why do we tighten out muscles against cold?
Consider most animals instinctively recoil from unpleasant sensations. Resisting this often involves tightening the muscles. Think about the last time you had to get a shot or pull a splinter out of the skin, you often tighten the muscles in an attempt to resist your own instinctual reaction. Even non-physical reactions like resisting fear or disgust often have similar responses, the mind and body do affect each other. Physical actions can effect mental states. Tensing muscles has been shown to have a direct effect on our willpower.