MECHANICAL-BEHAVIOR OF HAMSTRING MUSCLES IN LOW-BACK-PAIN PATIENTS AND CONTROL SUBJECTS

The purpose of this study was to measure and compare the passive elast ic moment, the stiffness and the damping coefficient of the hip joint, as functions of the hip and knee joint angles in men with and without low-back pain. Two conventional tests, the straight-leg-raising test and the trunk forward flexion, were also performed and compared betwee n these subjects. The passive elastic moment was measured using an iso kinetic device in the passive mode. This device raised the lower limb from the horizontal position to the straight-leg-raising angle at a sl ow and constant angular velocity. A custom-made splint connected with the lever arm of the isokinetic device maintained the knee in extensio n and the ankle in the neutral position. The damping coefficient of th e hip joint was measured for 0, 15, 45, 60, 75 and 90% of straight leg raising angle of each subject, using the suspension method based on s mall oscillation theory. To ensure that muscles were inactive during t he passive hip moment tests, muscle activity was monitored with surfac e EMG. The stiffness was computed as the ratio of the change in passiv e elastic moment to the change in the hip angle. The passive elastic m oment, the stiffness and the normalized trunk flexion were significant ly different between the two groups respectively. There was, however, no difference between the two groups in the results of straight-leg-ra ise and damping coefficient of the hip. The passive elastic moment was a nonlinear function of the hip flexion angle and showed large inters ubject differences, especially as the joint limit was approached. The damping coeffcient was a polynomial function of the hip flexion angle. The measured variables were analysed using a discriminant function an d it was shown that the two groups were clearly discriminable in a mea ningful manner.