Differences in Muscle Activation of Hip Extensors and Erector Spinae during the Good Morning Exercise for Selective Activation of Medial and Lateral Hamstrings according to Hip Rotation

Purpose: This study aimed to investigate differences in muscle activity of the medial and lateral hamstrings based on hip joint position during the Good Morning exercise, as well as the effects on muscle activity of synergist muscles, with implications for hamstring injury prevention and rehabilitation.Methods: Nineteen healthy males with no history of hamstring injury participated in the study.They performed the Good Morning Exercise under four hip rotation conditions.Muscle activity of the erector spinae, gluteus maximus, semitendinosus, and biceps femoris was measured during eccentric and concentric contractions.Results: During the concentric phase, the muscle activity of the erector spinae was significantly higher in conditions of hip internal rotation (p < 0.001), while the muscle activity of the gluteus maximus was significantly lower in conditions of internal rotation (p = 0.011).During the eccentric and concentric phases, the muscle activity of the semitendinosus was higher with internal rotation (p < 0.001), and the muscle activity of the biceps femoris was higher with external rotation (p < 0.001). Conclusion:Rotating the hip joint during the Good Morning exercise can induce selective activation of the hamstrings.