Answer:
ATP
Explanation:
ATP is essential for both contraction and relaxation of muscles, with calcium being the main factor for the affinity between actin and myosin, in order to reach the sarcomere shortening phase.
The function of ATP is to release energy for the occurrence of movement, in addition to other actions. While there is calcium bound to troponin, there is cross-bridge(projections of actin filaments) cycling and muscle contraction.
Thus, ATP has three main functions in muscle contraction:
Providing energy to shorten the sarcomere, enabling actomyosinic shutdown by physical-chemical interaction to the thick filament and supporting the calcium pump, which ends the cycling of the cross bridges.
