It is quite common for patients with hemiplegia caused by cerebrovascular diseases, diseases of the spine or the lower extremities to have an unstable pelvis when walking or using stairs. We frequently assess such patients and initiate exercise therapy for the gluteus maximus focused on lateral shifts of weight while standing before undertaking walking exercise. The muscle fibers of the gluteus maximus have been reported to have various actions: the upper and lower gluteus maximus both abduct the hip joint. Therefore, we used electromyography to investigate the action of the upper and lower gluteus maximus during lateral shifts of weight while standing for 7 healthy male subjects (14 lower extremities). We measured integrated electromyograms (IEMGs) of the upper and lower gluteus maximus of the shifted side as subjects shifted to a horizontal laterally changed quantity of shift 60%, 70%, 80%, 90% and 95% of the weight without tilting or rotating the trunk and pelvis while standing. IEMGs of the lower gluteus maximus of the shifted side tended to decrease as the lateral shift decreased, and it was significantly higher in the 60% shift than in the 95% shift. The lower gluteus maximus of the shifted side was considered to have been activated to provide anteroposterior stability for the pelvis for action in extending the hip joint in the 95% shift. IEMGs of the upper gluteus maximus of the shifted side was significantly increased as the lateral shift increased. The upper gluteus maximus was found to be activated to provide lateral stability for the pelvis to allow for the abduction of the hip joint in more than 80% shift. Moreover, the upper gluteus maximus was also found to have been activated to provide anteroposterior stability for the pelvis for action extending the hip joint in the 95% shift. We suggest that because of the various actions, as described above, that it is necessary to estimate the separate actions of the upper and lower gluteus maximus for exercises involving lateral weight shifts while standing.
