Final answer:
Complete recovery of muscle fibers after sustained, intense activity typically requires up to a day due to the repair of muscle fibers and restoration of energy stores.
Step-by-step explanation:
After sustained activity at higher levels, complete recovery for a muscle fiber can take up to a day or even longer in some cases.
This duration allows for the repair of tiny tears in muscle fibers, a condition known as delayed onset muscle soreness (DOMS), and the replenishment of energy stores.
Muscle fatigue is influenced by ATP depletion, lactic acid buildup that lowers intracellular pH, and possibly damage to the sarcoplasmic reticulum and sarcolemma, resulting in impaired calcium regulation.
These effects combined with necessary reparative processes mean that option 2, "A day," is the best answer to the question of how long complete recovery can take after intense physical activity.