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Does Stretching Training Influence Muscular Strength? A Systematic Review
With Meta-Analysis and Meta-Regression
Article in The Journal of Strength and Conditioning Research · August 2022
DOI: 10.1519/JSC.0000000000004400
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Brief Review
Does Stretching Training Influence Muscular
Strength? A Systematic Review With Meta-Analysis
and Meta-Regression
Ewan Thomas,1 Salvatore Ficarra,1 João Pedro Nunes,2 Antonio Paoli,3 Marianna Bellafiore,1 Antonio Palma,1
and Antonino Bianco1
1
Sport and Exercise Sciences Research Unit, Department of Psychology, Educational Science and Human Movement, University of
Palermo, Palermo, Italy; 2Metabolism, Nutrition, and Exercise Laboratory, Physical Education and Sport Center, Londrina State
University, Londrina, Brazil; and 3Department of Biomedical Sciences, University of Padova, Padova, Italy
Abstract
Thomas, E, Ficarra, S, Nunes, JP, Paoli, A, Bellafiore, M, Palma, A, and Bianco, A. Does stretching training influence muscular
strength? A systematic review with meta-analysis and meta-regression. J Strength Cond Res XX(X): 000–000, 2022—This aim of
this study was to review articles that performed stretching training and evaluated the effects on muscular strength. Literature search
was performed using 3 databases. Studies were included if they compared the effects on strength following stretching training vs. a
nontraining control group or stretching training combined with resistance training (RT) vs. an RT-only group, after at least 4 weeks of
intervention. The meta-analyses were performed using a random-effect model with Hedges’ g effect size (ES). A total of 35 studies
(n 5 1,179 subjects) were included in this review. The interventions lasted for a mean period of 8 weeks (range, 4–24 weeks), 3–4
days per week, applying approximately 4 sets of stretching of approximately 1-minute duration. The meta-analysis for the stretching
vs. nontraining control group showed a significant small effect on improving dynamic (k 5 14; ES 5 0.33; p 5 0.007) but not
isometric strength (k 5 8; ES 5 0.10; p 5 0.377), following static stretching programs (k 5 17; ES 5 0.28; p 5 0.006). When
stretching was added to RT interventions, the main analysis indicated no significant effect (k 5 17; ES 5 20.15; p 5 0.136);
however, moderator analysis indicated that performing stretching before RT sessions has a small but negative effect (k 5 7; ES 5 2
0.43; p 5 0.014); the meta-regression revealed a significant negative association with study length (b 5 20.100; p 5 0.004).
Chronic static stretching programs increase dynamic muscular strength to a small magnitude. Performing stretching before RT and
for a prolonged time (.8 weeks) can blunt the strength gains to a small-to-moderate magnitude. Performing stretching in sessions
distant from RT sessions might be a strategy to not hinder strength development.
Key Words: stretch, resistance training, flexibility, interset stretch, PNF, muscle stretching exercises
and isotonic contractions, jumps, and running tests. The authors
concluded that the influence of stretching on performance was unclear and stated that they were unable to perform a meta-analysis
because of the high heterogeneity in the results among the available
literature. Since the publication of this review, multiple studies were
subsequently published concerning the long-term effects of stretch
training, alone or combined with resistance exercises.
Stretching exercises have been increasingly included in resistance training (RT) programs within strength and conditioning
and rehabilitation settings; however, very few investigations have
evaluated the effects of these combined forms of exercise on
strength outcomes. Recent works posited that combining
stretching exercises with RT could potentiate the improvements
in muscle size, although this effect requires that stretch is placed
within the interset rest period (59,71). A more recent study
compared traditional RT vs. interset rest stretching RT protocols
and did observe a small advantage in muscle thickness for the
stretching group but no significant differences in strength gains
between the groups (18). Other groups have also tried to explore
the effects of stretching combined with RT on measures of
strength and observed conflicting results (8,64,81). However,
dissimilarities regarding study designs and exercise protocols
(e.g., stretch typology, duration, and timing) may explain the
dissimilar outcomes.
Introduction
In the past decades, research has explored the effects of acute
stretching on the performance of strength and power activities
(4,73). The inclusion of stretching exercises as part of workout
programs was advocated to improve the tolerance to a stretch and
therefore allow free movements and enhance performance (24,83).
However, recent works suggest that these effects may vary according
to stretch typology, duration, and timing of application, where short
dynamic stretches seem to induce positive effects on acute performance outcomes (72), whereas long-lasting static stretches may result in a small impairment on strength (5,9,54,72,84).
Despite the broad evidence regarding acute stretching, the
amount of studies exploring the effects of chronic stretching interventions on strength is limited. In a systematic review conducted in
2017, the effects of chronic stretching on general exercise performance were analyzed (55). The review took into account the influence of flexibility on performance in functional tasks, isometric
Address correspondence to Ewan Thomas, ewan.thomas@unipa.it.
Supplemental digital content is available for this article. Direct URL citations appear
in the printed text and are provided in the HTML and PDF versions of this article on
the journal’s Web site (http://journals.lww.com/nsca-jscr).
Journal of Strength and Conditioning Research 00(00)/1–12
ª 2022 National Strength and Conditioning Association
1
Copyright © 2022 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
The Effects of Stretching on Strength (2022) 00:00
search was performed using 3 online databases: Pubmed (NLM),
Scopus, and Web of Science. Reference lists and GoogleScholar citations of included studies and previous relevant reviews were also
examined. The full search strategy used on each database is presented in Supplemental Digital Content (see Supplementary Material
A, http://links.lww.com/JSCR/A362). The PRISMA flow diagram
illustrates the process by which the articles were selected (Figure 1).
Within this context, it is still unclear whether chronic stretching
may influence muscular strength. Therefore, this systematic review with meta-analysis and meta-regression aimed to examine
the available literature on the effects of chronic stretching interventions performed alone or combined within RT programs on
measures of muscular strength.
Methods
Experimental approach to the problem The rationale for this review with meta-analysis was to collect and synthesize studies
which aimed to address the effects of stretching on measures of
strength. Further, we will evaluate if specific exercise parameters
as stretch typology, stretch duration, the timing of the stretch
administration and duration of the intervention may have a potential effect on measures of strength. The results of this review
are intended to help trainers and practitioners in designing and
improving exercise prescriptions which include stretching alone
or combined with resistance training.
Inclusion and Exclusion Criteria
Studies were included for review if they fulfilled the following
selection criteria: (a) Published between January 1991 and December 2021; (b) published in English and peer-review journals;
(c) examined the effects of stretching vs. control on strength
(nontraining or RT-only control groups); (d) had a training intervention of at least 4-week duration; (e) had at least one measure of muscular strength assessed before and after intervention;
and (f) involved subjects with no known medical conditions, injury, or performing competitive sports. Any validated measure of
muscular strength was accepted, such as laboratory-based tests
(i.e., isokinetic concentric, eccentric, or isometric peak torque) or
field-based tests (i.e., performance on repetition maximum [RM]
tests) (70). The systematic search of the literature was performed
by 2 investigators (E.T. and S.F.), who eventually resolved disagreements about article inclusion by a discussion with a third
investigator (A.B.). Abstracts and unpublished material were not
Search Strategy
The Preferred Reporting Items for Systematic Reviews and MetaAnalyses (PRISMA) guidelines for conducting a systematic review
were adopted (60). The research questions were defined by the
PICOS model in accordance with PRISMA guidelines. A literature
Figure 1. PRISMA flow chart illustrating the article selection process. PRISMA 5 Preferred Reporting Items
for Systematic Reviews and Meta-Analyses.
2
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The Effects of Stretching on Strength (2022) 00:00
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(b) the timing of which the stretching exercises were performed in
relation to the RT exercises (i.e., before, after or apart, or within
the RT session in the interset rest period of RT exercises); and (c)
the influence of the length of interventions by means of metaregression analyses. Total time under stretching was calculated as
the number of training weeks 3 number of sessions per week 3
number of stretch sets of each session 3 time of each stretching
set (71).
Meta-analyses were performed using the Comprehensive
Meta-analysis software (version 2; Biostat Inc., Englewood, CO).
For each analysis, the effect size (ES) was calculated as the difference between posttest and pretest scores of the stretching group
minus the difference between posttest and pretest scores of the
control group, divided by the average pretest and posttest SDs of
both groups, with Hedges’ g adjustment for small sample bias (7).
The magnitude of the ES was classified according to the following
scale: 0–0.19 5 trivial effect, 0.20–0.49 5 small effect, 0.50–0.79
5 moderate effect, and $0.80 5 large effect (26). Positive effects
indicated benefit for stretching conditions. Significance was set at
p , 0.05. Heterogeneity was explored using the I2 statistic, in
which values of ,50% indicate low heterogeneity, 50–75% indicate moderate heterogeneity, and .75% indicate high heterogeneity. For studies with multiple strength analyses, the mean of
the selected outcomes was used assuming dependence (7). Data
are presented as Hedges’ g ES and 95% confidence interval (CI).
For main analyses, results are presented considering all studies
of each comparison. Sensitivity analyses to detect the validity of
the results were also performed considering study quality, excluding the effects from poor-quality studies. For subgroup
analyses, only medium-quality and high-quality studies were
considered and presented. Results for subgroup analyses, including the poor-quality studies, are displayed in Supplemental
Digital Content (see Supplementary Material D, http://links.lww.
com/JSCR/A362).
included. To identify duplicated studies, articles selected from
each database were uploaded to the EndNote software (EndNote
version X8.1; Thompson Reuters, New York, NY).
Data Extraction
The following data were extracted from the selected studies and
tabulated on a Microsoft Word file (Microsoft Corp., Redmond,
WA): lead author, sample size, age of subjects, stretch typology,
muscle or joint at which the stretching protocol was applied, the
characteristics of the training protocols (intervention length,
weekly frequency, number of sets, duration of each stretching set),
and the methodology of strength assessment. If a range of values
was used, the mean was considered (e.g., if 6–10 seconds, then
–.8 seconds). Figures were also used to extrapolate data through
the WebPlotDigitizer software (version 4.2) if relevant information for this review was not included in tables or the main
text of the articles. When necessary, the corresponding author of
the study was contacted by the authors (E.T. or J.P.N.) to request
the required information. Data extraction was performed by 3
authors (E.T., S.F., and J.P.N.) and was cross-checked between
them, while discussion and consensus resolved any discrepancies.
Table 1 reports the summary of the design of the included studies.
Quality Assessment
A modified version of the Downs and Black checklist was adopted
(15). The modified version had a maximum score of 16 (see online
Supplementary Material B, Supplemental Digital Content, http://
links.lww.com/JSCR/A362), where a total score of $13 (75%)
indicates high methodological quality, a score of 11 or 12
(60–74%) indicates moderate quality, and a score #10 (60%)
indicates low quality (30,63). Studies were rated independently
by 2 authors (E.T. and S.F.), and the intraclass correlation coefficient was calculated to assess the measurement agreement
between the 2 raters. The agreement between the 2 raters was
0.79, which is considered good (44). In the event of disagreement,
a decision was reached by vote, inclusive of a third author (M.B.).
Results
The systematic literature search identified 12.622 potential articles;
of which, 35 were included (1–3,8,13,16,18,19,29,35,38–43,
48–51,56,58,61,64–66,68,69,76,79–82,91,92). The included
studies, composed of 1,055 individuals (n total 5 1,179; control: n
5 545, stretching: n 5 634 [including each limb on intra-subject
design studies]; mean age 5 24.0 6 3.2 year-old), applied
stretching as a unique form of intervention (n 5 21)
(1–3,13,16,39,41–43,49,51,56,58,61,65,66,68,76,80,82,92), as a
combination
of
RT
and
stretching
(n
5
12)
(8,18,19,29,35,38,40,48,50,64,81,91) or both (n 5 2) (69,79). All
studies applied stretching on the lower body involving plantar
flexors, knee eor hip extensors and flexors, except two, shoulder
horizontal flexors and extensors (82), and shoulder adductors (91),
which performed stretching on the lumbar spine extensors. Six
studies also applied stretching in the upper body, involving the
elbow extensors and shoulder adductors (8,18,29,50,69,79). The
most frequent form of stretching intervention was static stretching,
although PNF (13,41,56,76), dynamic (3,19,50), and ballistic
(42,49) modalities were also performed. The interventions were
performed over a mean period of 8 weeks (range: 4–24 weeks), 3–4
days per week, applying approximately 4 sets of stretching of approximately 1-minute duration (mode and median 30 seconds).
The mean total stretching volume was approximately 101 minutes
(weekly volume: ;13 minutes). When considering only the studies
with stretch training as a unique form of intervention, training was
Statistical Analyses
Descriptive data from the included studies were analyzed through
a personalized Microsoft Excel spreadsheet (Microsoft Corp.)
containing the coded data of the studies. The following metaanalytic comparisons for the effects of stretch training on strength
were explored: (a) control vs. stretching (which included studies
performing stretching interventions as a unique form of intervention vs. a nontraining control group); and (b) RT vs. RT 1
stretching (which included studies performing stretching training
combined with RT vs. an RT-only group). For comparison of
control vs. stretching, the following moderator analyses were
explored: (a) the effects of each stretching typology subgroup
(i.e., static or dynamic or proprioceptive neuromuscular facilitation [PNF] vs. control [for analysis purposes, the 2 studies that
applied ballistic stretching were considered as dynamic]); (b) the
effects of stretch training on dynamic and isometric strength tests;
(c) laboratory-based or field-based strength measures; and (d) the
influence of the length of interventions and total time under
stretching by means of meta-regression analyses. For comparison
of RT vs. RT 1 stretching, the following moderator analyses were
explored: (a) the effects of stretch training added to RT programs
on measures of strength of laboratory-based or field-based tests;
3
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The Effects of Stretching on Strength (2022) 00:00
Table 1
Summary of the design of the included studies.*
Sample
Stretch/experimental characteristics
Sets
(n)
Duration
(s)
5
10
30
5
4
6
3
3
3
120
1. 30
2. 30
Knee flexors
4
3
1
Upper body
12
4
3
1. 30
2. 30†
16
Static
Knee flexors and extensors
10
3
3
15
22.7 6 2.6
Ballistic
Plantar flexors
6
5
4
30
41
22.9 6 2.7
Static
Plantar flexors
6
5
4
30
Konrad et al. (41)
40
23.4 6 2.8
PNF
Plantar flexors
6
5
4
30†
LaRoche et al. (49)
29
4
3
10
Leslie et al. (51)
16
31.6 6 15.2 1. Static
Hip extensors
2. Ballistic
22.5 6 4.2 Static
Knee flexors
4
3
5
1. 30
2. 30
180
Lima et al. (16)
23
19.1 6 1.4
Static
Knee flexors and extensors
8
3
3
30
Minshull et al. (56)
18
20.5 6 2.3
Knee flexors
8
3
3
Mizuno (58)
20
18.8 6 0.6
1. Static
2. PNF
Static
Plantar flexors
8
3
4
1. 20
2. 20†
30
Moltubakk et al. (61)
Nakamura et al. (64)
26
40
22.0 6 1.6
21.4 6 1.1
Static
Static
Plantar flexors
Plantar flexors
24
4
7
3
4
3
60
60
Nakao et al. (66)
30
22.7 6 2.2
Static
Knee flexors
4
3
1
300
Nelson et al. (67)
25
23.2 6 3.2
Static
Plantar flexors
10
3
4
30
Rees et al. (76)
20
19.7 6 1.6
PNF
Plantar flexors
4
3
4–6
6-10†
Simpson et al. (80)
21
22.0 6 3.0
Static
Plantar flexors
6
5
1
180
Yahata et al. (92)
16
21.4 6 1.5
Static
Plantar flexors
5
2
6
300
30
29.2 6 3.1
Static
Knee and shoulder:horizontal
flexors and extensors
10
3
1
30
Stretch
typology
Intervention
(wk)
n
Age (y)
Stretch training vs. control
Abdel-Aziem and
Mohammad (1)
75
22.3 6 3.0
Static
Plantar flexors
6
Akagi and Takahashi (2)
Barbosa et al. (3)
19
45
23.7 6 2.3
21.9 6 3.1
Plantar flexors
Knee flexors
Cini et al. (13)
18
24.0 6 2.5
Conceição et al. (82)
70
17.0 6 1.1
Static
1.
Dynamic
2. Static
1. Static
2. PNF
Static
Kokkonen et al. (39)
38
23.0 6 3.0
Konrad and Tilp (42)
39
Konrad and Tilp (43)
Authors
Stretch 1 RT training vs.
RT training
Bastos et al. (8)
Region of assessment
Frequency
(d·wk21)
Other details
Right limb was trained and
evaluated
1. Trained individuals
2. Untrained individuals
Single limb (intrasubject)
Both limbs were trained;
nondominant one was
evaluated
Both limbs were trained; one
was randomly evaluated
Stretch was performed for
lumbar spine extensors,
shoulder horizontal flexors
and extensors
Stretch was performed for
the major lower-limb
muscles (15 exercises), but
only the knee joint was
strength tested
Both were limbs trained; right
one was evaluated
Both were limbs trained; right
one was evaluated
Both were limbs trained; right
one was evaluated
Right limb was trained and
evaluated
Both limbs were trained;
dominant one was evaluated
Right limb was trained and
evaluated
Single limb (intrasubject)
Both limbs were trained and
evaluated; the mean
response was considered
Single limb (intrasubject)
Dominant limb was trained
and evaluated
1. High-intensity stretching
group
2. Normal-intensity
stretching group
Dominant limb was trained
and evaluated
Right limb was trained and
evaluated
Both limbs were trained and
evaluated; the mean
response was considered
Nondominant limb was
trained and evaluated
Dominant limb was trained
and evaluated
1. Stretch was performed
before first set of each RT
session
2. Stretch was performed
before 1st set of each RT
4
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The Effects of Stretching on Strength (2022) 00:00
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Table 1
Summary of the design of the included studies.* (Continued)
Sample
Authors
n
Age (y)
Stretch/experimental characteristics
Stretch
typology
Region of assessment
Intervention
(wk)
Frequency
(d·wk21)
Sets
(n)
Duration
(s)
Evangelista et al. (18)
29
28.0 6 6.4
Static
Upper body and knee
extensors
8
2
3
30
Ferreira-Junior et al. (19)
45
21.2 6 0.5
Knee flexors
8
2
4
1. 20
2. 20
Gollin and Beratto (29)
19
32.0 6 7.0
1. Static
2.
Dynamic
Static
Upper and lower body
8
3
4
15
Junior et al. (35)
9
25.4 6 5.3
Static
Knee extensors
10
2
2
25
Klinge et al. (38)
12
24.2 6 3.7
Static
Knee flexors
12
7
8
45
Kokkonen et al. (40)
32
23.0 6 2.5
Static
Lower limbs
8
2
3
15
Kubo et al. (48)
8
21.0 6 2.0
Static
Plantar flexors
8
7
10
45
Leite et al. (50)
21
46.0 6 6.5
Dynamic
Upper and lower body
12
4
3
30
Nakamura et al. (65)
16
21.3 6 1.1
Static
Knee extensors
5
2
2
30
Souza et al. (81)
16
22.6 6 2.1
Static
Elbow extensors and knee
extensors
8
3
3
30
Wadhi et al. (91)
26
25.2 6 6.4
Static
Shoulder adductors
8
2
8
30
43
21.0 6 4.0
Static
Upper and lower body
12
2
3
30
Stretch training vs. control
and stretch 1 RT training
vs. RT training
Nóbrega et al. (69)
Other details
exercise (8 exercises, 4 sets,
8–10 RM)
Stretch was performed in the
ISS RT rest period (6
exercises, 4 sets, 8–12 RM)
Stretch was performed
before the RT exercise (1
exercise, 4 sets, 8–12 RM)
Stretch was performed in the
ISS RT rest period (6
exercises, 5 sets, 6–15 RM)
Stretch was performed
before the RT exercise
(intrasubject) (1 exercise, 4
sets, 80% 1RM)
Stretch was performed after
or apart of the RT sessions (1
isometric exercise, 3 sets of
4 reps at ;80–100% MVIC)
Stretch was performed apart
of the RT sessions for the
major lower-limb muscles
(15 exercises), but only the
knee joint was strength
tested (3 exercises, 3 sets,
8RM)
Stretch was performed
before, after, or apart of the
RT sessions (intrasubject) (1
exercise, 5 sets, 10 reps at
70% 1RM)
Stretch was performed for
the whole body (60-min
stretching classes), but
strength was tested only on
bench and leg presses
1. Stretch was performed
before each RT session
2. Stretch was performed
after each RT session (8
exercises, 3 sets, 6–15 RM)
Stretch was performed in the
ISS RT rest period
(intrasubject) (1 exercise, 3
sets, 10 repetitions)
Stretch was performed in the
ISS RT rest period (6
exercises, 4 sets, 8RM)
Stretch was performed in the
ISS RT rest period (2
exercises, ;4 sets, 4–10
RM)
Stretch was performed for
the whole body (40-min
stretching classes), but
strength was tested only on
handgrip and bench and leg
presses
1. Stretch training vs. control
5
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The Effects of Stretching on Strength (2022) 00:00
Table 1
Summary of the design of the included studies.* (Continued)
Sample
Authors
Simão et al. (79)
Total/mean
n
80
Age (y)
34.5 6 1.8
1,055 24.0 6 3.2
Stretch/experimental characteristics
Stretch
typology
Region of assessment
Static
Upper and lower body
/
/
Intervention
(wk)
16
8
Frequency
(d·wk21)
Sets
(n)
Duration
(s)
3
4
15–60
3.5
4
56
Other details
2. Stretch 1 RT training vs.
RT training (stretch was
performed after the RT
sessions) (9 exercises, 3
sets, 8–12 RM)
Stretch was performed for
the whole body (40-min
stretching classes), but
strength was tested only on
bench and leg presses
1. Stretch training vs. control
2. Stretch 1 RT training vs.
RT training (stretch was
performed before the RT
sessions) (8 exercises, 3
sets, 6–15 RM)
/
*PNF 5 proprioceptive neuromuscular facilitation; RT 5 resistance training; RM 5 repetition maximum; ISS 5 stretching was applied between the sets of the RT exercises.
†To be noted that the value refers to the duration of the stretching phases of the PNF protocol (either the stretching of the agonist or the contraction of the antagonist muscles).
analysis did not affect the finding, so that the stretching programs (all
types considered together) demonstrated positive changes in dynamic
tests (k 5 18; ES 5 0.28 [0.08–0.49; p 5 0.008; I2 5 29%) but not in
isometric tests (k 5 12; ES 5 0.15 [20.04 to 0.34]; p 5 0.126; I2 5
0%). Significant positive effects were also observed when tests were
subcategorized as laboratory-based measures (k 5 20; ES 5 0.20
[0.04–0.34]; p 5 0.016; I2 5 24%) or field-based measures (k 5 5; ES
5 0.66 [0.26–1.07]; p 5 0.001; I2 5 31%). Figure 2 reports the forest
plot of the results. Meta-regressions indicated no significant effect for
any model with study length (b 5 0.007; p 5 0.607; k 5 29) or total
volume (b 5 0.001; p 5 0.609; k 5 29) as factors. These effects
remained the same when excluding the low-quality studies (b 5
0.005; p 5 0.695 [Figure 4A], b 5 20.001; p 5 0.492; k 5 24).
performed on average for 7 weeks, 4 days per week, in 4 sets of 70
seconds. When stretches were added to RT, they were performed
on average in 4 sets of 30 seconds in programs that lasted 10 weeks,
performed 3 days per week.
Quality Assessment
The Downs and Black quality assessment indicated that 13
studies were of high quality (37.1%), 12 were of medium quality
(34.3%), and 10 were of low quality (28.6%). The mean score
was 12 (out of 15), indicating an overall medium quality of the
included studies. Results of the Downs and Black for each study
can be found in Supplemental Digital Content (see Supplementary
Material C, http://links.lww.com/JSCR/A362).
Resistance Training vs. Resistance Training 1 Stretching. For the
comparison of RT vs. RT 1 stretching, 17 effects were included
(14 studies; 45 outcomes), and the result revealed no influence of
adding stretching exercises to RT programs on measures of
strength (ES 5 20.15 [20.35 to 0.05]; p 5 0.136; I2 5 7%; df 5
16; Figure 3). When excluding the low-quality studies, results
were not altered (k 5 9 [8 studies; 19 outcomes]; ES 5 20.03 [2
0.29 to 0.23]; p 5 0.809; I2 5 16%; df 5 8).
Subgroup analysis for the types of strength test showed no significant effect on laboratory-based (k 5 5; ES 5 0.19 [20.12 to
0.51]; p 5 0.235; I2 5 20%) or field-based (k 5 6; ES 5 20.10 [2
0.48 to 0.28]; p 5 0.598; I2 5 40%) RM strength tests. Subgroup
analysis for timing of stretch administration showed a small nonsignificant negative effect for stretches performed before RT (k 5 3;
ES 5 20.28 [20.90 to 0.33]; p 5 0.365; I2 5 60%), and no effects
for stretches performed in the interset rest period (k 5 3; ES 5 0.11
[20.30 to 0.52]; p 5 0.597; I2 5 0%) or after or apart RT (k 5 3;
ES 5 0.14 [20.30 to 0.58]; p 5 0.536; I2 5 0%). Figure 3 reports
the forest plot of the results. Meta-regression indicated a significant
influence of study length (b 5 20.100; p 5 0.004; k 5 17
[Figure 4B]). This effect remained the same when excluding the
low-quality studies (b 5 20.093; p 5 0.012; k 5 9). Figure 4
displays the scatterplots of the meta-regressions on study length.
Meta-Analyses
Control vs. Stretching. For the comparison of control vs. stretching, 29 effects were included (23 studies; 66 outcomes), and the
result revealed a significant small effect of stretching training on
improving muscular strength (ES 5 0.21 [0.07–0.36]; p 5 0.004; I2
5 16%; df 5 28; Figure 2). When excluding the low-quality
studies, results were not altered (k 5 24 [19 studies; 57 outcomes];
ES 5 0.26 [0.10–0.42]; p 5 0.002; I2 5 20%; df 5 23).
Subgroup analysis for stretch typologies showed improvements in
muscular strength following static stretching training (k 5 17; ES 5
0.28 [0.08–0.47]; p 5 0.006; I2 5 28%) but not for dynamic
stretching (k 5 4; ES 5 0.08 [20.29 to 0.45]; p 5 0.676; I2 5 0%),
whereas PNF model showed a small but nonsignificant positive effect
(k 5 3; ES 5 0.44 [20.35 to 1.23]; p 5 0.277; I2 5 61%). When
considering only the static stretching programs (which presented
significant positive results), subgroup analysis for types of strength
test showed significant positive changes only in dynamic tests (k 5 14;
ES 5 0.33 [0.08–0.58]; p 5 0.007; I2 5 42%) but not isometric tests
(k 5 8; ES 5 010 [20.13 to 0.33]; p 5 0.377; I2 5 0%). The
backward inclusion of dynamic and PNF stretching models in the
6
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The Effects of Stretching on Strength (2022) 00:00
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Figure 2. Meta-analyses of the effects stretching training interventions on muscular strength. ES 5 Hedges’
g effect size; Q 5 study quality; L 5 low; M 5 medium; H 5 high; HI/NI 5 high or normal intensity; Un/Tr 5
untrained or trained subjects. The relative weight of each study refers to the main analysis, highlighted in
bold.
within muscles release Ca1 in a tension-dependent manner. Increased Ca1 determines additional binding during cross-bridge
creation, allowing muscles to resist stretches of greater magnitudes (31,33,75). These rationales could make the basis to suggest
performing stretching exercise programs as a stimulus to increase
muscular strength. Our findings support such propositions indicating that stretching training increases strength.
It is important to observe that stretching produced significant
effects only in dynamic strength tests but not isometric tests. Our
results are in line with the suggestions made in a previous systematic review by Medeiros and Lima (55). Specifically, we observed a small effect for laboratory tests, which involve a single
muscle action (e.g., concentric, eccentric), and a moderate effect
for field-based RM tests. This distinction is important to be made
because RM tests consist of both muscle lengthening and shortening phases of the movement. Increased force is usually displayed when concentric contractions are preceded by longer
eccentric contractions, allowing more time for the formation of
cross-bridges (86,89). Thus, during a lengthening action (e.g., the
negative phase of a calf raise exercise RM test), tendons have
more time to store potential energy for subsequent concentric
contraction (89). Interestingly, this property of tendons can be
improved by stretching training (47,76), and in turn, it may
Discussion
This is the first systematic review coupled with meta-analysis and
meta-regression that explored the effects of stretching exercise
training on measures of muscular strength. Results indicate that
when stretching exercises are performed as a unique training activity, muscular strength could be significantly improved, although
to a small magnitude. Conversely, when stretching exercises are
added to RT training protocols, despite no significant effect being
observed overall, there might be a time-dependent negative effect
on the RT-induced strength gains. These main inferences were
driven based on a relatively high number of studies that had a
predominantly medium or high quality, which therefore strengthen
the confidence of the results.
Muscular strength production capacity depends on active and
passive muscle components and both may be modified by
stretching or shortening interventions (45). Muscle architecture,
stiffness, fibre size and length, number of parallel cross-bridges,
and action potentials, among other properties of the neuromuscular system are all factors that can influence force production
(20,32). Evidence indicates that when lengthening active contractions are performed, muscle force increases to a greater extent
compared with concentric or isometric contractions (33). Such
phenomenon is explained because a stretch of the titin filaments
7
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The Effects of Stretching on Strength (2022) 00:00
Figure 3. Meta-analyses of the effects of stretching exercises added to resistance training (RT) interventions
on muscular strength. ES 5 Hedges’ g effect size; Q 5 study quality; L 5 low; M 5 medium; H 5 high; RM
5 repetition maximum. The relative weight of each study refers to the main analysis, highlighted in bold.
the effects of stretching interventions in nonactive vs. active
subjects.
We previously observed that stretching typology influences
the changes in flexibility (85) and speculated that it could also
affect the strength responses. In this review, although only static
stretching showed significant positive effects (which could suggest that the type of stretching intervention plays a role in responses), it is important to note that the 95% CI of the 3
conditions overlapped each other (Figure 2). Static stretching
was the model with the majority of studies (k 5 17) included in
the analyses, followed by dynamic stretching (k 5 4) and PNF (k
5 3). Thus, the number of included studies, which determine the
power of each analysis, might have influenced the distinct responses regarding statistical significance (34). Although static
stretching resulted in a small and significant improvement in
dynamic strength, dynamic stretching led to no changes, and
PNF led to small nonsignificant gains. The reason dynamic
stretching did not confer increases in strength, in opposition to
the other stretch typologies, is uncertain. Nonetheless, results
from PNF should be analyzed with caution because they were
driven by heterogeneous responses among the few studies
available (I2 5 61%, k 5 3). In addition, the slightly greater
effect of PNF over static and dynamic models could be attributed
to the active contraction phase performed during PNF and not
to stretching-specific neuromuscular mechanisms (14,77).
Studies from Barbosa et al. (3) and LaRoche et al. (49) were the
only ones who directly compared the effects of dynamic vs. static
stretching, and both reported no benefit on strength for any
condition, as well as no significant difference between the conditions. Moreover, Cini et al. (13) and Minshull et al. (56)
compared static vs. PNF and showed no difference between the
conditions. Therefore, conclusions comparing the efficacy of the
3 stretching models should be made with caution. More studies
explain the observed effects. The greater range of movement
(ROM) achieved as a consequence of the stretching training could
allow for greater momentum occurring over the joints, enabling
the production of greater residual force (33). However, it is important to consider that stretching for up to 8-week duration does
not seem to change the structural properties of muscles and tendons (25), which may suggest that, similarly to increases in ROM,
the strength gains occur at a neural or sensory level (28).
In most of the stretching-only studies of this review, the samples composed of sedentary untrained subjects, and this detail
helps to explain the positive result on strength. It is wellestablished in the literature that untrained subjects could benefit
from exercise interventions to a greater extent than trained ones:
the so-called trainability principle (46). Thus, the brief stimulus
prompted by the stretch training was sufficient to improve their
muscular strength. However, the effect was of small magnitude
and much smaller than expected with RT interventions to improve strength in this population. Specifically, we observed average improvements of 0.21 ES, although the average effect
following RT is approximately 1.40 ES (74). Curiously, the study
with the minor effect on strength (in this case, an ES of 20.68)
was conducted on active men regularly exercising at least 3 times
per week in noncompetitive activities (3). Therefore, those who
experienced a decrease in their level of strength probably did not
reach a sufficient level of muscle strain stimuli through the
stretching intervention to maintain muscular strength (62,90).
This is in line with other findings reporting that the lower the
baseline muscular conditioning of the subjects, the higher the
strength gain following the same structured strength training (36).
Together, these results indicate that the physical activity background of the trainees may influence the efficacy of stretch exercise training. Because of the reduced number of studies in
physically active subjects, we could not run an analysis comparing
8
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Figure 4. Meta-regression of the effects of stretching interventions associated with
the length of the studies (panel A: stretching performed as a unique form of intervention; panel B: stretching added to resistance training [RT] programs).
Mondays, Wednesdays, and Fridays while stretching only on
Tuesdays and Thursdays). Conversely, the study of Bastos et al.
(8) was the only one that indicated significant negative effects
following the inclusion of stretching to RT. The authors conducted a 10-week RT program performed under 3 different
conditions in recreationally trained subjects. One group underwent RT only, a second group added multiple stretching exercises before each RT session, and a third group performed
stretching exercises on each muscle group before the respective
RT exercise. At the end of the intervention period, all groups
significantly increased strength levels. However, greater relative
improvements were achieved by the group performing RT without stretching. In a similar study regarding RT and stretching
protocols, Souza et al. (81) compared RT with interset stretch RT
programs and observed similar increases in strength between the
groups. In this sense, it seems that the timing of stretch application
to RT plays a role in the adaptations.
The timing of the stretching application to RT varied between
the studies included herein. Stretch exercises were included before
RT (8,19,35,48,50,79), in the inter-RT-set rest period
(8,18,29,64,82,91), after RT (38,48,50,69), or on alternate days
(40). The only study that compared distinct timings was that of
Leite et al. (50), where authors examined stretching before or after
RT sessions in comparison to an RT-only condition. The 3 groups
improved strength over time; however, the ES for the RT-only
group were slightly higher, indicating that combining stretching
with RT may hamper strength development. The ES for the beforeRT stretching group was the least. In a previous study (71), we
reviewed the effects of stretching on muscle hypertrophy, and the
are required to clarify whether stretching typology may determine different strength responses.
Results from the meta-regression analyses revealed no effect
neither for stretching volume nor for the length of the interventions. These findings are in accordance with results observed in a
recent review on muscle architecture, which suggested that
training volume does not seem to be a sole determining factor in
adaptations following stretch exercise training (71). Nonetheless,
only a few studies (n 5 6/21) conducted interventions oer more
than 8 weeks, which prevents us to infer whether chronically
expressed mechanisms other than sensory or neural (i.e., changes
in mechanical or architectural properties) could determine different outcomes. However, 2 studies investigating the effects of
passive stretching on architectural characteristics of the muscletendon unit complex by Longo et al. (52) and Beltrão et al. (6),
both lasting 12 weeks, did not observe any modification in architectural characteristics.
In regard to the stretch 1 RT interventions, no overall effect
was observed, indicating that including stretching exercises
within RT programs does neither provide benefit nor blunt the
strength gains induced by RT (Figure 3). However, an individual
inspection of the studies revealed similar increases in strength for
both RT and RT 1 stretch groups for almost all studies, although
the timing of stretching application seems to influence the results.
Only the study of Kokkonen et al. (40) highlighted an additional
benefit by including stretching on increasing strength. In this investigation, the combined exercise program (static stretching 1
RT) was performed on inactive subjects, and the stretching was
performed only during non-RT days (RT was performed on
9
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The Effects of Stretching on Strength (2022) 00:00
potential higher perceived state of vigor and effort if the stretching
is performed in the interset rest period (11). Chronic and acute
studies that directly compare the timing of stretch in RT are
needed to test such assumptions.
results suggested that when stretching is performed at a certain
intensity of tensile strain, some small effects on measures of muscle
hypertrophy can be observed; effects of which also seem to depend
on the timing of stretch administration. In particular, it seems that a
small negative effect on muscle size is observed when stretching is
performed before RT. In this review, a small nonsignificant effect
was observed for before-RT stretching on impairing RT-induced
strength gains (20.28, p 5 0.356). This finding should be analyzed
with caution because besides the nonsignificance, it was derived
from few effects of moderate heterogeneity (k 5 3; I2 5 60%). It is
also important to note that this result became significant when
including the low-quality studies within the analysis (k 5 7, ES 5 2
0.43, p 5 0.014; I2 5 25%; see Supplementary Material D, Supplemental Digital Content, http://links.lww.com/JSCR/A362).
Nonetheless, such findings are in agreement with several previous
studies and reviews that indicate that doing stretches before
strength tasks may blunt acute performances (5,22,37,53,67).
Thus, it seems that this acute impairing effect repeated multiple
times may also chronically impair strength gains (in approximately
210%), especially in prolonged training interventions (.8 weeks).
The groups that performed before-RT stretching improved
strength by approximately 27% compared with approximately
36% of their control counterparts. Importantly, the metaregression indicated a significant negative association of study
length with the stretch-induced impaired strength gain (Figure 4B).
That is, the longer the intervention, the greater the stretch-induced
impairing effect on strength gains. This time-dependent effect was
probably the result of the characteristics of the studies of longer
duration. Among the studies that lasted more than 8 weeks, all
presented negative ESs, except that of Nóbrega et al. (69). Interestingly, this latter was the only one (among the 8-week1
studies) that not implemented stretching before RT.
The reasons why performing stretches before RT may blunt
strength gains seem mainly to be related to neural mechanisms
(5,12,22,37,53,67,87,88). Recently, Trajano and Blazevich (87)
revised the neural mechanisms associated with the acute stretchinduced force loss and evidenced that reduced motoneuron excitability might occur. These neural alterations might mainly
originate at the motoneuron level (87). Therefore, after a
stretching exercise, decrements in estimates of maximal voluntary
muscle activation are expected, as well as a reduction in neural
drive and muscle excitation during the subsequent strength task
(87). In addition, it could be speculated that the impaired effect on
long-term strength gains may also be related to changes in muscle
architecture (21,23). For example, a reduction in muscle-tendon
stiffness could hamper strength production capacity, especially if
stretching is performed at high intensities (25,27,71). Studies
examining these factors with intervention durations greater than
8 weeks are warranted.
Moreover, the reasons as to why before-RT stretching has a
negative influence on strength gains could indicate similarities for
the inter-RT-set stretching; however, it is interesting to observe
that although the before-RT stretching impaired strength gains,
interset stretching did not. The dissimilar result between the 2
strategies is difficult to reconcile, especially because there are few
studies on this specific topic. Despite the difference in study duration as mentioned before (i.e., studies using before-RT stretch
were longer), the studies included using both methods analyzed
similar muscle groups and strength tests and conducted similar
stretching interventions (3x/wk, 3 3 30 s). It is likely that doing
stretching exercises before RT might result in a physiological relaxation effect (thus reducing the brainstem-derived neural drive)
by increased parasympathetic activity (87), as opposed to a
Limitations and Perspectives
The main limitation of this review is the relatively low number of
moderate and high-quality studies included for the RT 1 stretch
analyses and each stretch typology or timing in the stretch-only
and RT 1 stretch conditions. This aspect needs to be significantly
considered by future research because different ways of stretching
are commonly included either as a form of warm-up during
training or as a form of cooldown. Specifically, the majority of
studies performed static stretching, which does not allow us to
adequately understand the possible effects of other stretching
types. It would be also interesting if future studies consider performing experiments lasting more than 8 weeks and evaluating
neural and architectural adaptations of the skeletal muscle system
after stretching and RT. This would help to better understand the
mechanisms behind the conflicting long-term effects. Furthermore, the use of different assessment methods to evaluate strength
is an important aspect to consider when investigating the effects
on strength gains (10). In addition, recent studies have observed
that muscle activation and performance tend to increase subsequently to a stretch applied to antagonist muscles of the tested
one (17,57,78); in opposition to what is observed for stretching
the agonist muscle (12). It would be interesting if future studies
consider exploring RT plus stretch of the antagonist muscle group
on muscular adaptations. Also, studies comparing the effects of
different stretching intensities are required for both stretch-only
and RT 1 stretch bodies of literature (27).
Practical Applications
Chronic static stretching programs increase dynamic muscular strength to a small magnitude. Incorporating stretching
exercises in RT programs does not affect strength overall.
Despite no effect observed if performing stretching inter-RT
sets or after RT, a low-to-moderate hampering effect is seen if
stretching is performed before RT, especially if it is performed
for long periods (.8 weeks). Performing stretching in sessions
distant from RT sessions may be a strategy to not hinder
strength development. Our results provide relevant information for strength and conditioning professionals to improve the design of exercise training programs.
Acknowledgments
Conceptualization and design: E. Thomas and S. Ficarra.
Literature search: E. Thomas, S. Ficarra and J. P. Nunes.
Analysis: E. Thomas and J. P. Nunes. Manuscript writing, first
draft: E. Thomas, S. Ficarra and A. Paoli. Manuscript writing,
revision: J. P. Nunes, M. Bellafiore, A. Palma and A. Bianco.
Supervision: A. Palma and A. Bianco. All authors read and
approved the final manuscript. Availability of data: Full data
coded of the included studies can be shared upon reasonable
request from the corresponding author. The authors have no
conflicts of interest to disclose. The results of this study do not
constitute endorsement of the product by the authors or the
National Strength and Conditioning Association.
10
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