The concentration of calcium in the myofibril must decrease.
Explanation:
This is because calcium makes myosin, actin and ATP to interact which actually lead to the contraction of muscle fibre.
Relaxation of muscle fibres occurs when the nerves stop stimulating. The result in the pumping of Calcium into the sarcoplasmic reticulum which then cause a break in the link that connect actin and myosin. Actin and myosin return to their unbound state leading to the relaxation of the muscle .
