During intense running, oxygen supply to muscle cells becomes insufficient. The muscles then break down pyruvate into lactic acid (anaerobic respiration) instead of carbon dioxide and water. The build-up of lactic acid causes muscle cramps.
Difference from aerobic respiration:
Source: Life Processes, Section 5.3 Respiration
---
The examiner expects two points: (1) reason for cramps = lactic acid build-up due to lack of oxygen, and (2) difference = no oxygen used, product is lactic acid (not CO₂ + H₂O), and less energy released. Both points are needed for full 2 marks. Don't confuse anaerobic in muscles (→ lactic acid) with anaerobic in yeast (→ ethanol + CO₂).