Short intense exercise utilizes anaerobic metabolic mechanisms more than more sustained activities. Excitation-Contraction Coupling Contraction in skeletal muscle begins with an action potential in the muscle fiber. This causes the release of calcium from the sacroplasmic reticulum. The point of contact of the nerve and muscle is called the neuromuscular junction which is normally located in the middle of the muscle fiber. Therefore an action potential initiated here spreads toward the ends of the fiber making it possible for all sarcomeres to contract at the same time.
Skeletal muscle has an adaptation that allows the action potential to spread deep within the fiber.
The T or transverse tubules are internal extensions of the sarcolemma that penetrate through the fiber such that action potentials in the t-tubules cause the release of calcium from the nearby sarcoplasmic reticulum in the immediate vicinity of the myofibrils. The sarcoplasmic reticulum contains calcium ions in very high concentration that are released when the adjacent T-tubule is excited. Pumps within the walls of the sarcoplasmic reticulum return the calcium within the cytoplasm to levels below those needed to activate the contractile process. The association of the motor nerve and the muscle fiber occurs at the neuromuscular junction. Here, the neuron ends in a terminal button that contains small vesicles filled with the neurotransmitter acetylcholine.