How does a muscle shorten during its contraction and return to its original form during relaxation ?

Explanation

(i) Shortening of muscle during contraction : Hint :
(a) Reaching o nerve impulse at neuromuscular junction.
(b) Initiation and spread of action potential on muscle fibre.
(c) Release of calcium ions from sarcoplasmic reticulum.
(d) Role of calcium in muscle contraction.
(e) Rotation of heads of myosin filaments.
(ii) Relexation of muscle fibre:
(a) Active trapping of calcium by sarcoplasmic reticulum.
(b) Breaking of actomyosin complexes.
(c ) Outward sliding actin over myosin filaments.