Influences of Hamstring Stretching on Passive Muscle Stiffness Vary Between Hip Flexion and Knee Extension Maneuver: A Pilot Study

抄録

Introduction: Muscle injury such as muscle strain frequently occurs in the hamstring, especially in the long head of the biceps femoris (BFl). Pre-exercise stretching has been widely performed as one of the methods for preventing the hamstring muscle strain in sports. It has been reported that less flexibility of the hamstring muscle-tendon unit assessed by passive range of motion (ROM) of the hip joint increases the risk of the hamstring muscle strain. In contrast, according to some recent literature, stretching appears to have no or little protective effect for sport injuries including the hamstring muscle strain. Taken together, no consensus has been reached as to whether stretching exercise can have positive effects for prevention of sports injuries. One of the possible reasons for the discrepancies is due to the joint at which the stretching exercise and flexibility test of the hamstring are performed. Since the hamstring which comprises the BFl, semitendinosus (ST), and semimembranosus (SM) is a biarticular muscle group, the muscles can be stretched by passive hip flexion or knee extension. Nevertheless, no study has examined the substantial effect of each of the two stretching maneuver on passive stiffness of the individual muscles, perhaps because it is impossible from a conventional evaluation of the torque-angle relationship to quantify the passive stiffness of a specific muscle. One of the methods to resolve the problem is to use ultrasound shear wave elastography which can quantify localized tissue stiffness along the principal axis of the probe. As a pilot study, we used this technique and compared passive stiffness of BFl, ST, and SM before and after an acute bout of static stretching by either hip flexion or knee extension.

Methods: In one healthy male subject, before and after 5 sets of 90-s stretching, passive lengthening measurements where the knee or hip joint was passively rotated to the maximal range of motion (ROM) were performed. During the passive lengthening, muscle stiffness (defined as shear modulus) of each muscle was measured by using ultrasound shear wave elastography.

Resuts: Both stretching maneuvers increased maximal ROM and decreased passive torque at a given hip joint angle. Passive muscle stiffness was prominently reduced in all of BFl, ST, and SM after passive knee extension stretching maneuver, whereas the stretching effect by passive hip flexion maneuver was pronounced in ST and SM, but not in BFl.

Conclusion: The present findings suggest that 1) the effects of hamstring stretching on individual passive muscles’stiffness vary between passive knee extension and hip flexion stretching maneuvers, and 2) stretching of the hamstring should be performed by passive knee extension rather than hip flexion which is commonly used as a warm-up exercise. Further research with large number of subjects is required to confirm our conclusion.