pennation angle in the lateral and medial
gastrocnemius.
Muscle thickness inlateral andmedial head
ofthegastrocnemius
Figure7 illustrates changes in the muscle thickness meas-
ured via sonography in the lateral and medial gastrocnemius
in all three groups.
Results for muscle thickness measurement in the lateral
head of the gastrocnemius showed a significant increase
from pre- to post-test with a time effect of ƞ2 = 0.19,
p < 0.001 with a moderate, significant interaction effect
(time × group, ƞ2 = 0.11, p = 0.021). In the medial head of
the gastrocnemius, there was a high, significant increase
in muscle thickness showing a time effect of ƞ2 = 0.228,
p < 0.001 with a significant time x group interaction
(ƞ2 = 0.142, p = 0.006).
For the lateral head of the gastrocnemius, post hoc
testing pointed out significant differences in favor
of the intervention group(IG2) with moderate effect
sizes between the hypertrophy training group and (IG2)
Fig. 6 Comparison of maximal strength from pre- to post-test in the
stretching group (IG1), the hypertrophy training group (IG2) and
the control group (CG) with extended (a) and flexed knee joint (b).
** indicates a significant increase compared to the control group of
p < 0.001, * indicates a significant increase compared to the control
group of p < 0.05
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1782 European Journal of Applied Physiology (2023) 123:1773–1787
1 3
and the control group (CG) (p = 0.021, d = 0.61) but
no significant differences could be observed between
the stretching group (IG1) and the control group (CG)
(p = 0.36, d = 0.32) and the stretching group (IG1)
and the hypertrophy training group (IG2) (p = 0.37,
d = 0.32). Therefore, no significant increase was found
for the stretching group compared with the control
group, while there was a significantly greater increase
in the muscle thickness of the lateral head in the IG2.
In the medial head of the gastrocnemius, no signifi-
cant difference was found between the stretching (IG1)
and the hypertrophy training group (IG2) (p = 0.979,
d = 0.03), however, there were significant differences
in favor of the intervention groups with moderate effect
sizes between the stretching group (IG1) and the control
group (CG) (p = 0.027, d = 0.6) as well as between the
hypertrophy training group (IG2) and the control group
(CG) (p = 0.014, d = 0.646), showing significant hyper-
trophy in IG1 and IG2 without a difference between the
groups, while no significant changes could be obtained
in the control condition.
Individual progressions of the listed parameters are illus-
trated in separate figures in the supplemental material.
Pennation angle inthelateral andmedial head
ofthegastrocnemius
For the pennation angle in the lateral head of the gastrocne-
mius, no significant increase from pre- to post-test could be
observed (time effect of p = 0.549, ƞ2 = 0.006, time × group
interaction p = 0.625, ƞ2 = 0.015). In the medial head of
the gastrocnemius, there was a high, significant time effect
(p < 0.001, ƞ2 = 0.163), however, no significant time × group
interaction (p = 0.077, ƞ2 = 0.075) could be found.
Discussion
The present study compared the effects of a one hour daily
stretching intervention in the plantar flexors with a com-
monly used hypertrophy training routine over a period of six
weeks. Results showed an increase in maximal strength with
moderate to high effects (ƞ2 = 0.143–0.572, d = 0.6–1.17,
p < 0.001–0.006), low to moderate effects for increases
in muscle thickness (ƞ2 = 0.11–0.228, d = 0.32–0.65,
p < 0.001–0.021) as well as low to high effects for
increases in flexibility (ƞ2 = 0.089–0.426, d = 0.42–0.9,
p < 0.001–0.046) irrespective of performing a commonly
used hypertrophy training or long-lasting stretching for
the calf muscle. The control group exhibited no signifi-
cant changes in any measured value. Results showed that
there was no significant difference in adaptations between
the stretching and hypertrophy training group regarding
increases in maximal strength, muscle thickness and flex-
ibility (p = 0.37–0.99, d = 0.03–0.4). Therefore, performing
stretch training can be assumed to provide a sufficient stimu-
lus to increase maximal strength and hypertrophy in the calf
muscle if performed with adequate training volume (stretch
duration × weekly frequency), which is comparable to adap-
tations of commonly used resistance training.
Previous studies were able to show stretch-mediated
strength increases as well. Nelson etal. (2012) and Yahata
etal. (2021) pointed out improvements in maximal strength
of up to 29% (d = 1.24) and 6.6% (d = 0.35) using lower
stretching durations of4 x 30s three times per week and
30min per session two times per week, respectively. Con-
sidering a stretch-induced increase in maximal strength of
29% by using 4 × 30s of stretching, the included participants
should be stated as untrained, as listed increases would be
higher as expectable effects of resistance training programs.
Fig. 7 Muscle thickness comparison from pre- to post-test in the
stretching group (IG1), the hypertrophy training group (IG2) and the
control group (CG) in the lateral (a) and medial (b) gastrocnemius.
** indicates a significant increase compared to the control group of
p < 0.001, * indicates a significant increase compared to the control
group of p < 0.05
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1783European Journal of Applied Physiology (2023) 123:1773–1787
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Since Nelson etal. (2012) described their participants as
physically inactive or “minimally recreationally active” by
performing training less than five times per months for less
than 60min per session, the training level of the partici-
pants included in the present studymust be considered as
significantly higher. While Nunes etal. (2020) reviewed
current literature pointing out no significant influence of
stretch training on hypertrophy, the only studies that used
long-lasting stretching (> 30 min of stretch per session)
with a daily frequency showed significant, stretch-mediated
hypertrophy and maximal strength increases (Warneke etal.
2022a), comparable with previous animal studies (Kelley
1996; Warneke etal. 2022b).
In animal studies (Frankeny etal. 1983) and also in
human studies (Warneke etal. 2022a, c; Yahata etal. 2021)
higher adaptations were found by increasing stretching
duration and volume. Since in resistance training, previ-
ous authors pointed out increases in strength capacity of
about 17.0 ± 8.75% (d = 1.0) (Green and Gabriel 2018; Grgic
etal. 2018) and Warneke etal. (2022a) showed comparable
increases in strength and muscle thickness in response to
one hour of daily stretching, these long durations seem to
be necessary to achieve an adequate stimulus.
It is well known that mechanical tension (intensity)
plays a crucial role in physiological adaptations when
aiming to induce hypertrophy but especially for maxi-
mal strength improvements, which is accompanied by a
stimulation of anabolic signaling pathways (Schoenfeld
etal. 2015; Wackerhage etal. 2019). Literature points
out the possibility to induce similar mechanical tension
and therefore anabolic signaling due to the activation of
so-called stretch-activated channels (Suzuki and Takeda
2011), resulting in stimulating mTOR signaling pathways
(Tyganov etal. 2019). Therefore, increases in maximal
strength are possibly explained by mechanical tension-
induced adaptations which one could speculate to be
similar to adaptations of a common hypertrophy training,
including increases in muscle quality, muscle thickness and
architecture and/or elongation of the muscle-tendon unit.
Accordingly, in animal studies, Devol etal. (1991) referred
to mechanical tension per sarcomere as an important fac-
tor to induce stretch-related responses in the muscle, how-
ever, in humans the underlying physiological processes of
stretch-activated increases in maximal strength and muscle
thickness remain unclear. A previous study (Warneke etal.
2022a) found no relationship between increases in muscle
thickness and maximal strength.
Noticeable, even though there are several similarities
regarding the adaptations over the six-week period fol-
lowing stretching and hypertrophy training in the results
reported in this study (regarding maximal strength, muscle
thickness and flexibility), it can be assumed that resistance
training would lead to further health-related benefits, such as
improved cardiovascular function (Schjerve etal. 2008; Yu
etal. 2016) and bone mineral density (Westcott 2012). To
this point, it remains unclear whether and to which extend
long-lasting stretching would be effective concerning health-
related parameters.
It is well known that neuronal factors play an essential
role in maximal strength increases in the first weeks of
training (Del Vecchio etal. 2019) while structural adap-
tations might play a secondary role (Gabriel etal. 2006).
Consequently, it can be assumed that enhanced strength
capacity could be primally explained by neuronal changes.
The potential neuromuscular adaptations leading to stretch-
mediatedincreases in maximal strength capacity still remain
unclear. Holly etal. (1980) pointed out that no significant
increase in central nervous activity was found when induc-
ing long-term stretching in animal models, while Sola etal.
(1973) pointed out significant stretch-mediated hypertrophy
even if the muscle was previously denervated. Therefore,
further investigations are requested to clarify the physi-
ological mechanism of stretch-induced maximal strength
increases. In contrast, benefits of central nervous innervated
muscle contraction such as motor learning effects can be
hypothesized to occur in a lower magnitude compared to
active training protocols.
However, even though transferability of results from
animal research should be considered carefully, in animal
model the morphological adaptations are investigated more
frequently, pointing out a serial accumulation of sarcomeres
in response to chronic stretching interventions even after a
few days (Antonio etal. 1993) which could also be respon-
sible for increased muscle mass and, due to optimizing the
length–tension relationship, for changes in force production
capability of the muscle. Hypothesizing a general transfer-
ability to humans, these adaptations could also indicate
changes in muscle morphology which could contribute to
significant maximal strength increases. Furthermore, since
an increased muscle thickness was measured, an enhance-
ment in the pennation angle was reasonably hypothesized
(Cormie etal. 2011). Accordingly, the pennation angle
seems to increase with enhancement in muscle thickness in
both groups. This may also be responsible for improvements
in maximal strength as an increase in the number of contrac-
tile filaments in parallel and a higher strength capacity can
be assumed (Cormie etal. 2011).
However, even without a significant difference between
the stretching group (IG1) and the hypertrophy training
group (IG2) the comparatively high time-effort of the stretch
training should be considered, as the time spent with training
for IG1 was long compared with IG2. While IG2 performed
their training routine within a weekly duration of about
45min (3 × 15min), IG1 had to stretch the plantar flexors
for up to seven hours per week. Furthermore, the stretch-
ing group performed their training routine more frequently
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1784 European Journal of Applied Physiology (2023) 123:1773–1787
1 3
(sevendays per week) than the hypertrophy training group
(three times per week). Even with (non-significant) higher
increases in maximal strength in the stretching group, the
time-effort of this group can be assumed to be unpropor-
tionally high compared with the hypertrophy training group.
However, the training of IG1 could be integrated in the daily
life or prolonged times of immobilization, which was not
possible for IG2, as the hypertrophy orientated training pro-
tocol required a leg press machine.
It is well accepted that performing stretch training
results in improved flexibility (Medeiros and Lima 2017).
There are many hypotheses trying to explain increases
in range of motion after a stretch training. While authors
hypothesize an increased tolerance of stretching tension
via a reduced pain sensitivity (Freitas etal. 2018), animal
models show evidence of structural adaptations by a serial
accumulation of sarcomeres (Antonio and Gonyea 1993).
However, when resistance training is performed over full
range of motion, improvements in range of motion can
be assumed as well (Afonso etal. 2021). There are many
theories explaining the increases in muscle flexibility and
joint range of motion, pointing out neuromuscular changes
(Freitas etal. 2018; Freitas and Mil-Homens 2015) and
structural changes in the muscle–tendon unit and reduc-
tion in passive peak torque (Moltubakk etal. 2021; Naka-
mura etal. 2017). The described increased number of
serialsarcomeres in animals(Antonio etal. 1993; Warneke
etal. 2022b)was, to the best knowledge, not confirmed in
humans.
In the supplemental material, the individual progres-
sions were reported for the significant results of this study,
showing no difference in consistency of the increment of
maximal strength between stretch-mediated hypertrophy and
resistance training-induced hypertrophy as well as maximal
strength increases (Suppl. Fig. A–F). Since most previous
studies were performed with untrained participants, this
study was conducted with (recreationally) active partici-
pants, showing a comparatively wide range of strength and
flexibility level as well as in muscle thickness. Although
lower adaptations can be assumed in trained participants,
the stretch-mediated hypertrophy was also effective in par-
ticipants with higher strength levels and/or muscle thickness.
However, since the study was conducted over a period of
only sixweeks, investigations using longer training dura-
tions are requested to exclude strong adaptations because of
an unfamiliar training stimulus.
Limitations
Since testing of maximal strength was performed under iso-
metric conditions, higher increases in the stretching group
might be explained with contraction-specificity because of
proximity to the intervention stimulus (Lanza etal. 2019).
To improve comparability to dynamic conditions, dynamic
one repetition maximum testing should be included in
future testing as hypertrophy training of IG2 was performed
dynamically but tested under isometric conditions. There
is limited transferability of isometric strength to one repe-
tion maximum measurements (Murphy and Wilson 1996).
In contrast to maximal strength increases, there was higher
hypertrophy in the gastrocnemius in the resistance training
compared to the stretching group. This may be explained due
to the use of different joint angles and, therefore, used stim-
uli in different muscle length while stretching used maximal
range of motion only. In both groups, the interventions seem
to be more effective for increases in muscle thickness of the
medial head of the gastrocnemius. To rule out adaptations
based on an unfamiliar stimulus or only adaptations in the
first phase of training, investigations examining longer inter-
vention periods are requested. As this study compared the
effects of a onehour daily stretching routine to the effects of
a hypertrophy training using 5 × 10-12 repetitions performed
three times per week, obviously, the time under tension as
well as the intensities cannot be compared with one another.
However, this was not the aim of this study as the effects of
two different training routines are contrasted. Furthermore,
inconsistency in the wording to describe the training status
of included participants throughout the studies should be
considered when interpretating the results of these studies.
No statement can be given about the effects in highly trained
participants, as no previous research investigated long-last-
ing stretching in elite athletes.
Furthermore, ultrasound imaging to investigate hypertro-
phy following training interventions seems to be biased by
limited objectivity and a lack of accuracy (Warneke etal.
2022e). Therefore, using magnetic resonance imaging meas-
urements to confirm morphological adaptations should be
considered in future study designs.
In general, there is no “real” quantification of stretching
intensity in many studies in humans. Using stretching pain
as an indicator for stretch intensity seems to be biased, as
Lim and Park (2017) pointed limited correlations between
stretching pain and passive peak torque. Assuming mechani-
cal tension is of crucial importance for adaptations in maxi-
mal strength and hypertrophy, the passive torque of the mus-
cle should be considered as relevant. Therefore, no studies
could be found addressing the effects of different intensities
which could be of high impact for the practicability of the
stretching routine, since it might be hypothesized that using
higher intensities could reduce the required stretching dura-
tion to reach comparable adaptations.
Lastly, the influence of training level, sex and age was
not investigated in this study. However, the sex-dependent
adaptations were previously investigated by Warneke etal.
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1785European Journal of Applied Physiology (2023) 123:1773–1787
1 3
2023. To investigate further independent variables’ influence
such as age and training level, a more heterogeneous group
of participants should have been included to the study.
Practical applications
Results point outlong-lasting stretch training (onehour daily,
high elongation stress)as apromisingalternativetoresist-
ancetraining (e.g., hypertrophy training) in different set-
tings over a six-week period, especially if commonly
usedresistancetraining is contraindicated, e.g., after injury
and surgery. There are some advantages of long-lasting stretch
training for athletes and patients to perform their training rou-
tine independent of training equipment like the leg press or
calf muscle machines which are required for traditionalresist-
ancetraining of the plantar flexorsto achieve hypertrophy.
Outlook
Long-lasting stretching interventions produced significant
hypertrophy and maximal strength gains in animal studies
(Antonio etal. 1993; Bates 1993; Warneke etal. 2022b).
In humans, more evidence regarding long-lasting stretch-
ing interventions and its impact on maximal strength and
muscle thickness is required. Even though Nunes etal.
(2020) showed that short-lasting stretching is not sufficient
to induce hypertrophy, previous research shows that long-
lasting stretching interventions can induce sufficient tension
to improve maximal strength, range of motionand muscle
thickness (Warneke etal. 2022a). The present study also
showed significant increases over a six-week period in the
measured parameters which are comparable to those of a
commonly used resistance training in the plantar flexors.
Since significant decreases in strength capacity, flexibility
as well as muscle thickness due to immobilization (Stevens
etal. 2004) after injury and/or surgery can be assumed, the
results of this study are promising as a method with high
potential in rehabilitation of orthopedic indications. There-
fore, studies including clinical trials and older participants
should be performed. To investigate the underlying physio-
logical adaptations leading to increased strength capacity as
well as hypertrophy, neuromuscular adaptations (for exam-
ple via EMG) as well as further morphological adaptations
should be addressed in further studies.
Supplementary Information The online version contains supplemen-
tary material available at https:// doi. org/ 10. 1007/ s00421- 023- 05184-6.
Author contributions KW1 and KW2 developed the idea of the study.
KW1 carried out the experiment with the help of LHL and TW. Statisti-
cal analysis was performed by KW1 and MK. KW1 and AB determined
the muscle thickness from the sonography measurement. MH provided
the measurement devices and programs. KW1 wrote the first draft of
the manuscript which was discussed and reworked by KW1, KW2,
MK, and SS. Results were discussed by all authors. SS supervised
the study.
Funding Open Access funding enabled and organized by Projekt
DEAL.
Data availability Data can be provided by the corresponding author
due to reasonable request.
Declarations
Conflict of interest No involved authors declares a conflict of interest.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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