The human action of standing up is regarded as a key aspect of rehabilitation for 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 therefore performed electromyography (EMG) kinesiology to analyze the action of standing up and model the results in order to clarify the function of the biarticular muscle in the lower extremity. During standing up with a vertically upward movement of the trunk, EMGs demonstrated full activity of the monoarticular extensors of the knee joint and the biarticular muscle of rectus femoris. In experimental analysis with this model, parallel linkage function of rectus femoris allowed standing up with monoarticular extensors of the knee joint alone providing the driving force. The analysis further showed that in the dynamic action of standing up, parallel linkage retains the inertial force generated by contraction of the monoarticular extensors of the knee joint and thus retains the force of rearward trunk falling and effectively functions to direct floor reaction force toward the center of gravity.
