Answer. The legs contain the most muscle groups in your body. Too much sitting causes the quads, hamstrings, and calves to change. In fact, these muscles become less toned as the muscles get smaller and less defined. Indeed, the effects are first seen on your upper legs and hips, where fat deposits are common. These changes are largely because fat in this area tends to be less metabolically active than fat in other areas of the body. Next, the buttocks are made up of three major muscle groups: the gluteus maximus, gluteus medius, and gluteus minimus. Too much sitting causes these muscles to soften and become undefined, leaving the butt looking flat and misshapen. Moreover, as these muscles lose their tone, you may have difficulty maintaining good balance.