What happens to our body when we exercise?

Emma White - Certified Personal Trainer | 12 Sep, 2022

Anyone who has done some level of activity before – whether that be walking or lifting weights – will know that exercising affects how your body feels.

It's common to experience an increase in breathing rate and heart rate, plus your body may feel warmer. Depending on the type of exercise, you may have also experienced aching muscles or that burning sensation when performing repetitions of an exercise as your muscles start to fatigue.

The question is, do you know why these things happen during exercise? To help answer that, here's a little guide to what is happening inside your body to your brain, muscles, lungs and heart when you exercise.

Note: click on the circles at the top of the box to scroll through the guide

  • Brain

    In order for our body to do anything, the brain must send a signal to the relevant body part to perform a specific action. When it comes to exercise, if we're running, our brain sends signals via motor neurons to the muscles in our legs and arms, to activate them so that they cause movement. The relevant muscles will then contract and relax to result in a running motion.

    There is a feedback loop between the brain and muscles – as we exercise, the brain receives information from the muscles which signals for the capacity to increase or decrease as necessary. So essentially, the fatigue we experience when we exercise is not only governed by changes happening within our muscles, but also via our brain signals.

  • Muscles

    As our muscles receive signals from the brain to move, this activates the fibres within our muscles to contract and relax to produce movement. The process requires energy in the form of ATP – adenosine triphosphate. ATP is sometimes referred to as the 'energy currency' of the body, as it is the molecule needed to provide energy to all cells. For our body to make lots of ATP it needs oxygen, so our lungs are signalled to start taking in more oxygen.

    The production of ATP results in waste products, one of which is lactic acid. This is produced when ATP is made in the absence of oxygen, during very high-intensity exercise for example. Lactic acid is responsible for the achy feeling we get in our muscles as they begin to fatigue. Resting for a short time allows the body time to remove the lactic acid, so the aching subsides.

  • Lungs

    Once our muscles start working and needing more ATP, more oxygen is required. This means a signal goes to the lungs to increase our breathing rate so that we take in more oxygen. At rest we typically breathe around 15 times per minute, but this can increase to over 40 breaths per minute during exercise, in order to cope with the oxygen demand. As we take in more air, our lungs absorb oxygen and this is passed into the blood where it is supplied to our working muscles. At the same time, our lungs collect carbon dioxide from blood and remove it from the body – another waste product of energy metabolism. In order for this exchange to happen, blood needs to be circulating around our body so that deoxygenated blood is transported from the muscles to the lungs, and oxygenated blood is taken from the lungs to the working muscles – this is where the heart comes in.

  • Heart

    Our heart is the very special muscle that pumps blood around our body so that our cells can receive oxygen and remove any necessary substances. The increased demand for oxygen from muscles working hard triggers the heart to beat more frequently and sometimes more forcefully – so that it can provide more blood to the body in one pump. This is why we notice an increase in our heart rate when we exercise and why we can sometimes feel it beating out of our chest if we're working at a high intensity. With each beat, the heart pumps oxygenated blood that it's received from the lungs all around the body, and deoxygenated blood from the body is taken to the lungs to receive oxygen.


Long-term effects of exercise

As you can see, there's a lot of work going on to create the movement required to exercise. The above processes require energy, which is why we burn more calories when we exercise. They also place demand on our muscle fibres, lungs and heart which is what leads to the adaptations that make us fitter.

Within our cells we have little organelles called mitochondria, which use oxygen and nutrients to produce ATP. It's adaptations in the efficiency and number of mitochondria in our cells that lead to us becoming fitter over time. The need for more energy from exercise results in the body creating more mitochondria to meet this demand. When we have more mitochondria, we can use oxygen and create energy more efficiently, meaning our heart and lungs don't have to work as hard for the same activity going forward. This enables us to work at a higher intensity and for longer durations. Over time as our body becomes more efficient at providing oxygen and producing energy, our exercise performance improves.

As well as this, our heart strength improves with regular exercise because it's a muscle – and like any other muscle, it gets stronger the more it is worked!

Final word

Understanding the science behind exercise helps us to appreciate what an incredible machine our body is! It should inspire us to do the very best we can to take care of ourselves, our heart in particular. Even just a small amount of activity is better than nothing. So forget the all or nothing mentality, your body will thank you for anything you can do – little and often is way more beneficial than the occasional big workout.

Emma White (Certified Personal Trainer) has always loved fitness. She's passionate about the many benefits of regular exercise, particularly the positive impact on mental health and overall quality of life, as well as how it provides the key to successful weight management.

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