Regardless of whether you are a recreational runner or an elite athlete, running pushes every physiological system to the max. It can leave even the fittest athlete feeling sore and achy from head to toe. The body undergoes a number of significant changes to cope with the metabolic, thermoregulatory and physiological demands of running 26.2 miles. 

The starting line

The starting line at the 1980 Dublin city marathon. Only 26.1 miles to go

As you wait anxiously in anticipation for the race to start, the stress hormone adrenaline will increase and cause your heart rate to increase.  Your brain will also send a signal to your lungs to increase your breathing.

As the race begins, all of your physiological support systems - the musculoskeletal, cardiovascular, respiratory, digestive, immune, nervous, and endocrine system - will immediately go into overdrive. Your heart will have to pump three to four times the amount of blood around your body during a marathon than when you are resting. The blood that is pumped from your heart will be redistributed around your body with more going to your muscles and less to your stomach and abdominal organs such as the liver, kidneys and spleen.

Heat and hydration

The amount of heat produced by the body can increase 30 to 40 fold during a marathon. In order to keep your body temperature between 36 and 38 degrees Celsius, some blood will be diverted from your muscles to the skin, where sweat glands will produce moisture that evaporates into the atmosphere to cool your body

Over the entire course of the marathon, you will lose three to six litres of sweat. Dehydration will occur if the fluid lost in sweat is not adequately replaced. Depending on the environmental conditions, your body may be forced to choose between sending blood towards your heart and working muscles, or into a system of capillaries underneath the skin that act as a cooling system. Either way, your performance may be negatively affected. 

You will take about 40,000 steps during the marathon

In the absence of proper hydration, heat loss will be impeded, blood volume will decrease and your heart rate may start to drift upwards without any change in effort or breathing. Dehydration may occur more rapidly and the risk of early fatigue and heat-related illnesses may increase, especially if the race day turns out to be unexpectedly warmer than normal or if the humidity level is high.

Hitting the wall

The other side of the wall 

During a marathon, your body will use ten times more energy than when doing normal daily activities. The extra energy will come from burning a combination of glucose and fat, with glucose being the preferred fuel. However, humans can only store about 2,000 calories of glucose compared to the 700,000 to 100,000 calories we can store as fat.  

Over the entire course of the marathon, you will lose three to six litres of sweat

Runners burn roughly 100 calories of glucose for every mile of the marathon. That means you will have burned your entire supply of glucose after 32 km and will start to rely almost exclusively on fat for fuel. Because fat provides less energy per unit time than glucose, you will have to slow down. That’s when you’re in danger of hitting the infamous "runner’s wall". Sometimes blood glucose levels may drop to very low levels resulting in a condition called hypoglycemia. Taking sports drinks or sugary gels during the race can help to maintain normal blood glucose levels. 

40,000 steps

You will take about 40,000 steps during the marathon. Each time your foot strikes the ground, a large impact force is created that places extra stress on your muscles and joints. As a result, you may start to feel soreness in your thighs and calves at some stage during the marathon. Some runners also experience soreness in their forearms, shoulders and upper back. Don’t be concerned. Muscle soreness is completely normal during and particularly after a marathon, and the soreness usually goes away within a week. You’ll probably also experience joint pain when you stop running.  

The finishing line

You're nearly there. Photo: William Murphy https://www.flickr.com/photos/infomatique/

After completing the marathon, your immune system may be compromised for several months after running a marathon and leave you susceptible to colds and infections. In some runners, heart function may become impaired for a time after completing the marathon. These changes in heart function tend to occur in recreational runners more often than in trained athletes.  

Your immune system may be compromised for several months after running a marathon and leave you susceptible to colds and infections

Blood flow to your kidneys will decrease during the marathon and may impair kidney function for up to two weeks. Damage to the kidneys will be compounded by dehydration and a rise in core body temperature 

But no matter how you look at it, running a marathon is a huge accomplishment. Crossing the finish line is a surreal feeling that very few ever get to experience. The aches and pains that you experience during and after the race quickly subside and you will be left with a wonderful sense of fulfilment.