As the London Marathon approaches, many runners are training for the 26.2-mile challenge. But what exactly happens to the body during a marathon? Performance physiologist Francesca Bagshaw explains the key physiological changes.
During a marathon, breathing and heart rate increase significantly to deliver more oxygen to working muscles. The heart also pumps more blood per beat, a phenomenon known as increased stroke volume. Some runners experience 'cardiac drift', where heart rate rises disproportionately by 5–20 beats per minute, often a sign of dehydration or overheating.
Running heavily engages the lower body muscles—hamstrings, calves, quadriceps—as well as the abdominals and arms. Repetitive movement causes micro-tears in muscle fibres, leading to inflammation and delayed onset muscle soreness (DOMS) 24–72 hours post-race. Light walking can help recovery by maintaining blood flow.
The body generates significant heat during exercise, raising core temperature. To cool down, blood is redirected to the skin, and sweat is produced. This fluid and electrolyte loss can lead to dehydration if not managed properly.
Energy metabolism shifts during the marathon. In the early miles, glycogen is the primary fuel source, but as stores decline, the body switches to fat oxidation. This transition increases perceived effort and may slow pace, especially if fuelling is inadequate.
