Release of which one of the following chemicals is responsible for causing fatigue after muscular activity in human beings?

Muscle fatigue and cramps during or after intense physical activity are primarily attributed to the accumulation of lactic acid. During vigorous exercise, the body's demand for oxygen exceeds the supply, creating an anaerobic environment in muscle cells [c1, t2]. In this state, the normal aerobic pathway is bypassed, and pyruvate is converted into lactic acid to allow energy production to continue [c1, t6]. This buildup increases the acidity (lowers pH) within the muscle tissue, leading to the characteristic burning sensation and fatigue [t2, t6]. While lactic acid is eventually cleared or converted back to pyruvate once oxygen becomes available, its immediate accumulation acts as a natural defense mechanism to prevent permanent muscle damage by slowing down contraction systems [t6]. Other substances like uric acid are metabolic waste products of purines, and glucuronic acid is involved in liver detoxification, neither of which causes acute muscle fatigue [t5, t7].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Activity 5.5 > p. 88
2. [2] https://www.scientificamerican.com/article/why-does-lactic-acid-buil/