Sit-to-stand motion is an important daily activity and disability of the motion can significantly reduce quality of life. Therefore, it is important to understand the mechanism of sit-to-stand motion to prevent such scenarios. It was clarified that sit-to-stand motion is generated by four muscle groups called muscle synergy. However, it is unclear how the muscle synergies are controlled. Human sit-to-stand motion may be planned based on body cognition before the motion. In this study, we aim to clarify the relationship between body condition and muscle activity in sit-to-stand motion. Accordingly, we measured muscle activity when knee movability is disturbed as the condition of body changed. Moreover, we measured muscle activity of normal sit-to-stand motion and the sit-to-stand motion with disturbed knee movability using surface electromyogram. Subsequently, we extracted muscle synergy from measured muscle activity and compared the activity level of muscle synergy. Consequently, it was revealed that the muscle activity contributing to bending forward became larger and the muscle activity contributing to rise hip and stabilize became smaller when knee movability was disturbed. The result suggests that humans compensate the disturbed knee movability with the forward momentum with bending motion. Moreover, it may imply that humans adjust their motion to various environments or body conditions by adjusting the level of activity of bending forward.