The muscle shortens during its contraction and returns to its original form during relaxation by the following steps:

i) The neurotransmitter acetylcholine is released when a neural signal reaches the neuromuscular junction and acetylcholine creates an action potential in the sarcolemma.

ii) A release of calcium ions occurs due to the action potential, which is spread through the muscle fibres.

iii) The increase in the calcium ion concentration causes the binding of the calcium ion to a sub-unit of troponin on the actin filaments. This causes the removal of active sites in myosin.

iv) The myosin head binds with the exposed active sites of actin and forms a cross-bridge.

v) This causes the attached actin filaments to move towards the centre of A-band.

vi) The Z-line also moves inwards, and a contraction of the sarcomere occurs in the muscles.

vii) During the muscle contraction, the I-band gets reduced while the A-band retains its original length.

viii) Breaking of cross-bridge occurs when the ADP and Pi releasing myosin goes back to its relaxed state and another new ATP binds which breaks the cross-bridge.

ix) Further sliding occurs when the ATP is hydrolysed again by myosin head, and repetition of cross-bridge formation and breakage occurs.

x) This process continues as the calcium ion concentration causes the formation of an action potential.

The above diagram represents the various stages involved in the cross bridge formation, sliding of filaments and breaking of the cross bridge. These processes occur during muscle shortening during its contraction and returning to its original state during relaxation.