Lumbar multifidus layers stiffness during static stretching measured by shear wave elastography

Muscle stretching is widely applied to increase the range of motion and modulate muscle stiffness, partly due to stress-relaxation mechanisms during sustained elongation. This study investigated the acute effects of a 3-minute static stretching protocol on the shear modulus (μ) of the superficial and deep layers of the lumbar multifidus muscle and examined inter-individual variability in stiffness responses. Twenty-two healthy men (23.1 ± 3.9 years) participated. Shear modulus was assessed using shear wave elastography at baseline, immediately after, and 5 min following stretching, as well as continuously during a 3-minute static trunk flexion stretching. Functional flexibility was evaluated using the Schober and Wells tests, and inter-individual variability was explored using K-means clustering. Significant improvements in functional flexibility were observed after stretching (Schober: p < 0.001; Wells: p = 0.004), whereas no significant changes in μ were detected at the group level. However, an exploratory cluster analysis two distinct response patterns: responders exhibited a progressive reduction in stiffness during stretching, whereas non-responders showed an increase in stiffness. These divergent temporal responses are consistent with viscoelastic behavior such as stress-relaxation, but may also reflect differences in neuromuscular activation and stretch tolerance. Overall, these findings indicate that a 3-minute static stretching protocol improves functional flexibility without altering lumbar multifidus stiffness at rest, suggesting that the flexibility gains are not explained solely by changes in multifidus stiffness. They also highlight substantial inter-individual variability in mechanical responses, which should be considered when interpreting stretching interventions.