Biology · Locomotion and Movement · NEET
NCERT (Chapter 17, Locomotion and Movement) says it in one line: repeated activation of the muscles leads to the accumulation of lactic acid due to anaerobic breakdown of glycogen, and this causes fatigue. For NEET, learn all three parts together: repeated use + lactic acid build-up + anaerobic breakdown of glycogen.
It is linked to anaerobic conditions. When a muscle is worked hard for a long time, oxygen supply becomes inadequate. Under this low-oxygen (anaerobic) condition, glycogen is broken down and pyruvic acid is reduced to lactic acid. Under normal aerobic conditions pyruvic acid is formed instead, and fatigue does not build up in the same way.
In muscle fatigue, NCERT specifically says the anaerobic breakdown of glycogen (the stored form of glucose in muscle). The general biochemistry chapter talks about glucose going to lactic acid, but for the Locomotion chapter answer, the word to write is glycogen. NEET can test this exact word choice.
Muscle fatigue is the decline in muscle activity caused by lactic acid accumulation after repeated use. Tetanus (tetanic contraction) is a sustained continuous contraction that happens when stimuli arrive so fast that the muscle cannot relax between twitches. Do not confuse fatigue (tiredness from lactic acid) with tetany (a disorder from low calcium in NCERT's disorder list).
Lactic acid does not store oxygen. The oxygen-storing red pigment in muscle is myoglobin. Muscles rich in myoglobin (red fibres) get more oxygen and resist fatigue better; muscles low in myoglobin (white fibres) rely on anaerobic breakdown of glycogen and tire faster. This is why muscle fatigue and red-and-white muscle fibres are studied together.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Accumulation of lactic acid due to the anaerobic breakdown of glycogen after repeated activation of the muscles.
During heavy exercise oxygen supply to the muscle becomes inadequate, so glycogen is broken down anaerobically and pyruvic acid is reduced to lactic acid instead of being fully oxidised.
Red muscle fibres resist fatigue because they have a high content of myoglobin, an oxygen-storing pigment that supports aerobic respiration. White fibres have less myoglobin and fatigue faster.
No. Lactic acid accumulation causes the tired, weak feeling of fatigue. Muscle cramps and sustained contractions are separate ideas; NEET usually tests fatigue as lactic acid build-up, not as cramp.
NEET usually gives a fill-in or match question where you must pick the correct cause of fatigue. The safe answer is: lactic acid accumulation from anaerobic breakdown of glycogen. Watch for the glucose-vs-glycogen swap.
