Hi there!

Muscle fatigue and muscle heat are different things.

Muscle fatigue is a condition in which the muscle fibres of an individual performing a continuous strenuous physical activity fails to respond to any stimulus. It arises due to accumulation of lactic acid produced as a result of anaerobic respiration during this strenuous period as the muscle fibres does not get sufficient oxygen to meet its needs and thus it switches over to anaerobic respiration. When a fatigued muscle fibre is allowed to rest, it responds again by contracting.

Muscle heat, on the other hand, is the heat produced by our muscles during its contraction. 

Thanks