The human action of standing up is regarded as a key aspect of maintaining a basic standard of daily life. At clinical sites, a different phenomenon that departs from the standard theory for this action has been observed, suggesting that a distinctive biarticular muscle function is involved. We have previously showed by dynamic electromyographic analysis of the action of moving the trunk to the vertical that the main agonists are the knee joint monoarticular extensor and the rectus femoris. In the present study, we focused on the function of the biarticular muscle acting in a parallel linkage to direct the floor reaction force toward the center of gravity, by constructing a model with the rectus femoris linked in parallel and measuring these changes to clarify the contribution of the rectus femoris to trunk stability during standing up.
