INFLUENCE OF LONG-LATENCY REFLEX MODULATION ON THE PERFORMANCE OF QUICK ADJUSTMENT MOVEMENTS

The effect of long-latency reflex modulation on the performance of a q uick adjustment movement following a muscle stretch was studied in 26 healthy male subjects. When the subjects felt a sudden angle displacem ent in the direction of a wrist extension they were required to make a n adjustment movement by moving a handlebar, held in the hand? to alig n with a target position as quickly and as accurately as possible. The index of performance (adjustment time) was the time taken to move the handle to the target position from stretch onset. A DC torque motor w as used to evoke electromyographic (EMG) reflex responses on a wrist f lexor. Averaging of the rectified EMC, recorded from surface electrode s placed over the flexor, showed short-and long-latency reflexes !Mi a nd M2 components). For all subjects, the amplitudes of the refer compo nents decreased during the adjustment movement because the target posi tion for this study was fixed to the extension side of the wrist joint . The decrease in the M2 component, which is considered to be a transc ortical reflex, was significantly larger than the decrease in the M1 c omponent, which is spinal reflex. The main finding was of a positive c orrelation between the length of adjustment time and the degree of red uction of M1 and M2 with the adjustment movement (r = 0.602 for M1, P < 0.01: r = 0.850 for M2, P < 0.001). Moreover, there were correlation s between the consistency of the voluntary response onset and the degr ee of M2 decrease (r = 0.5771 P < 0.01), and between the consistency o f the voluntary response onset and the length of the adjustment time ( r = 0.603, P < 0.01). Therefore, we have concluded that the subjects w ho where able to perform adjustment movements within a short time coul d modulate the long-latency reflex of the muscle involved in such move ments in order to make the function of their voluntary muscle activity more effective, and thus were able to respond appropriately.