Effect of initial force level on the force-EMG relationship during relaxation

The characteristics of force relaxation have been compared with those of force generation in terms of differences in the magnitude of force control at the target level. However, the effect of the initial force level (before adjustment) on the characteristics of force relaxation remains unclear. In addition, the relationship between force and muscle activity during force relaxation is not well understood. Therefore, this study aimed to investigate the effect of initial force level during force relaxation, focusing on the relationship between force and muscle activity. Participants were instructed to decrease the applied force from starting levels of 70%, 50%, and 30% maximum voluntary contraction (MVC) to a target level of 20% MVC as quickly and accurately as possible. Force and electromyographic (EMG) data for the vastus lateralis (VL) and biceps femoris were recorded and analyzed. The accuracy of force adjustment, speed of application, and the relationship between force and EMG amplitude were evaluated. Errors tended to be greater at the starting level of 30% MVC. Adjustment time increased as the starting level rose from 30% to 50% and 70% MVC. In the VL, the ratio of EMG amplitude to force was highest at a starting level of 70% MVC. These findings show that the characteristics of force relaxation and the relationship between force and EMG amplitude are influenced by the initial force level.