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danilosanmartin
If there is no energy left for the muscle, then the muscle is tired and needs to rest. This is because there will be no more energy (in the form of ATP) to supply the excitation-contraction coupling mechanism (the mechanism for muscle contraction, which needs energy).

In the event of this situation, the person will not be able to move the muscle with depleted energy source at all.
The supply of energy-rich molecules in muscles are used up, then the muscles become tired, and they need to rest.

 Lactic acid is being produced from anaerobic respiration. If compared to cellular respiration, fermentation does not produce much energy.