If ATP use exceeds the ability of muscle cells to produce ATP aerobically, the cells switch to anaerobic respiration. An important source of energy for muscle contraction is glycogen. When a muscle cell is stimulated once, it contracts and relaxes, a response called a twitch. If the stimulation rate increases, the muscle cell does not completely relax between pulses, and the response strengthens. At a high rate of stimulation, muscle cells reach a sustained state of maximal contraction called tetanus.
Joints attach bones.
Some joints, such as those holding the skull bones in place, are immovable. Freely moving synovial joints consist of cartilage and connective tissue ligaments that contain lubricating synovial fluid. Most voluntary muscles attach to bones, forming lever systems. When a muscle contracts, a bone moves. The muscle end attached to the stationary bone is its origin.