What happens to our muscles when we run

What happens to our muscles when we run?

As an athlete, whatever level you are it’s useful to have a basic understanding of how our bodies work and in particular our muscles and their impact on how we perform. The following is a simplified guide as to what happens to our muscles when we run.

Our muscles, in fact every cell in our body uses a source of energy molecules called adenosine triphosphate (ATP), these molecules are made from the food/drinks you digest. There are essentially 3 stages to our biochemical energy sources and can be broken down as follows:

Sprint start / Initial burst (Phosphagen System)

We only store a limited amount of ATP in our muscles at any one time, essentially enough for a few second burst of energy. Now it’s going to be a pretty short race if we only have power for a few seconds so another clever chemical reaction is needed to help maintain this burst of energy. To replenish the ATP levels quickly, muscle cells contain a high-energy phosphate compound called creatine phosphate. The muscle cell turns ATP into another high powered molecule, adenosine diphosphate (ADP), and the phosphagen rapidly turns the ADP back into ATP. While our muscle cells are expert recyclers this process is also limited and will only enable you to continue to sprint or maintain power for a further 8 to 10 seconds so great if you’re Usain Bolt running 100m, but for the rest of us mere mortals who run a bit slower and also for a bit longer we need to find energy from another source. 

Very fast pace (Glycolysis)

So we want to run for more than 10 seconds, this is where ‘Glycosis’ kicks in, which is the second fastest way to resynthesize ATP. Imagine our muscles are like tanks of fuel, they store energy in the form of complex carbohydrate called muscle glycogen. Being complex they are made up of a chain of glucose molecules. The muscle cell uses anaerobic metabolism (anaerobic means “without oxygen”) to make ATP and a byproduct called lactic acid from the glucose. There is a trade off here, the benefit is that this process can help power us at a fast pace for up to 2 minutes, however the down side is that the chemical reaction and production of lactic acid means the muscles eventually lose their ability to contract effectively due the increase in lactic acid, and muscle force production and exercise intensity ultimately decreases leaving us feeling like we’re running through treacle. Now, if we just want our 1 minute of fame, appearing on the start line video or pictures leading the race for 60 seconds, or you happen to find yourself running a 200-400m race then fantastic. By not requiring oxygen this energy system is very useful for that moment of glory or very short race as it takes the heart and lungs a little while to catch up and start pumping oxygen into our system however most of us run more than a few hundred metres and find it much more fun over taking people throughout a race than being over taken by the whole field so this is where oxygen plays it’s part…

Slow, medium & fast pace (Aerobic System)

Oxygen, don’t you just love it, not only does it keep us alive but it plays a huge part in our energy system. Typically by two minutes our heart rate is up and our breathing increases resulting in our body triggering the switch to supply our working muscles with oxygen. When oxygen is present, glucose (remember this is the fuel our cells need to function) can be completely broken down into carbon dioxide and water in a process called aerobic respiration.

There are 3 sources of glucose;

  • Glycogen supplies in the muscles
  • Breakdown of the liver’s glycogen into glucose, which gets to working muscle through the bloodstream
  • Absorption of glucose from food in the intestine, which gets to working muscle through the bloodstream

Aerobic respiration can also use fatty acids from fat reserves in muscle and the body to produce ATP. Have you heard of fat burning or ketosis? Now that’s a whole new article… however understand that your body naturally draws energy from your fat stores as it triggers the most effective energy system to utilise depending on your circumstances. There are times, like in extreme cases (for example starvation) where proteins can also be broken down into amino acids and used to make ATP, however unless you are involved in a very long multi day ultra and not able to eat at all you needn’t worry about this! Aerobic respiration uses carbohydrates first, then fats and finally proteins, depending on the circumstances.

Aerobic respiration is the most complex of the three energy systems and is the system mainly responsible for half-marathons, Marathons and Sprint triathlons. It takes more chemical reactions to produce ATP than either of the above systems. Aerobic respiration produces ATP at the slowest rate of the three systems, but it can continue to supply ATP for several hours or longer, so long as the fuel supply lasts. 

In summary

As you exert yourself your muscles gobble up their glycogen and release lactic acid, also known as ‘the burn’, which in turn signals to the brain that you’re under physical stress. Your heart rate rises along with your core temperature and there becomes an internal struggle for blood flow as your working muscles, vital organs and your bodies natural cooling system (whereby blood flows to the skin, that’s why we go red in the face) all vie for blood flow. But that’s not all, depending on when you last ate or drank, or more importantly, what you last ate or drank your stomach is also vying for blood flow but as your working muscles, vital organs and cooling system are more important your body simply starts to shot your stomach down leading to cramps and worse case scenario a visit to the toilet. This again emphasizes the importance of nutrition and correctly fueling for your exercise. Visit our Sports Nutrition Guides for easy to read fuelling tips for many different disciplines and distances.

But there is more to heavy breathing than just a sign of over exertion, your muscles require oxygen to make the best use of glucose. The fitter you are the more efficient this process is and your body efficiently transports oxygen, burns glucose and even manages to utilse stored fat as an energy source through a process called Gluconeogenesis. However fitness levels aside if you are pushing yourself beyond your comfort zone and not actively replenishing your energy stores your ATP supply will struggle to keep up with the demand. You can’t breath in or process oxygen fast enough, and lactic acid starts to flood-your body. Every step feels harder and harder until you hit the wall and grind to a slow jog, walk or eventual stop. 

An athlete’s ability to use his/her energy systems efficiently will have a great influence on the athlete’s performance, be it effective training or utilising fuelling strategies that can ensure a steady supply of ATP to the muscle. The Mountain Fuel Sports Nutrition system is the perfect way to ensure that you’re muscles are energised and replenished to help you maximise training and achieve your race day goals.

Additional reading:
Another useful article is how your performance can be affected by the heat.