The muscles work together with the bones and joints as a team to maintain our upright posture and allow ample movement. The muscles and ligaments surrounding joints provide stability. Joints require regular use to maintain flexible, but they should not be overextended. Joint cartilage, ligaments and tendons are not repaired as quickly and easily as other parts of the body since they are not well supplied with blood vessels. Even the simplest motions involve complex coordination of muscles, all controlled by the brain via the nerves. Some muscles, like the ones controlling the intestinal tract and blood vessel walls, work involuntarily, so we are not even aware that the muscles are being used. Muscles aid the Venous system with the return of blood to the heart, preventing sluggishness. They also provide somebody warmth. Muscles require oxygen from fuel and regular used to remain conditioned. Muscles convert chemical energy into heat and movement. Condition skeletal muscles perform better, and prevent arthritic changes in the joints and spine that are common with age.