On getting signal from the brain the muscles contract as the body pulls calcium from the blood to the muscle cells. Calcium binds with troponin drawing it out of its position and tropomyosin follows troponin as these two proteins are linked together. On movement of the troponin and tropomyosin, actin and myosin activation occurs and they move towards each other thus contracting the muscle. Therefore, free calcium is required for muscle contraction.