Acute and Chronic Effects of Static Stretching on Neuromuscular Properties: A Meta-Analytical Review

: The aim of this review was to provide an overview of the recent ﬁndings on the acute and chronic effects of static stretching on joint behaviors and neuromuscular responses and to discuss the overall effects of acute and chronic static stretching on selected outcomes via meta-analyses, using a total of 50 recent studies. The results of our meta-analyses demonstrated that acute static stretching results in increased range of motion (ROM), decreased passive resistive torque (PRT), increased maximum tolerable PRT (PRT max ), decreased maximum voluntary isometric torque, decreased muscle–tendon unit stiffness, decreased muscle stiffness, decreased tendon stiffness, and decreased shear elastic modulus. Moreover, the chronic effects of static stretching included increased ROM, increased PRT max , decreased muscle stiffness, and decreased shear elastic modulus (or shear wave speed). These results suggest that static stretching interventions have the potential to increase ROM and reduce the mechanical properties of muscle–tendon tissue, but they may not change corticospinal excitability and spinal reﬂex excitability or muscle architecture


Introduction
Static stretching is commonly used in athletic environments with the specific aims of increasing joint range of motion (ROM) and reducing injury risk [1].Briefly, static stretching involves keeping a target muscle for a prescribed period of time in a lengthened position at where a stretch sensation or the point of discomfort is reached [2].Considering that static stretching is relatively easy to implement even for professionally untrained people, static stretching may have potential in many clinical applications, especially for individuals with increased muscle tightness or severe weakness, preventing them from developing undesired disuse-induced motor impairments.Indeed, static stretching has widely been applied to treat and prevent spasticity and contracture in individuals with neurologic diseases (for review see [3][4][5]).However, its underlying mechanisms and thus stretching protocols remain unclear, probably because functional outcome measures (e.g., joint mobility, walking performance) have mainly been discussed.
Understanding how static stretching affects the neuromuscular system may help clinicians develop more effective, subject-specific protocols that can maximize functional outcomes.Specifically, the excitability of spinal motor neurons (i.e., their ability to fire action potentials) is closely associated with motor behavior and performance.It has been shown that after neurological diseases, there are significant changes in the excitability of the corticospinal pathway (as measured by motor evoked potential [MEP]) [6][7][8] and in spinal excitability (as measured by Hoffman reflex [H-reflex] and tendon reflex [T-reflex]) [9,10].In addition, muscle contractile properties (i.e., the ability of muscles to generate force and movement) are linked to motoneuron excitability as well as motor performance.For Appl.Sci.2023, 13, 11979 2 of 31 instance, muscle architectural parameters (e.g., fascicle length, pennation angle, muscle thickness) are important determinants of muscle function (such as force-generating capacity and shortening velocity) [11][12][13].It has also been suggested that muscular contraction performance appears to be influenced by an interaction between contractile and passive connective tissues (e.g., stiffness of muscle/tendon tissues) in a pennate muscle [14][15][16][17][18]. Additionally, muscle bulging condition (e.g., the material properties of extracellular connective tissues, transverse loading) can also change muscle performance [19][20][21][22][23]. Therefore, it is essential to understand whether and to what extent the neuromuscular factors could respond to static stretching.
The aim of this review was to provide an overview of the recent findings on the acute and chronic effects of static stretching on joint behaviors (e.g., range of motion, joint torque) and neuromuscular responses (e.g., motor evoked potentials, reflex responses, muscle architecture, muscle stiffness, tendon stiffness, shear elastic modulus) and to discuss the overall effects of acute and chronic static stretching on the aforementioned outcomes across the studied articles via meta-analyses.The findings of this review could help clinicians obtain a more complete picture of the effects of acute and chronic static stretching on neuromuscular properties, potentially providing the foundation for designing a more subject-specific, effective protocol that might improve neuromuscular responses.

Materials and Methods
For this study, we utilized the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines [48].Using the PubMed, Scopus, and ProQuest databases, all authors independently performed a literature search using the following keyword combinations: static stretching, passive stretching, neural response, motor evoked potential, H-reflex, tendon reflex, stretch reflex, excitability, muscle architecture, fascicle length, pennation angle, muscle thickness, muscle stiffness, tendon stiffness, shear elastic modulus, shear wave speed, and shear wave elastography; for example, "("static stretch" OR "passive stretch" OR "static stretching" OR "passive stretching") AND ("neural response" OR "motor evoked potential" OR "H-reflex" OR "tendon reflex" OR "stretch reflex" OR excitability OR "muscle architecture" OR "fascicle length" OR "pennation angle" OR "muscle thickness" OR "ultrasound" OR "muscle stiffness" OR "tendon stiffness" OR "shear elastic modulus" OR "shear wave speed")."We also found some relevant studies from the references of the selected articles and from a personal library.The PRISMA flow diagram is shown in Figure 1.
The inclusion criteria were as follows: (1) peer-reviewed research articles published in English between January 2012 and December 2022; (2) adults aged 18-50 with no history of serious injury or an ongoing injury (non-athletic and athlete population); (3) static/passive stretching in a single session (acute) or multiple sessions (chronic), targeted lower extremity joints, not combined with other interventions (e.g., voluntary contraction, electrical stimulation); (4) each intervention session dedicated to only one muscle group, not multiple joints or different muscle groups; (5) pre-post comparison of at least one of the aforementioned outcome variables.The exclusion criteria were as follows: (1) participants with neurologic or musculoskeletal injuries; (2) any of the aforementioned outcome measures By extracting the necessary information from the reviewed studies (e.g., the number of participants, mean and standard deviation of available outcomes), meta-analyses were conducted to provide more precise estimates of the acute and chronic effects of static stretching on neuromuscular properties.Considering that many studies also reported joint behaviors such as range of motion (ROM), passive resistance torque (PRT), maximum tolerable PRT (PRTmax), maximum voluntary isometric torque (MVIT), and muscletendon unit (MTU) stiffness, these parameters were also considered in the meta-analyses.
All the statistical analyses were performed using IBM SPSS Statistics (Version 28, IBM SPSS Inc., Chicago, IL, USA) [49].Briefly, a random effects model with restricted maximal likelihood estimation was chosen to account for the heterogeneity between the studies.The effect size was calculated using Cohen's d, and d values of 0.2, 0.5, and 0.8 are considered to indicate small, moderate, and large effect sizes, respectively [50].The I 2 statistic was used to assess the degree of heterogeneity, and I 2 values of 25%, 50%, and 75% are considered to indicate low, moderate, and high heterogeneity, respectively [51].If an outcome measure was not available from at least two studies, a meta-analysis for the outcome measure was not performed.

Results
The literature search identified 564 studies, and 7 studies from other sources (i.e., the references of the studies and a personal library) were added (Figure 1).After the removal By extracting the necessary information from the reviewed studies (e.g., the number of participants, mean and standard deviation of available outcomes), meta-analyses were conducted to provide more precise estimates of the acute and chronic effects of static stretching on neuromuscular properties.Considering that many studies also reported joint behaviors such as range of motion (ROM), passive resistance torque (PRT), maximum tolerable PRT (PRT max ), maximum voluntary isometric torque (MVIT), and muscle-tendon unit (MTU) stiffness, these parameters were also considered in the meta-analyses.
All the statistical analyses were performed using IBM SPSS Statistics (Version 28, IBM SPSS Inc., Chicago, IL, USA) [49].Briefly, a random effects model with restricted maximal likelihood estimation was chosen to account for the heterogeneity between the studies.The effect size was calculated using Cohen's d, and d values of 0.2, 0.5, and 0.8 are considered to indicate small, moderate, and large effect sizes, respectively [50].The I 2 statistic was used to assess the degree of heterogeneity, and I 2 values of 25%, 50%, and 75% are considered to indicate low, moderate, and high heterogeneity, respectively [51].If an outcome measure was not available from at least two studies, a meta-analysis for the outcome measure was not performed.

Results
The literature search identified 564 studies, and 7 studies from other sources (i.e., the references of the studies and a personal library) were added (Figure 1).After the removal of duplicates and the screening process, a total of 50 studies (i.e., 31 studies for acute static stretching and 19 studies for chronic static stretching) were eligible for inclusion.
PRT max was reported in five studies [37,41,[58][59][60], showing a significant increase in all but one study [37].Of these five studies, one study examined three different rest intervals [41], and the raw values from two studies were not available [59,60], which led to a total of five data points being available.A meta-analysis revealed a significant overall moderate to large effect with low heterogeneity (d = 0.78; 95% CI = 0.41 to 1.16; p < 0.001; I 2 = 18%; Figure 2C).
Of these studies, one study examined two stretching methods [67], one study examined two different intensities [66], and the raw values from one study were not available [58], leading to a total of 12 data points from 10 studies.A meta-analysis revealed a significant low to moderate effect with no heterogeneity (d = −0.33;95% CI = −0.54 to −0.13; p < 0.001; I 2 = 0%; Figure 2E).
Regarding the T-reflex amplitude, two studies showed a significant decrease in the soleus [52,53], but a meta-analysis was not conducted because raw values were only available from one study [52].
PRT was reported in seven studies [47,71-74,78,83], showing no significant change in all but three studies [47, 73,83].Of these studies, one study examined two stretching methods in terms of stretching intensity [78], and the raw values from one study [73] were not available, leading to a total of seven data points being available from six studies.A metaanalysis revealed no significant overall small effect with low to moderate heterogeneity (d = −0.19;95% CI = −0.57to 0.18; p = 0.31; I 2 = 43%; Figure 5B).
PRT max was reported in eight studies [72,76,78,[82][83][84][85][86], and three studies examined two stretching methods in terms of stretching intensity [78,84,86].This led to the attainment of a total of 11 data points from the studies, and seven out of the 11 data points showed a significant increase.A meta-analysis revealed a significant overall moderate to large effect with low to moderate heterogeneity (d = 0.71; 95% CI = 0.39 to 1.02; p < 0.001; I 2 = 40%; Figure 5C).
Four studies reported tendon stiffness [72,74,85] or tangent modulus [83], and all the studies did not show a significant change.Of these studies, two studies reported two values that were obtained from two different sites (i.e., free/whole Achilles tendon [83]) or from the middle and final third parts of the stress-strain curve [85], leading to a total of six data points from the four studies.A meta-analysis revealed no significant overall small effect with no heterogeneity (d = 0.05; 95% CI = −0.22 to 0.33; p = 0.70; I 2 = 0%; Figure 6E).

Discussion and Conclusions
The purpose of this review was to evaluate the acute and chronic effects of static stretching on neuromuscular properties by summarizing the selected joint behaviors and neuromuscular responses based on data from the recent literature.The results of our meta-analyses based on the reviewed studies suggested that (1) acute static stretching may change joint behaviors (e.g., increased ROM, decreased PRT, increased PRT max , decreased MVIT, decreased MTU stiffness) and muscular responses (e.g., decreased muscle stiffness, decreased tendon stiffness, decreased shear elastic modulus), and that (2) chronic static stretching may change joint behaviors (e.g., increased ROM, increased PRT max ) and muscular responses (e.g., decreased muscle stiffness, decreased shear elastic modulus or shear wave speed).
It has been suggested that a stretching-induced increase in ROM may be a result of an increase in stretch tolerance (i.e., the ability to tolerate the discomfort at the end of ROM) [87] and/or changes in MTU mechanical properties (e.g., reduced stiffness of muscle and connective tissues) [88].The results from our meta-analyses support the findings that both acute and chronic static stretching appear to be effective in increasing ROM and PRT max (a measure of an increase in stretch tolerance), suggesting that the increased ROM may be explained, in part, by an increase in stretch tolerance, as reflected in the increased PRT max .Moreover, considering the significant overall effect for the MTU stiffness and muscle/tendon stiffness after acute static stretching, increased ROM in response to acute static stretching may also be associated with reduced MTU mechanical properties.However, as the results of our meta-analyses showed no significant overall effect for the MTU stiffness and tendon stiffness after chronic static stretching, the potential associations between the increased ROM and changes in MTU mechanical properties in response to chronic static stretching are unclear.Interestingly, despite no significant changes in MTU stiffness and tendon stiffness, muscle stiffness and shear elastic modulus or shear wave speed (i.e., muscle-specific variables) were significantly reduced in response to chronic static stretching.These results may imply that in the reviewed studies, changes in the mechanical properties of muscle tissue did not lead to changes in mechanical behavior at joint levels, especially for chronic static stretching.
Indeed, based on the presumption that non-muscular structures (e.g., fasciae, nerves, vessels, skin) sitting in/around/across muscles can also contribute to the mechanical properties measured at joint levels, the non-muscular structures have been considered a possible contributor to changes in joint behavior in response to static stretching.For example, it has been evidenced that ankle joint ROM is substantially influenced by different hip and knee joint positions [89], and such notable hip joint position-dependent ankle ROM was observed even without significant changes in ankle joint torque and MG shear elastic modulus for a given ankle angle [90].These findings have emphasized the possible role of fasciae which connect inter-muscles from ankle to hip/spine joints, also referred to myofascial tissue connectivity (for review see [91]).Acute static stretching protocols aiming to load the sciatic nerve also led to a significant increase in ankle ROM and a significant decrease in the sciatic nerve shear wave velocity (an index of stiffness) without significant changes in MG and BF shear wave velocity, indicating that changes in the mechanical properties of nerves only can directly affect a joint's ROM without changes in the mechanical properties of the muscles across the joint [92].Together with the fact that the increased ROM was strongly correlated with age-related increased fascia thickness [93] and with decreased sciatic nerve shear wave velocity [92], it seems possible that static stretching-induced changes in passive joint behavior is attributable, in part, to changes in the mechanical properties of the non-muscular structures (for review see [91,94]).In addition to the possible mechanical contribution of the non-muscular structures, given that fasciae are densely innervated with sensory nerves, including proprioceptors and nociceptors [95,96], the sensitivity of the sensory neurons responsible for the sensation of pain (e.g., nociceptors) may be altered in response to static stretching and thus may contribute to an increase in stretch tolerance.
It is also plausible that an increase in stretch tolerance might result from some neural adaptations, which could be reflected in altered neural responses to static stretching.However, the results of our meta-analyses demonstrated no significant change in neural responses.Although a meta-analysis for the effects of chronic static stretching on neural responses was not conducted due to the limited data available from the reviewed studies, neural responses to chronic static stretching are seemingly not associated with increased stretch tolerance, which was evaluated at a stretched position.For example, the spinal reflex excitability (e.g., H max /M max ratio) was significantly reduced in the resting lengths but not in the stretched lengths of both the SOL and MG after 3 weeks static of stretching training [72], and the maximum voluntary activation (e.g., RMS EMG) of the triceps surae did not change after a 12-week static stretch intervention [82].Collectively, it appears that acute and chronic static stretching may not lead to a significant change in neural responses, and an increase in stretch tolerance may be possible without significant changes in reflex and voluntary responses.
Decreases in maximum isometric force generation after acute static stretching, so called stretch-induced force loss, have received consistent attention, probably because reduced muscle performance after static stretching as a means of a warm-up before competitive exercise is undesirable.Two primary hypotheses have been developed to explain such stretching-induced force deficits: (1) mechanical factors such as altered MTU mechanical properties and (2) neural factors such as a reduced efferent neural drive and altered motor control strategies (for review see [97]).First, the results of our meta-analyses appear to support the mechanical hypothesis that more compliant tendons can lead to stretchinginduced force loss, presumably shifting the fiber/fascicle operating range toward the ascending limb of the active force-length curve [98,99].In addition, the fact that the overall effect size was significant for the MTU mechanical properties and shear elastic modulus but not for muscle fascicle length suggests the possibility of stretching-induced changes in the mechanical properties of the intramuscular connective tissue (e.g., endomysium, perimysium, epimysium) [73,100].As these connective tissues connect adjacent fibers, fascicles, and muscles directly and indirectly, they are known to provide a route by which active contractile forces can be transmitted to the adjacent fibers/fascicles/muscles, also referred to as myofascial force transmission (for review see [101][102][103]).In this regard, more compliant connective tissues, as a result of static stretching if any, could compromise the efficiency of the force transmission, potentially resulting in decreased MVIT.
The results of our meta-analyses did not support the possible contribution of corticospinal excitability and spinal reflex excitability to the stretching-induced force loss, as also suggested by previous studies [54,56,104].However, based on the fact that the EMG amplitude normalized to M max and the relative voluntary muscle activation (estimated using the interpolated twitch technique) significantly reduced after stretching [104,105], there seem to be other potential neurophysiological mechanisms that may influence the effective, voluntary neural drive.For example, afferent signals from some peripheral proprioceptive structures, such as muscle spindles, Golgi tendon organs, and free nerve endings, are likely integrated in the process of muscle force control (for review see [106]).In particular, the amplification of the motor command provided by persistent inward currents in motor neurons seems to be reduced after static stretching, which might prevent the motoneurons from discharging at higher firing rates (for review see [31]).Indeed, Mazzo et al. [107] reported that the magnitude of neural drive (estimated from the cumulative spike train) to the plantar flexors significantly increased after static stretching, albeit during low isometric contraction at 10% of MVIT.Future studies are required to study the effects of static stretching on spinal motoneuron firing behaviors and the association between motoneuron excitability and stretch-induced force loss.
The present review has several limitations.First, this review does not include all the relevant studies, probably due to the limitations in the keyword searches and search engine algorithms used.This may have biased the results of the current meta-analyses.Second, the static stretching protocols in each of the studies varied in terms of dosage, intensity, and target joint and muscle, and this was not considered in the meta-analyses.Third, the participants in the studies were mostly young healthy male individuals, so the current results may not be generalizable to other populations, such as older adults, female individuals, and patients with neurologic diseases or musculoskeletal injuries.Lastly, many data points in the meta-analyses came from the same articles or research teams, which may lead to an overestimation of the effect size.
In conclusion, static stretching may be effective for increasing range of motion and reducing muscle, tendon, and muscle-tendon unit stiffness.However, the reduced stiffness of tendons and the muscle-tendon units is likely an acute response, suggesting that longterm (chronic) static stretching may be beneficial for improving the flexibility of joint and muscle tissue than the flexibility of tendons and the muscle-tendon units.Moreover, static stretching alone may not significantly change neural excitability and muscle architecture.Given that long-term disuse (e.g., due to neurological diseases) likely leads to measurable changes in neural excitability and muscle contractile properties, further studies are required to explore the impact of various aspects of static stretching protocols, such as intensity, duration, and dosage.Furthermore, it is essential to investigate how the integration of static stretching with various sensorimotor stimuli can reshape the neuromuscular systems.

Figure 1 .
Figure 1.The systematic process for including studies relevant to the scope of this review.

Figure 1 .
Figure 1.The systematic process for including studies relevant to the scope of this review.
Appl.Sci.2023, 13, x FOR PEER REVIEW 23 of 31 from the three studies, and 12 out of the 13 data points showed a significant decrease.A meta-analysis revealed a significant overall moderate to large effect with no heterogeneity (d = −0.58;95% CI = −0.76 to −0.40; p < 0.001; I 2 = 0%; Figure6F).

Figure 6 . 6 .
Figure 6.Forest plots of the effect sizes and 95% confidence intervals of chronic static stretchinginduced changes in fascicle length (A), pennation angle (B), muscle thickness (C), muscle stiffness Figure 6.Forest plots of the effect sizes and 95% confidence intervals of chronic static stretchinginduced changes in fascicle length (A), pennation angle (B), muscle thickness (C), muscle stiffness (D), tendon stiffness (E), and shear elastic modulus or shear wave speed (F).Note that the red diamond

Table 1 .
Summary of acute effects of static stretching.

Table 2 .
Summary of chronic effects of static stretching.