Recently, declining physical ability of elderly people has become an extremely important social issue. To improve their daily living activities, the standing-up motion is emphasized in this study as an important daily motion. Synergy analysis is applied to the standing-up motion to extract four important groups of muscle activations (synergies). Furthermore, the effect of synergies on body movement is calculated based on a musculoskeletal model of the human body. Results suggest that the first synergy works as preparation of the motion by pulling the ankle and flexing the hip. The second synergy controls the joint moment of the hip and knee joints to raise the hip and move the center of mass forward. The third synergy controls the ankle joint according to movement of the center of mass. The last synergy stabilizes the posture change from a seated to a standing position. Our findings imply that it is important to train those functional muscle activity to enhance the ability of standing-up motion.