Effects of daily static stretch training over 6 weeks on maximal strength, muscle thickness, contraction properties, and flexibility

Optimising the Dose of Static Stretching to Improve Flexibility: A Systematic Review, Meta-analysis and Multivariate Meta-regression

BACKGROUND

Static stretching is widely used to increase flexibility. However, there is no consensus regarding the optimal dosage parameters for increasing flexibility.

OBJECTIVES

We aimed to determine the optimal frequency, intensity and volume to maximise flexibility through static stretching, and to investigate whether this is moderated by muscle group, age, sex, training status and baseline level of flexibility.

METHODS

Seven databases (CINAHL Complete, Cochrane CENTRAL, Embase, Emcare, MEDLINE, Scopus, and SPORTDiscus) were systematically searched up to June 2024. Randomised and non-randomised controlled trials investigating the effects of a single session (acute) or multiple sessions (chronic) of static stretching on one or more flexibility outcomes (compared to non-stretching passive controls) among adults (aged ≥ 18 years) were included. A multi-level meta-analysis examined the effect of acute and chronic static stretching on flexibility outcomes, while multivariate meta-regression was used to determine the volume at which increases in flexibility were maximised.

RESULTS

Data from 189 studies representing 6654 adults (61% male; mean [standard deviation] age = 26.8 ± 11.4 years) were included. We found a moderate positive effect of acute static stretching on flexibility (summary Hedges' g = 0.63, 95% confidence interval 0.52-0.75, p < 0.001) and a large positive effect of chronic static stretching on flexibility (summary Hedges' g = 0.96, 95% confidence interval 0.84-1.09, p < 0.001). Neither effect was moderated by stretching intensity, age, sex or training status, or weekly session frequency and intervention length (chronic static stretching only) [p > 0.05]. However, larger improvements were found for adults with poor baseline flexibility compared with adults with average baseline flexibility (p = 0.01). Furthermore, larger improvements in flexibility were found in the hamstrings compared with the spine following acute static stretching (p = 0.04). Improvements in flexibility were maximised by a cumulative stretching volume of 4 min per session (acute) and 10 min per week (chronic).

CONCLUSIONS

Static stretching improves flexibility in adults, with no additional benefit observed beyond 4 min per session or 10 min per week. Although intensity, frequency, age, sex and training status do not influence improvements in flexibility, lower flexibility levels are associated with greater improvement following both acute and chronic static stretching. These guidelines for static stretching can be used by coaches and therapists to improve flexibility.

CLINICAL TRIAL REGISTRATION

PROSPERO CRD42023420168.

Chronic Effects of Static Stretching Exercises on Skeletal Muscle Hypertrophy in Healthy Individuals: A Systematic Review and Multilevel Meta-Analysis

BACKGROUND

The chronic effect of static stretching (SS) on muscle hypertrophy is still unclear. This study aimed to examine the chronic effects of SS exercises on skeletal muscle hypertrophy in healthy individuals.

METHODS

A systematic literature search was conducted in the PubMed, Web of Science, Cochrane Library, and SPORTDiscus databases up to July 2023. Included studies examined chronic effects of SS exercise compared to an active/passive control group or the contralateral leg (i.e., utilizing between- or within-study designs, respectively) and assessed at least one outcome of skeletal muscle hypertrophy in healthy individuals with no age restriction.

RESULTS

Twenty-five studies met the inclusion criteria. Overall, findings indicated an unclear effect of chronic SS exercises on skeletal muscle hypertrophy with a trivial point estimate (standardised mean difference [SMD] = 0.118 [95% prediction interval [95% PI] = - 0.233 to 0.469; p = 0.017]) and low heterogeneity (I2 = 24%). Subgroup analyses revealed that trained individuals (β = 0.424; 95% PI = 0.095 to 0.753) displayed larger effects compared to recreationally trained (β = 0.115; 95% PI = - 0.195 to 0.425) and sedentary individuals (β = - 0.081; 95% PI = - 0.399 to 0.236). Subanalysis suggested the potential for greater skeletal muscle hypertrophy in samples with higher percentages of females (β = 0.003, [95% confidence interval [95% CI] = - 0.000 to 0.005]). However, the practical significance of this finding is questionable. Furthermore, a greater variety of stretching exercises elicited larger increases in muscle hypertrophy (β = 0.069, [95% CI = 0.041 to 0.097]). Longer durations of single stretching exercises (β = 0.006, [95% CI = 0.002 to 0.010]), time under stretching per session (β = 0.006, [95% CI = 0.003 to 0.009]), per week (β = 0.001, [95% CI = 0.000 to 0.001]) and in total (β = 0.008, [95% CI = 0.003 to 0.013]) induced larger muscle hypertrophy. Regarding joint range of motion, there was a clear positive effect with a moderate point estimate (β = 0.698; 95% PI = 0.147 to 1.249; p < 0.001) and moderate heterogeneity (I2 = 43%). Moreover, findings indicated no significant association between the gains in joint range of motion and the increase in muscle hypertrophy (β = 0.036, [95% CI = - 0.123 to 0.196]; p = 0.638).

CONCLUSIONS

This study revealed an overall unclear chronic effect of SS on skeletal muscle hypertrophy, although interpretation across the range of PI suggests a potential modest beneficial effect. Subgroup analysis indicated larger stretching-induced muscle gains in trained individuals, a more varied selection of SS exercises, longer mean duration of single stretching exercise, increased time under SS per session, week, and in total, and possibly in samples with a higher proportion of females. From a practical perspective, it appears that SS exercises may not be highly effective in promoting skeletal muscle hypertrophy unless a higher duration of training is utilized. PROSPERO registration number: CRD42022331762.

Static Stretch Training versus Foam Rolling Training Effects on Range of Motion: A Systematic Review and Meta-Analysis

BACKGROUND

Long-term static stretching as well as foam rolling training can increase a joint's range of motion (ROM). However, to date, it is not clear which method is the most effective for increasing ROM.

OBJECTIVE

The purpose of this systematic review and meta-analysis was to compare the effects of static stretching and foam rolling training on ROM.

METHODS

The literature search was performed in PubMed, Scopus, and Web of Science to find the eligible studies. Eighty-five studies (72 on static stretching; and 13 on foam rolling) were found to be eligible with 204 effect sizes (ESs). For the main analyses, a random-effect meta-analysis was applied. To assess the difference between static stretching and foam rolling, subgroup analyses with a mixed-effect model were applied. Moderating variables were sex, total intervention duration, and weeks of intervention.

RESULTS

Static stretch (ES =  - 1.006; p < 0.001), as well as foam rolling training (ES =  - 0.729; p = 0.001), can increase joint ROM with a moderate magnitude compared with a control condition. However, we did not detect a significant difference between the two conditions in the subgroup analysis (p = 0.228). When the intervention duration was ≤ 4 weeks, however, a significant change in ROM was shown following static stretching (ES =  - 1.436; p < 0.001), but not following foam rolling (ES =  - 0.229; p = 0.248). Thus, a subgroup analysis indicated a significant favorable effect with static stretching for increasing ROM compared with foam rolling (p < 0.001) over a shorter term (≤ 4 weeks). Other moderator analyses showed no significant difference between static stretch and foam rolling training on ROM.

CONCLUSIONS

According to the results, both static stretching and foam rolling training can be similarly recommended to increase joint ROM, unless the training is scheduled for ≤ 4 weeks, in which case static stretching demonstrates a significant advantage. More studies are needed with a high-volume foam rolling training approach as well as foam rolling training in exclusively female participants.

Discussing Conflicting Explanatory Approaches in Flexibility Training Under Consideration of Physiology: A Narrative Review

The mechanisms underlying range of motion enhancements via flexibility training discussed in the literature show high heterogeneity in research methodology and study findings. In addition, scientific conclusions are mostly based on functional observations while studies considering the underlying physiology are less common. However, understanding the underlying mechanisms that contribute to an improved range of motion through stretching is crucial for conducting comparable studies with sound designs, optimising training routines and accurately interpreting resulting outcomes. While there seems to be no evidence to attribute acute range of motion increases as well as changes in muscle and tendon stiffness and pain perception specifically to stretching or foam rolling, the role of general warm-up effects is discussed in this paper. Additionally, the role of mechanical tension applied to greater muscle lengths for range of motion improvement will be discussed. Thus, it is suggested that physical training stressors can be seen as external stimuli that control gene expression via the targeted stimulation of transcription factors, leading to structural adaptations due to enhanced protein synthesis. Hence, the possible role of serial sarcomerogenesis in altering pain perception, reducing muscle stiffness and passive torque, or changes in the optimal joint angle for force development is considered as well as alternative interventions with a potential impact on anabolic pathways. As there are limited possibilities to directly measure serial sarcomere number, longitudinal muscle hypertrophy remains without direct evidence. The available literature does not demonstrate the necessity of only using specific flexibility training routines such as stretching to enhance acute or chronic range of motion.

Influence of 8-weeks of supervised static stretching or resistance training of pectoral major muscles on maximal strength, muscle thickness and range of motion

OBJECTIVES

Current research suggests static stretch-induced maximal strength increases and muscle hypertrophy with potential to substitute resistance-training routines. However, most studies investigated the plantar flexors. This study explored the effects of a static stretching program on maximal strength, hypertrophy and flexibility of the pectoralis major and compared the effects with those of traditional resistance training.

METHODS

Eighty-one (81) active participants were allocated to either a static stretching, strength-training or control group. Pectoralis stretching was applied 15 min/day, 4 days per week for 8 weeks, while resistance training trained 3 days per week, 5 × 12 repetitions.

RESULTS

There was an increase in all parameters (strength: p < 0.001, ƞ2 = 0.313, muscle thickness: p < 0.001, ƞ2 = 0.157-0.264, flexibility: p < 0.001, ƞ2 = 0.316) and a time*group interaction (strength: p = 0.001, ƞ2 = 0.154, muscle thickness: p = 0.008-0.001, ƞ2 = 0.117-0.173, flexibility: p < 0.001, ƞ2 = 0.267). Post-hoc testing showed no difference between both intervention groups regarding maximal strength and muscle thickness (p = 0.905-0.983, d = 0.036-0.087), while flexibility increased in the stretching group (p = 0.001, d = 0.789).

CONCLUSION

Stretching showed increases in maximal strength and hypertrophy, which were comparable with commonly used resistance training. Based on current literature, the influence of mechanical tension as the underlying mechanism is discussed. Furthermore, as equipment and comparatively long stretching durations are requested to induce meaningful strength increases in recreationally active participants, practical application seems limited to special circumstances.

Resistance Training Causes the Stretch-Induced Force Deficit-A Randomized Cross-Over Study

PURPOSE

Stretch-induced force deficit suggests an acute stretch-specific strength capacity loss, which is commonly attributed to EMG reductions. Since those deficits could also be attributed to general fatigue induced by overloading the muscle, this study aimed to compare stretching with an exhausting calf raise programme to compare strength and stretching responses.

METHOD

This study included 16 participants with different, high-duration calf muscle stretching effects (10, 20, 30 min of stretching) with resistance training (RT) (3 × 12 repetitions) performed until muscle failure, by using a cross-over study design with pre-post comparisons. Strength was tested via isometric plantar flexor diagnostics, while flexibility was assessed using the knee-to-wall test (KtW) and an isolated goniometer test.

RESULTS

Using a three-way ANOVA, RT strength decreases were greater compared to 10 and 20 min of stretching (p = 0.01-0.02), but similar to those of 30 min of stretching. ROM in the KtW showed no specific stretch-induced increases, while only the stretching conditions enhanced isolated tested ROM (p < 0.001-0.008). No RT-related isolated ROM increases were observed.

CONCLUSIONS

The results showed both interventions had similar effects on strength and ROM in the calf muscles. More holistic explanatory approaches such as fatigue and warm-up are discussed in the manuscript and call for further research.

Effects of Chronic Static Stretching on Maximal Strength and Muscle Hypertrophy: A Systematic Review and Meta-Analysis with Meta-Regression

BACKGROUND

Increases in maximal strength and muscle volume represent central aims of training interventions. Recent research suggested that the chronic application of stretch may be effective in inducing hypertrophy. The present systematic review therefore aimed to syntheisize the evidence on changes of strength and muscle volume following chronic static stretching.

METHODS

Three data bases were sceened to conduct a systematic review with meta-analysis. Studies using randomized, controlled trials with longitudinal (≥ 2 weeks) design, investigating strength and muscle volume following static stretching in humans, were included. Study quality was rated by two examiners using the PEDro scale.

RESULTS

A total of 42 studies with 1318 cumulative participants were identified. Meta-analyses using robust variance estimation showed small stretch-mediated maximal strength increases (d = 0.30 p < 0.001) with stretching duration and intervention time as significant moderators. Including all studies, stretching induced small magnitude, but significant hypertrophy effects (d = 0.20). Longer stretching durations and intervention periods as well as higher training frequencies revealed small (d = 0.26-0.28), but significant effects (p < 0.001-0.005), while lower dosage did not reach the level of significance (p = 0.13-0.39).

CONCLUSIONS

While of minor effectiveness, chronic static stretching represents a possible alternative to resistance training when aiming to improve strength and increase muscle size. As a dose-response relationship may exist, higher stretch durations and frequencies as well as long program durations should be further elaborated.

Comparison of Supervised versus Self-Administered Stretching on Bench Press Maximal Strength and Force Development

PURPOSE

While there is reported superior effectiveness with supervised training, it usually requires specialized exercise facilities and instructors. It is reported in the literature that high-volume stretching improves pectoralis muscles strength under supervised conditions while practical relevance is discussed. Therefore, the study objective was to compare the effects of volume equated, supervised- and self-administered home-based stretching on strength performance.

METHODS

Sixty-three recreational participants were equally assigned to either a supervised static stretching, home-based stretching, or control group. The effects of 15 min pectoralis stretching, 4 days per week for 8 weeks, were assessed on dynamic and isometric bench press strength and force development.

RESULTS

While there was a large magnitude maximal strength increase (p < 0.001-0.023, ƞ2 = 0.118-0.351), force development remained unaffected. Dynamic maximal strength in both groups demonstrated large magnitude increases compared to the control group (p < 0.001-0.001, d = 1.227-0.905). No differences between the intervention group for maximal strength (p = 0.518-0.821, d = 0.101-0.322) could be detected.

CONCLUSIONS

The results could potentially be attributed to stretch-induced tension (mechanical overload) with subsequent anabolic adaptations, and alternative explanatory approaches are discussed. Nevertheless, home-based stretching seems a practical alternative to supervised training with potential meaningful applications in different settings.

Effects of chronic static stretching interventions on jumping and sprinting performance-a systematic review with multilevel meta-analysis

When improving athletic performance in sports with high-speed strength demands such as soccer, basketball, or track and field, the most common training method might be resistance training and plyometrics. Since a link between strength capacity and speed strength exists and recently published literature suggested chronic stretching routines may enhance maximum strength and hypertrophy, this review was performed to explore potential benefits on athletic performance. Based on current literature, a beneficial effect of static stretching on jumping and sprinting performance was hypothesized. A systematic literature search was conducted using PubMed, Web of Science and Google scholar. In general, 14 studies revealed 29 effect sizes (ES) (20 for jumping, nine for sprinting). Subgroup analyses for jump performance were conducted for short- long- and no stretch shortening cycle trials. Qualitative evaluation was supplemented by performing a multilevel meta-analysis via R (Package: metafor). Significant positive results were documented in six out of 20 jump tests and in six out of nine sprint tests, while two studies reported negative adaptations. Quantitative data analyses indicated a positive but trivial magnitude of change on jumping performance (ES:0.16, p = 0.04), while all subgroup analyses did not support a positive effect (p = 0.09-0.44). No significant influence of static stretching on sprint performance was obtained (p = 0.08). Stretching does not seem to induce a sufficient stimulus to meaningfully enhance jumping and sprinting performance, which could possibly attributed to small weekly training volumes or lack of intensity.

Effects of a Home-Based Stretching Program on Bench Press Maximum Strength and Shoulder Flexibility

Recent research showed significant stretch-mediated maximum strength increases when performing stretching between 5 to 120 minutes per day with the calf muscle. However, since the practical applicability of these long stretching durations was questioned and studies exploring the transferability to the upper body are scarce, the aim of this study was to investigate the possibility of using a home-based stretching program to induce significant increases in maximum strength and flexibility. Therefore, 31 recreationally active participants (intervention group: 18, control group: 13) stretched the pectoralis major for 15min/day for eight weeks, incorporating three different stretching exercises. The maximum strength was tested isometrically and dynamically in the bench press (one-repetition maximum: 1RM) as well as shoulder range of motion (ROM) performing bilateral shoulder rotation with a scaled bar. Using a two-way analysis of variance (ANOVA) with repeated measures, the results showed high magnitude Time effects (ƞ² = 0.388-0.582, p < 0.001) and Group*Time interaction (ƞ² = 0.281-0.53, p < 0.001-0.002), with increases of 7.4 ± 5.6% in 1RM and of 9.8 ± 5.0% in ROM test in the intervention group. In the isometric testing, there was a high-magnitude Time effect (ƞ² = 0.271, p = 0.003), however, the Group*Time interaction failed to reach significance (p = 0.75). The results are in line with previous results that showed stretch-mediated maximum strength increases in the lower extremity. Future research should address the underlying physiological mechanisms such as muscle hypertrophy, contraction conditions as well as pointing out the relevance of intensity, training frequency and stretching duration.

Physiology of Stretch-Mediated Hypertrophy and Strength Increases: A Narrative Review

Increasing muscle strength and cross-sectional area is of crucial importance to improve or maintain physical function in musculoskeletal rehabilitation and sports performance. Decreases in muscular performance are experienced in phases of reduced physical activity or immobilization. These decrements highlight the need for alternative, easily accessible training regimens for a sedentary population to improve rehabilitation and injury prevention routines. Commonly, muscle hypertrophy and strength increases are associated with resistance training, typically performed in a training facility. Mechanical tension, which is usually induced with resistance machines and devices, is known to be an important factor that stimulates the underlying signaling pathways to enhance protein synthesis. Findings from animal studies suggest an alternative means to induce mechanical tension to enhance protein synthesis, and therefore muscle hypertrophy by inducing high-volume stretching. Thus, this narrative review discusses mechanical tension-induced physiological adaptations and their impact on muscle hypertrophy and strength gains. Furthermore, research addressing stretch-induced hypertrophy is critically analyzed. Derived from animal research, the stretching literature exploring the impact of static stretching on morphological and functional adaptations was reviewed and critically discussed. No studies have investigated the underlying physiological mechanisms in humans yet, and thus the underlying mechanisms remain speculative and must be discussed in the light of animal research. However, studies that reported functional and morphological increases in humans commonly used stretching durations of > 30 min per session of the plantar flexors, indicating the importance of high stretching volume, if the aim is to increase muscle mass and maximum strength. Therefore, the practical applicability seems limited to settings without access to resistance training (e.g., in an immobilized state at the start of rehabilitation), as resistance training seems to be more time efficient. Nevertheless, further research is needed to generate evidence in different human populations (athletes, sedentary individuals, and rehabilitation patients) and to quantify stretching intensity.