Contraction is triggered by calcium ions and during contraction the length of the sarcomere changes. M due to sequestring of calcium ions SR by calsequestrin binding. The calsequestrin concentration is higher in fast contracting muscles. During contraction phase the cytosol Calcium ion concentration rises to 5 x 10 -6M .
Muscle shortens against constant load.
Isometric contraction Muscle generates increased tension in an attempt to shorten. The necessities for contraction are decided by voluntary or reflex activities from brain or spinal cord respectively. Action potential is carried from spinal nerve by axon to all its link muscles. In the muscles at neuromuscular junction the action potential releases quanta of acetylcholine in the synapses located between the axon and muscle fiber . The acetylcholine changes the electrical potential of resting muscle fiber membrane and initiates new actin potential.