Lactic acid is formed in the muscles during exercise or running. It is formed:

Lactic acid is formed in the muscles during intense exercise when the body's demand for oxygen exceeds the available supply. Under these anaerobic conditions, the body cannot perform aerobic respiration, which is the more efficient method of breaking down glucose [1][3]. Instead, muscle cells switch to an alternative pathway called anaerobic glycolysis [1][2]. In this process, glucose is broken down into pyruvate, which is then converted into lactic acid [1]. This pathway allows for the rapid, though less efficient, production of ATP (energy) to sustain muscle contraction during short bursts of high-intensity activity [1][5]. While aerobic respiration produces significantly more energy, anaerobic glycolysis provides the necessary 'extra' energy from glucose when oxygen is lacking, despite the resulting buildup of lactic acid which can lead to muscle fatigue and cramps [1][2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Activity 5.5 > p. 88
2. [3] Science-Class VII . NCERT(Revised ed 2025) > Chapter 9: Life Processes in Animals > How does the exchange of gases happen? > p. 132
3. [2] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > What you have learnt > p. 99
4. [5] https://www.nature.com/articles/s42255-020-0251-4