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What Happens to Your Body During Exercise

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You can feel the changes as soon as you take your first running step onto the pavement. Your heart pounds, your muscles tighten up, your breath feels shallow. Fortunately, that feeling doesn’t last for long. Ten minutes later, you’ve moved beyond that initial hump of shock and floated into a world where push-ups and burpees are (almost) as comfortable as walking.

Like they say, a body in motion stays in motion. But beyond simple inertia, what is actually happening inside you when you begin to move? Let’s investigate.

When you begin to work out, every part of your body works together to make your movements effective—meaning some parts of you will shut down while other parts work harder. A good example is that your heart will begin to beat faster to pump blood to your muscles, while your stomach will slow down because digestion is not the body’s main priority anymore. In addition, your body tries to accomplish three main things:

  • increase oxygen flow
  • eliminate metabolic wastes
  • eliminate heat

As a result of trying to make all three of those things happen, your body creates something called ATP, scientifically known as adenosine triphosphate. ATP is the basis of function for your body and helps us understand what happens from there. Depending on what workout you are doing, your body will kick into one of three states:

  • phosphagen system
  • glycogen/lactic acid system
  • aerobic respiration system

Let’s go through it.

Phosphagen System: In this state, every one of your cells has enough ATP to last 5-15 seconds. This is crucial because it helps you react immediately to any situation, such as running away, throwing a punch, or lunging out of the way. Hopefully, you never really have to be in any of these positions, but it’s comforting to know that the phosphagen system will be there as you need it. Within those first few seconds of intense physical movement, your body is ready to react.

According to Jason Karp, PhD, “An effective workout for this system is short, very fast sprints on the treadmill or bike lasting 5–15 seconds with 3–5 minutes of rest between each.”

Glycogen/Lactic Acid System: Since 5-15 seconds of physical movement gets used up pretty quickly, your muscles also have a reserve called glycogen. Glycogen is made up of a chain of glucose molecules. From here, it takes 12 different chemical reactions to create ATP, a pretty slow process that lasts for about 90 seconds. The exercise done in this system is “anaerobic” because no oxygen is required. This state can’t last very long because of lactic acid build-up, a common soreness or “burn” that you’ll feel in the first minute of high-intensity movement. Sprinters are known to use this system the most.

Karp says of glycogen, “This system can be trained using fast intervals lasting 30 seconds to 2 minutes with an active-recovery period twice as long as the work period (1:2 work-to-rest ratio).”

Aerobic Respiration System: After you’ve been working out for about two minutes and your body realizes that you’re not stopping anytime soon, it goes into aerobic respiration and responds with oxygen. This is what helps breaks down glucose into carbon dioxide and water. Glucose is available from glycogen in your muscles, through the blood stream, and from food in the intestines. Aerobic respiration lets you work out for a much longer time than the first two systems, gathering its energy from carbs, fats, and then if absolutely necessary, protein.

Many of us use our aerobic systems when we commit to our long workouts, but if you’re looking for a specific training plan, Karp notes that, “the aerobic system can be trained with both continuous exercise and intervals.”

With that in mind, your body will naturally know which system to use when you work out. Many professional athletes will train specific systems to improve in their sport, but that doesn’t mean you can’t also manipulate your routine to match. For a more detailed description of how to format those workouts, try this.

Now knowing all of that, what is actually happening to the rest of your body when you start to move?


Blood flow increases as your body supplies additional blood cells to your rapidly beating heart.


As you warm up, your body is trying its best to release heat. Your blood vessels dilate, bring heat towards the skin, and then release it. This is why your skin feels warm when you work out: It’s your body’s way of getting all the inner heat out. Some people’s faces will turn red during a workout, signifying that heat is leaving the body.


In addition to calling on one of the previously mentioned systems to gain energy and ATP, your muscles also tear. But don’t worry — these are tiny “micro tears” that take a day or two to rebuild. The tears explain why your muscles feel sore, and the rebuilding is how they get stronger over time.


VO2 Max is a term you may have heard around the gym, and it represents the maximum amount of oxygen that a person can use. When you work out, your lungs work quickly to take in all the oxygen that your body requires. Over time, as you get more fit, your VO2 Max will be higher.


Remember how working out for more than two minutes takes your body into aerobic respiration?

This means that oxygen is needed throughout the whole body. As a result, your heart rate will increase to efficiently move the oxygen to your muscles.


Your brain loves exercise. The extra blood and oxygen helps you become more alert, awake, and focused. It releases endorphins, the “feel good” hormones in our body. There are some cool graphics that show how the brain literally lights up in more areas even after a 20-minute walk.

Craving a good ol’ fashioned workout now? Check out our favorite 7-minute workouts to give your mind and body an extra boost of energy.

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