The rate of skeletal muscle relaxation is related to the rate:

The rate of skeletal muscle relaxation is related to the rate at which free Ca++ is removed from the sarcoplasm. A fraction of a second after its release Ca++ is pumped back into the sarcoplasmic reticulum. This removes Ca++ from the troponin thus allowing the tropomyosin to cover the active sites of the actin. This prevents the formation of cross bridges and causes relaxation. Acetylcholine is secreted at the neuromuscular junction and is necessary for initiation of action potentials at the muscle endplate. Phosphocreatine is a source of energy used to create ATP. ATP hydrolysis occurs in the myosin heads Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong's Review of Medical Physiology, 24e.