Global Running Day serves as a reminder of the profound physical challenges a marathon presents. As the London Marathon approaches on April 26, many are inspired to take up the 26.2-mile challenge. But what exactly happens to the body during such an endurance event? Francesca Bagshaw, a performance physiologist at Nuffield Health MIHP, provides insights.
Increased Breathing and Heart Rate
During a marathon, both breathing and heart rate escalate significantly. Bagshaw explains that muscles require more oxygen and nutrients to produce energy for running. “We naturally breathe more to get more air into the lungs, and the heart beats faster to transport oxygen to the muscles,” she says. Stroke volume—the amount of blood pumped per beat—also increases to meet demands. Some runners experience ‘cardiac drift,’ where heart rate rises disproportionately by 5 to 20 beats per minute, often indicating dehydration or overheating.
Micro-Tears in Muscles
Running a marathon heavily engages lower body muscles like hamstrings, calves, and quadriceps, as well as abdominal and arm muscles. Repetitive movements cause micro-tears in muscle fibers. “These micro-tears stimulate inflammation and adaptations, making muscles stronger over time,” Bagshaw notes. However, during a marathon, they can lead to delayed onset muscle soreness (DOMS) 24-72 hours post-race. Light walking helps maintain blood flow to aid recovery.
Increased Sweat and Thermal Regulation
Exercise generates heat, raising core body temperature from 36-37°C. The body cools itself by redirecting blood flow to the skin, releasing sweat that evaporates. This process leads to fluid and electrolyte loss, risking dehydration if not managed properly.
Metabolic Changes
During a marathon, metabolism shifts from burning glycogen to fat. In early miles, glycogen is the primary fuel due to its rapid energy turnover. As stores deplete mid-race, the body switches to fat oxidation, which produces energy more slowly, increasing perceived effort. Around mile 18-20, runners may ‘hit the wall’—a sudden fatigue and drop in pace due to glycogen depletion. Proper fueling is crucial to avoid this.
