Effect of 12-week dynamic stretching of hamstrings on joint flexibility and musculoskeletal properties

PURPOSE: This study aimed to examine the effects of a 12‑week hamstrings dynamic stretching (DS) program on joint flexibility, muscle strength, and musculoskeletal properties in the stretched limb, with the non‑stretched limb assessed for comparison. METHODS: The study comprised 18 healthy young adults (12 males, 6 females) who performed hamstrings DS on one leg three times per week for 12 weeks, with the contralateral leg serving as the control. Pre- and post-intervention assessments included knee extension range of motion (KEROM), peak passive torque during KEROM (stretch tolerance), muscle stiffness, and knee flexor strength. Muscle stiffness was evaluated using shear-wave elastography targeting the biceps femoris (BF). Knee flexor strength was assessed using the rate of torque development (RTD) and isokinetic knee flexion torque. RESULTS: KEROM significantly increased in the DS condition (p = 0.025; d = 0.61), with no significant change in the control condition (p = 0.55; d = 0.15). Peak passive torque significantly increased following DS (p = 0.041; d = 0.68), with no change in the contralateral limb (p = 0.21; d = 0.21). BF stiffness significantly decreased in the DS condition and remained unchanged in the contralateral limb (70° knee flexion: DS, p < 0.01, d = 0.79; control, p = 0.30, d = 0.30; 50° knee flexion: DS, p = 0.044, d = 0.55; control, p = 0.18, d = 0.34. RTD and isokinetic knee flexion strength did not change significantly in either condition. CONCLUSION: Twelve weeks of hamstrings DS enhanced stretch tolerance, reduced muscle stiffness, and improved joint flexibility in the stretched limb, with no notable changes observed in the contralateral limb. TRIAL REGISTRATION: Name of trial registry Effects of training and conditioning methods on mechanical properties of skeletal muscle Identifying number (URL) UMIN000051819 ( https://center6.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000058080 ) Date of registration 5th Aug 2023.