Acute effects of different stretching techniques on regional muscle stiffness of the gastrocnemius medialis assessed with shear-wave elastography

Stretching can acutely increase the maximum range of motion (ROMmax). However, its effects on muscle mechanical properties vary depending on the type, intensity, and duration of the stretching maneuver. Shear-wave elastography (SWE) was used to assess muscle stiffness by measuring ultrasound wave speed through the tissue. This speed can be converted into modulus (μ, kPa), providing an estimate of tissue elasticity. Recently, SWE has been applied to investigate stretch-induced changes in muscle stiffness. Since different muscle regions may exhibit different μ values, we examined whether three stretching protocols affected μ differently in two muscle regions. METHODS: Twelve participants (8 men, 4 women; mean±SD: age = 23.9±2.4 years, body mass = 69.8±10.6 kg, height = 172.0±8.5 cm) completed three stretching sessions on separate days. ROMmax was assessed. GM stiffness (μ) was measured using SWE (V8, Samsung Healthcare, Seoul, Korea) at both a neutral ankle angle (90°) and at 80%ROMmax in the middle muscle portion (GMmid) and the muscular region of the GM myotendinous junction (MTJ). Measurements were taken PRE and immediately POST 5×30-s passive static stretching (PS30, 30-s inter-set rest), 5×45-s PS (PS45, 15-s inter-set rest), and 5×30-rep dynamic stretching (DS, 30-s inter-set rest). The order was randomized. The same absolute angle at 80%ROMmax was used in POST evaluations. Within-day reliability was assessed before stretching using intraclass correlation coefficient (ICC) analysis. Data were analysed with repeated-measures ANOVA (within-factors: stretching type, region, ankle angle, time) at p<0.05, with Bonferroni correction for pairwise comparisons. RESULTS: ICCs were >0.80 for all parameters. No significant differences were found at PRE across the three days (p>0.05). No main interactions were observed (p>0.05). PS30 increased ROMmax (8.4%, p<0.01) and decreased GMmid μ at 80%ROMmax (−17.6%, p<0.01). PS45 increased ROMmax (7.8%, p<0.01) and decreased GMmid μ at 80%ROMmax (−14.2%, p<0.01), as well as MTJ μ at both 90° (−13.4%, p<0.01) and 80%ROMmax (−8.6%, p<0.01). DS increased ROMmax (11.1%, p<0.01) and reduced GMmid μ at 80%ROMmax (−12.5%, p<0.01) and MTJ μ at 90° (−12.7%, p<0.01), while the decrease at 80%ROMmax was not significant (−8.3%, p>0.05). CONCLUSION: This study highlights that stretching-induced changes in muscle stiffness can be region-specific and depend on the protocol used. Longer-duration static stretching was more effective in reducing stiffness across GM regions. Dynamic stretching improved ROMmax but had a more limited impact on muscle stiffness, particularly at the MTJ. However, the small sample size and variability in μ may have influenced these findings.