Their limbs are adapted for hanging and grasping, not for supporting their weight. Muscles make up only 25 to 30 percent of their total body weight. Most other mammals have a muscle mass that makes up 40 to 45 percent of their total body weight. Their specialised hands and feet have long, curved claws to allow them to hang upside down from branches without effort, and are used to drag themselves along the ground, since they cannot walk. On three-toed sloths, the arms are 50 percent longer than the legs.