The sensation of fatigue in the muscles after prolonged strenuous physical work is caused by

Muscle cells convert glucose to pyruvate; when oxygen is insufficient during prolonged or intense work, pyruvate is reduced to lactic acid, causing its accumulation in muscles [1]. This lactate build-up (and associated increase in hydrogen ions) is produced during anaerobic metabolism when demand outstrips oxygen delivery and glycogen oxidation, and is linked to the burning sensation and reduced ability of muscle to sustain contraction by inhibiting further ATP production. Consequently, the immediate sensation of fatigue after strenuous activity is classically attributed to lactic acid accumulation rather than minor fiber wear or simple glucose depletion, though multiple factors can contribute to overall fatigue.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Activity 5.5 > p. 88