Effect of different knee flexion angles with a constant hip and knee torque on the muscle forces and neuromuscular activities of hamstrings and gluteus maximus muscles

Purpose This study examined the effect of different knee flexion angles with a constant hip and knee torque on the muscle force and neuromuscular activity of the hamstrings and gluteus maximus. Methods Twenty healthy males lay in prone position and held their lower limb with hip flexion at 45° and knee flexion at either 10° or 80°. At these angles, the hip and knee torques are identical. Under three load conditions: passive (referred to as Unloaded ), active ( Loaded ), and active with 3-kg weight added to the shank ( Loaded + 3 kg ), the muscle stiffness (i.e., an indicator of muscle force) and neuromuscular activity of the hamstrings and gluteus maximus were measured using shear wave elastography and surface electromyography. Results The muscle stiffness and neuromuscular activity of the hamstrings and gluteus maximus increased significantly with the load. Muscle stiffness in the hamstrings was significantly lower at knee flexion of 80° than at 10° for Unloaded , but not for either Loaded or Loaded + 3 kg . The neuromuscular activity of the hamstrings was significantly greater at knee flexion of 80° than at 10° for both Loaded and Loaded + 3 kg . The muscle stiffness or neuromuscular activity of the gluteus maximus showed no significant differences between knee angles. Conclusions When the passive force in the hamstrings decreases with knee flexion, sufficient muscle force to maintain the hip and knee torques against an external load is generated by preferentially increasing the neuromuscular activity of the hamstrings, rather than increasing the synergetic muscle force.