During intense muscle exercise, the breathing rate increases, but even so, the amount of oxygen we inhale is not sufficient to oxidize glucose to provide the energy necessary to maintain activity for a long time.
However, the muscles have a mechanism that ensures the continuation of the effort, even without oxygen: glucose breaks down to produce lactic acid and release energy, which is used by the muscles to contract.
The more intense and prolonged the muscle activity, the more lactic acid will accumulate in the muscle. For this process there is a limit.
The amount of lactic acid in resting muscles is 0.02%. In a violent exercise, it reaches 0.25%. Above this amount, the muscle is fatigued and unable to continue to contract.
Muscle fatigue is the accumulation of lactic acid in the muscles.