A student is performing repeated bicep curls in a gym. During the first few repetitions, oxygen is available to the muscle cells and aerobic respiration occurs. As the student continues, the muscles begin to fatigue and lactic acid builds up. Explain how skeletal muscle cells obtain the energy needed for contraction during aerobic respiration, and explain what happens in the muscle cells and the body when lactic acid accumulates.

During aerobic respiration glucose is oxidised in the mitochondria using oxygen to form CO₂ and H₂O, producing about 36 ATP per glucose molecule. The ATP released is used directly by myosin ATPase to power the cross‑bridge cycle and thus muscle contraction.

When oxygen supply falls, the muscle switches to anaerobic glycolysis. Glucose is converted to pyruvate and then to lactate (lactic acid) by lactate dehydrogenase, yielding only 2 ATP per glucose. The accumulation of lactate and H⁺ ions lowers pH, causing muscle fatigue and pain, which reduces contraction efficiency.

After exercise the excess lactate is carried in the blood to the liver, where it is converted back to glucose via gluconeogenesis (the Cori cycle). This process requires additional oxygen, creating an oxygen debt that must be repaid during recovery.