Effect of Different Muscle Relaxation on Motor Control: Comparison Between Ballistic and Ramp Task Methods

The control of muscle relaxation is not simply the cessation of a muscle contraction, but a dynamic control mechanism for the next movement. Muscle relaxation is triggered by neurophysiological control of the central nervous system. Here, two relaxation strategies were compared, Ballistic and Ramp conditions, and the dynamics of excitability changes between the two relaxation strategies were analyzed. The Ballistic and Ramp conditions were established using visual triggers and response sounds from 20% of the maximum voluntary contraction output of the right wrist joint. The excitability of the motor cortex involved in the onset of relaxation was assessed using muscle tension or electromyography as an indicator for the onset of relaxation at -100, -50, and 0 ms. Transcranial magnetic stimulation was used to assess the excitability of the motor cortex at -100, -50, and 0 ms during the response onset tone and visual trigger. The H-reflex was derived from the flexor carpi radialis. There were no differences in the H-reflex between the conditions. In the Ballistic condition, the motor evoked potential was significantly higher at 0 ms, just before muscle relaxation, than at other time points (p < .05). In the Ramp condition, motor evoked potential was attenuated at 0 ms immediately before muscle relaxation compared with that at -100 ms (p < .05). During the onset of muscle relaxation, the excitability of the primary motor cortex showed a transient increase at different timings, depending on the relaxation method used. Thus, we can understand the key timing and methodology for the initiation of relaxation.