Primary Fuel for Muscle Contraction During Prolonged Endurance Exercise

The primary fuel for muscle contraction during prolonged endurance exercise is:

Correct Answer: Fatty acids

During prolonged endurance exercise (>2–4 minutes, especially marathon-type activities), the body shifts its primary fuel from glycogen to fatty acids (free fatty acids) via the aerobic/oxidative system. Fat has the highest caloric density and can sustain energy for several hours.

Energy Source Shift During Exercise

• 0–4 sec: Stored ATP (immediate).
• 4–10 sec: Creatine phosphate (ATP-CP system).
• 10 sec–2 min: Muscle glycogen (anaerobic glycolysis / lactic acid system).
• 2–4 min: Muscle glycogen (aerobic system begins).
• >4 min / endurance: Fatty acids (aerobic β-oxidation) — primary fuel for sustained activity.

β-Oxidation of Fatty Acids

Fatty acids undergo β-oxidation in the mitochondria (also the site of Krebs cycle) to produce large amounts of acetyl-CoA, which enters the Krebs cycle for ATP generation. One mole of palmitic acid (16C) yields ~106 ATP molecules.