Muscular System Structure and Contraction (3B) Practice Test

During repetitive stimulation of a skeletal muscle at 50 Hz, intracellular ATP falls from 5.0 mM to 2.0 mM while inorganic phosphate rises. The muscle begins to fail to relax fully between stimuli, producing a sustained increase in baseline tension. Assume cytosolic free Ca$^{2+}$ transients remain normal in amplitude.

Which factor is most likely to limit muscle contraction in the described conditions?

In an experiment on skeletal muscle, extracellular Mg$^{2+}$ is increased substantially. The primary observed effect is reduced quantal content of acetylcholine release at the NMJ, with smaller end-plate potentials. Muscle fiber resting potential is unchanged.

Which factor is most likely to limit muscle contraction in the described conditions?