Cooperative activities of forearm muscles under loading applied to thumb or each finger

Human thumb and fingers are usually subjected to an external loading during daily activity. The information of how muscles in the forearm cooperate with each other in order to response to the external loading is still unknown. Such information may be helpful in understanding muscle function pathology and motor disorder. A novel method called electromyography computed tomography (EMG-CT) was developed to visualize muscle activity within a whole cross-section of the forearm by measuring surface EMG signals around the forearm. The current study aimed to extend the previous work by using the EMG-CT to investigate muscle cooperative activity under loading application to thumb or each finger. Loads of 0.98-9.8 N were applied to the thumb or each finger of four subjects in eight loading directions. The loading directions on thumb and index, middle, and little fingers were inner, outer, and upper directions. EMG signals around the subject’s forearm were recorded during the loading by using EMG band consisting of 40 pairs of bipolar electrodes. The results show different muscle cooperative activity pattern between loading conditions. During load was applied to thumb, muscle in lower region in pronation cross-section were highly active. When load was applied to a finger, muscles in lateral-lower region were highly active. In all subjects, total muscle activity in the whole cross-section and the maximum value of muscle activity increased in proportion to loading. This study demonstrates effectiveness of EMG-CT method by showing that the muscle cooperative activity of an individual is specific to force application conditions.