The act of sitting up does not actually require much use of the abdominal muscles. And their insufficient functioning results in weak abdominals in people with sedentary lifestyles. The hip also remains more or less immobile during the prolonged periods of sitting and they tighten and can hardly stretch or extend their range of motion. This results in the degeneration of the hip muscles and may lead to falls due to decreased mobility of the hip. Also your glutes weaken affecting your stability and stride while walking or jumping.