Purpose: The aim of the present study was to evaluate the effects of 6 weeks of a constant-angle hamstring muscle flexibility training on muscle-tendon stiffness and the range of motion (ROM) in young men with limited hamstring ROM.
Methods: 13 participants performed unilateral stretching training (EL), while the contralateral limb acted as control (CL). ROM, relative and peak passive torque, passive stiffness, dynamic knee flexion strength, and active optimum joint angle were assessed before and after the last training session. In addition, participants were tested during the first and last training sessions for first stretch sensation during the stretching procedure only in the EL.
Results: Straight-leg raise and isokinetic knee ROM tests (both p < 0.0001; from 59.4 ± 8.1 to 70.3 ± 9.8, from 28.3 ± 7.6 to 18.5 ± 5.2, respectively) and peak passive torque (p = 0.001; from 53.1 ± 11.7 to 64.9 ± 12.3) increased only in EL and no changes in relative passive torque, passive stiffness, dynamic knee flexion strength, and active optimum joint angle (p > 0.05) were observed. At the point of first stretch sensation, significant increases in passive torque (p = 0.004) and angle (p < 0.001) were found from pre- to post-training.
Conclusion: The flexibility training induced significant increases in ROM alongside increases in peak passive torque (stretch tolerance) and the ROM at which stretch was first perceived. However, this occurred without changes in muscle-tendon mechanical properties or transfer to the untrained limb (CL). These results suggest that limb-specific ROM increases were underpinned by neural adaptations.
Keywords: Knee flexors; Passive muscle stiffness; Passive torque; Range of motion.