Muscle fatigue is due to the accumulation of:

Muscle fatigue and cramps during sudden or intense physical activity are primarily attributed to the accumulation of lactic acid [1]. When the body undergoes strenuous exercise, the demand for energy exceeds the oxygen supply available to the muscle cells. Consequently, the muscles switch from aerobic respiration to anaerobic glycolysis to produce ATP [5]. In this anaerobic pathway, pyruvate is converted into lactic acid, a three-carbon molecule [1]. The buildup of this acid leads to a decrease in muscle pH, causing the characteristic 'burning' sensation and fatigue. While modern research suggests fatigue is a complex process involving various ionic and metabolic factors, lactic acid remains the standard clinical marker for anaerobic exhaustion and muscle fatigue in general physiology [4]. Other substances like cholesterol, lipoic acid, and triglycerides are involved in long-term metabolism but do not cause acute muscle fatigue [2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Activity 5.5 > p. 88
2. [3] https://www.ncbi.nlm.nih.gov/books/NBK546695/
3. [5] https://pubmed.ncbi.nlm.nih.gov/1450885/
4. [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC11396987/
5. [2] Science , class X (NCERT 2025 ed.) > Chapter 6: Control and Coordination > 6.3 HORMONES IN ANIMALS > p. 109