Motor reaction time as a measure of rapid force generation of a muscle

Motor reaction time (MT) of the vastus medialis, latency from the EMG onset to initiation of the actual movement of the lower leg, was examined for 19 normal subjects during rapid extension of the knee joint at various angles from 90 degrees to 0 degrees in the sitting posture. Muscular tension produced by maximal voluntary contraction (MVC) was measured at 80 degrees of the knee joint for 19 subjects. Also, EMG activities during period of MT were integrated at each angle of the knee for five subjects. MT correlated linearly with the integrated EMG, indicating that MT was a variable representing muscular force required for initiation of the movement. MT increased with extension of the knee and the increment was attributed to the increased effective load of the lower leg due to gravity in the range from 90 degrees to 30 degrees of knee flexion. Thus, MT could be defined solely by the external load against the movement in the mid-range of the joint angle. Moreover, at 80 degrees knee flexion MT did not relate to MVC. It is suggested that MT provides a useful means to evaluate a muscular function in rapid movement and is applicable to both normal and pathological situations.