While we all know that staying physically active is essential to a long, healthy, productive life, we don’t often understand exactly what’s happening behind the scenes of our bodies while we are at the gym. The reality is, exercise changes the body and brain. However, these changes are for the better, not for the worse.


When you exercise, your body calls on the glucose and sugar it has stored away from the foods you eat in the form of glycogen, for the energy required to contract muscles and encourage movement.

It also uses adenosine triphosphate, or ATP, but the body only has small stores of both glucose and ATP. After quickly using up these supplies, the body requires extra oxygen to create more ATP. More blood is pumped to the exercising muscles to deliver that additional O2. Without enough oxygen, lactic acid will form instead. Lactic acid is typically flushed from the body within 30 to 60 minutes after finishing up a workout.

Tiny tears form in the muscles after enough exercise, which helps them grow bigger and stronger as they heal. Soreness only means there are changes occurring in those muscles, which typically lasts for a couple of days.


As your muscles call for more oxygen (as much as 15 times more oxygen than when you’re at rest), your breathing rate increases. Once the muscles surrounding your lungs cannot move any faster, you’ve reached what’s called your VO2 max—your maximum capacity of oxygen use. The higher your VO2 max, the fitter you are.

Like any muscle, the diaphragm can grow tired with all that heavy breathing. Some say that as the diaphragm fatigues, it can spasm, causing a dreaded side stitch, but it’s still not clear what causes a side stitch. Deep breathing and stretching can alleviate discomfort in the middle of a workout, and pre-emptive strengthening in the gym can ward off future issues.

When your lungs are healthy, you keep a large breathing reserve. You may feel ‘out of breath’ after exercise, but you will not be ‘short of breath’. When you have reduced lung function, you may use a large part of your breathing reserve. This may make you feel ‘out of breath’, which can be an unpleasant feeling, but it is not generally dangerous.


Your heart rate increases with physical activity to supply more oxygenated blood to your muscles. The fitter you are, the more efficiently your heart can do this, allowing you to work out longer and harder. As a side effect, this increased efficiency will also reduce your resting heart rate. Your blood pressure will also decrease as a result of new blood vessels forming.

How much your heart rate increases depends on the intensity of your exercise. Many fitness experts differ on how high your heart rate should be during exercise, but they agree that it needs to be substantially higher than your resting heart rate to improve your cardiovascular fitness.


The increased blood flow from exercise also benefits your brain, allowing it to almost immediately function better. As a result, you tend to feel more focused after a workout. Furthermore, exercising regularly will promote the growth of new brain cells. These new brain cells help boost memory and learning.

When you work out regularly, the brain gets used to this frequent surge of blood and adapts by turning certain genes on or off. Many of these changes boost brain cell function and protects you from diseases such as Alzheimer’s, Parkinson’s or even stroke, and wards off age-related decline.

Exercise also triggers a surge of chemical messengers in the brain called neurotransmitters, which include endorphins, often cited as the cause of the mythical “runner’s high.” You’ll also likely feel better thanks to a bump in serotonin, a neurotransmitter well known for its role in mood and depression.

What happens to your body when you exercise?
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