linguistic:paraphrasing
Intense activity

Each muscle contraction involves an action potential that activates voltage sensors, and so releases Ca 2+ ions from the muscle fibre 's sarcoplasmic reticulum. The action potentials that cause this also require ion changes: Na influxes during the depolarization phase and K effluxes for the repolarization phase. Cl − ions also diffuse into the sarcoplasm to aid the repolarization phase. During intense muscle contraction, the ion pumps that maintain homeostasis of these ions are inactivated and this (with other ion related disruption) causes ionic disturbances. This causes cellular membrane depolarization, inexcitability, and so muscle weakness. Ca 2+ leakage from type 1 ryanodine receptor) channels has also been identified with fatigue.