Rate-dependent hysteresis in the EMG-force relationship: A new discovery in EMG-force relationship

In this study, we analyzed the existence of rate-dependent hysteresis in the electromyography (EMG)-force relationship. Eight able-bodied subjects participated in the experiment. Surface EMG signals were acquired from flexor pollicis longus muscle from 0% to 100% maximum voluntary contraction (MVC). The subject was asked to gradually increase grasping force from 0% to 100% MVC and decrease grasping force from 100% to 0% MVC at five different frequencies (1.5, 1, 0.5, 0.25 and 0.125 Hz). Mean absolute value (MAV) was chosen to represent the EMG signals and force signals. In order to compare differences in force between contraction and relaxation periods to EMG activity among different frequency conditions, a hysteresis index (HI), defined as an area inside the hysteresis cycle, was adopted. The results showed that all mean values of HI in different frequency conditions were larger than 0, which proved that hysteresis cycles existed in all frequency conditions. The results also showed that the HI values in different frequency conditions were significantly different from each other (p < 0.005), which proved hysteresis effects in EMG-force relationship were rate-dependent. The rate-dependent hysteresis in EMG-force relationship has a huge potential to improve the estimation performance of grasping force from EMG.