Final answer:
ATP depletion in a muscle fiber would lead to an inability to contract, as this energy-reserve molecule is essential for both the muscle contraction and relaxation processes. Without ATP, muscles fail to contract and cannot detach from their contracted state, leading to rigor.
Step-by-step explanation:
If ATP was completely depleted in a muscle fiber, the muscle would suffer from an inability to contract. ATP is crucial for muscle contraction; it provides the energy needed for the cross-bridge cycling that enables muscle fibers to slide and generate tension. Without ATP, not only do muscles fail to contract, they also cannot relax if they are in a contracted state. ATP is necessary to detach the myosin heads from the actin filaments, a step that must occur for muscles to relax after contraction. This challenge posed by ATP depletion leads to a state called rigor, where the muscle remains in a contracted and stiff condition.
Moreover, muscle fatigue is associated with reduced availability of ATP and manifests when a muscle can no longer contract in response to neural signals. This makes the preservation and timely replenishment of ATP via mechanisms like creatine phosphate, anaerobic glycolysis, and aerobic metabolism essential for sustained muscle function and avoidance of fatigue.