At the beginning, aerobic respiration is used by his legs to generate energy when there was still sufficient oxygen intake for energy to be generated via oxidative phosphorylation.
Towards the end of the ride, his legs are generating energy through anaerobic respiration due to insufficient oxygen intake to keep up with the energy requirement of his muscles as he cycles continuously. Through anaerobic respiration, pyruvate from glycolysis is converted to lactate by lactate dehydrogenase instead of being channelled to the mitochondria for oxidative phosphorylation. The accumulation of lactate in his muscles is what causes the pain in his leg. After he stops the activity, the pain would reduce as lactate is transported to the liver to be reconverted back into pyruvate.
