In this paper, the aim is to clarify the mechanism of generat- ing muscle activity for sit-to-stand motion by analyzing the relationship between muscle activity and sitting and standing posture. For efficient prevention and rehabilitation of disability in sit-to-stand movement, it is important to understand the control mechanism involved in the movement. To clarify the mechanism, it was hypothesized that the muscle activity involved in sit-to-stand movement is generated using sitting and standing posture as input, and the aim is to define the relationship between posture and muscle activity in sit-to-stand motion. Sit-to-stand motion in flat condition was compared with that in tilted ground condition. Through an experiment for one subject, the muscle activity contributing towards raising the hip and pushing the body forward was higher in the tilted condition than in the normal condition. This implies that human standing posture could be recognized based on the feet.