Short and long latency response due to transition from bilateral to unilateral contraction

The aim of this study was to investigate the effect of fast and slow relaxation of the knee extensor muscle of the dominant leg on torque-time curve of the unilaterally contracting contralateral muscle. Eight adult male subjects were recruited without bilateral deficit. In Task 1 subjects exerted bilateral and unilateral torque and bilateral index was calculated. In Task 2 subjects relaxed the right knee extensors as slow as possible while maximum activation of the contralateral muscle had to be maintained. In Task 3 the relaxation was as fast as possible. During slow relaxation the short latency response (dM1) was 6.6% torque reduction in the left leg. On the contrary fast muscle relaxation resulted in a 7.3% increase. During long latency response the torque increased in both tasks, but did not exceed the torque measured in Task 1. Significant correlation was found between the rate of torque reduction (RTR) and dM1 (r = 0.95, P < 0.001), time to peak (t1) and dM1 (r = 0.812, P < 0.01). The regression analysis indicated that RTR greater or less than -1.0 Nm/ms results in opposite short latency response. We concluded that the different tasks for two knee extensor muscles result in transient interhemispheric effects which are time and rate of torque reduction dependent.