Final answer:
Muscle fatigue is the reversible exercise-induced inability of a muscle to generate force, often due to ATP depletion and electrolyte imbalances. By contrast, sarcopenia is an irreversible age-related muscle atrophy that leads to loss of muscle mass and strength, especially affecting sedentary individuals.
Step-by-step explanation:
The reversible, exercise-induced reduction in the ability of a muscle to generate force is referred to as muscle fatigue. This condition occurs when a muscle can no longer contract effectively in response to nervous system signals. During intense exercise, ATP reserves may be depleted hindering normal muscle function, lactic acid buildup can lower intracellular pH affecting enzymes and proteins, and imbalances in Na+ and K+ levels can disrupt Ca++ flow out of the sarcoplasmic reticulum (SR), leading to impaired Ca++ regulation. However, sarcopenia, the age-related atrophy of muscles, is irreversible and results in a significant decline in muscle mass and strength, especially in those who are sedentary, leading to functional impairments such as issues with locomotion, balance, and posture.