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.