What happens to your body when you run a marathon?

The longer, brighter days may have encouraged you out of winter hibernation, and provided the much-need push to head back onto the trails and pavements for some refreshing springtime runs.

As well as the welcomed blue skies and mild weather, the upcoming London Marathon (April 26) will also undoubtedly inspire many of us to lace up our running shoes and get moving.

While the thought of posing with a shiny finisher’s medal might be enough to  tempt you to enter next year’s ballot, have you ever thought about what actually happens to the body during a marathon?

We spoke to Francesca Bagshaw, a performance physiologist at Nuffield Health MIHP, to find out…

Increase in breathing rate and heart rate

Both our breathing and heart rate pick up in pace significantly during a run, and Bagshaw explains that this is because the muscles demand greater oxygen delivery and nutrients when we exercise, especially during a marathon.

“For us to make and produce enough energy to run and exercise, our muscles require more oxygen,” she explains. “Therefore, during a marathon we tend to naturally breathe more to get more air into the lungs, and then the heart will beat more to transport that oxygen from the lungs into the muscles, so it can be used to make energy for us to keep running.”

Alongside this, runners will also experience a significant increase in stroke volume.

“The volume of blood that leaves the heart per beat will also increase to help us meet the demands of running,” says Bagshaw.

Some marathon runners will also experience a ‘cardiac drift’.

“A cardiac drift is when your heart rate increases disproportionately, usually by five to 20 beats per minute, during exercise,” says Bagshaw. “This is usually a sign that you are dehydrated and are getting too hot, which puts your cardiovascular system under greater strain. You might feel like your heart rate in your chest is running faster compared to the effort that you’re putting out.”

Micro-tears in the muscles

Training and running a marathon engages a lot of muscles in the body.

“Running heavily engages your lower muscles – especially your hamstrings, calves and quadriceps – but then also engages your abdominal muscles to help keep you running in a straight line and your arms to help drive you forward,” says the performance physiologist.

She explains how these repetitive movements and demand on the body will often lead to micro-tears in the muscle.

“These little micro-tears that we get during training will stimulate inflammation and various other responses around the muscle that adapt so the next time you go out and run, the muscle will be stronger and will be able to cope with the load and the intensity that you put on those muscles,” says Bagshaw.

“What can happen during a marathon is that you can sustain micro-tears and then get something called ‘delayed onset muscle soreness’ in the 24-72 hours after a marathon, where the muscles become really sore. To overcome that, do some light walking to keep the blood flowing to the muscles.”

Increase in sweat due to thermal regulation

“As you start to exercise, a massive majority of the energy you’re producing is not actually going into the energy to run – it’s being produced as heat, and that heat from the muscles drives up your core body temperature, which is usually around 36 to 37 degrees,” explains Bagshaw.

“As you get warmer, that triggers your body to want to cool itself down and get back down to its normal temperature by redirecting your blood flow to the skin.

“When you’ve got more blood going to the skin, it heats up the temperature of your skin and the sweat glands will release sweat as you get warmer that will come down the skin, and will evaporate away to try and cool your core body temperature.”

She notes that as a result, there is an increased fluid and electrolyte loss which can lead to dehydration if not properly managed.

Increase in metabolism

“During a marathon, your metabolism – which means how we burn either glycogen or fats as a fuel – increases,” says the performance physiologist. “In the early miles, glycogen is the dominant fuel source for most runners due to its rapid ATP (energy) turnover and efficient oxidation (burn) rate. This enables a runner to sustain pace with little metabolic delay.”

However, she highlights how the glycogen stores will start to decline mid-race and if not sufficiently replaced with fuelling, the body will switch towards fat oxidation and using fats as a fuel.

“Although energy rich, it produces energy and ATP slowly, increasing perceived effort (i.e. heavy legs) at the same pace, or may even result in slowing down pace,” says Bagshaw.

Towards the back end of the race, runners can run out of energy.

“For most recreational marathon runners, their primary fuel source during the marathon is glycogen stored in the muscles and liver. However, these stores are finite,” notes Bagshaw. “As the race duration increases, these stores are depleted and if not sufficiently replaced, a phenomenon known as ‘hitting the wall’ is experienced around mile 18-20 or 28-35km. ‘Hitting the wall’ will often result in the earlier onset of fatigue and ultimately a drop in pacing and performance.”