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

PurposeThis 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.MethodsTwenty healthy males lay in prone position and held their lower limb with hip flexion at 45 degrees and knee flexion at either 10 degrees or 80 degrees. 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+3kg), 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.ResultsThe 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 degrees than at 10 degrees for Unloaded, but not for either Loaded or Loaded+3kg. The neuromuscular activity of the hamstrings was significantly greater at knee flexion of 80 degrees than at 10 degrees for both Loaded and Loaded+3kg. The muscle stiffness or neuromuscular activity of the gluteus maximus showed no significant differences between knee angles.ConclusionsWhen 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.