Effects of contract–relax, static stretching, and isometric contractions on muscle–tendon mechanics

Introduction: Loading characteristics of stretching techniques likely influence the specific mechanisms responsible for acute increases in range of motion (ROM). Therefore, the effects of a version of contract–relax (CR) proprioceptive neuromuscular facilitation stretching, static stretching (SS), and maximal isometric contraction (Iso) interventions were studied in 17 healthy human volunteers. Methods: Passive ankle moment was recorded on an isokinetic dynamometer, with EMG recording from the triceps surae, simultaneous real-time motion analysis, and ultrasound-imaging-recorded gastrocnemius medialis muscle and Achilles tendon elongation. Subjects then performed each intervention randomly on separate days before reassessment. Results: Significant increases in dorsiflexion ROM (2.5°–5.3°; P < 0.01) and reductions in whole muscle–tendon stiffness (10.1%–21.0%; P < 0.01) occurred under all conditions, with significantly greater changes detected following CR stretching (P < 0.05). Significant reductions in tendon stiffness were observed after CR stretching and Iso (17.7%–22.1%; P < 0.01) but not after SS (P > 0.05), whereas significant reductions in muscle stiffness occurred after CR stretching and SS (16.0%–20.5%; P < 0.01) but not after Iso (P > 0.05). Increases in peak passive moment (stretch tolerance) occurred after Iso (6.8%; P < 0.05), CR stretching (10.6%; P = 0.08), and SS (5.2%; P = 0.08); no difference in changes between conditions was found (P > 0.05). Significant correlations (rs = 0.69–0.82; P < 0.01) were observed between changes in peak passive moment and maximal ROM under all conditions. Conclusions: Although similar ROM increases occur after Iso and SS, changes in muscle and tendon stiffness are distinct. Concomitant reductions in muscle and tendon stiffness after CR stretching suggest a broader adaptive response that likely explains its superior efficacy in acutely increasing ROM. Although mechanical changes appear tissue-specific between interventions, similar increases in stretch tolerance after all interventions are strongly correlated with changes in ROM.