You feel it the moment you enter the water. Your heart pounds, your muscles contract, your lungs tighten up! Fortunately, that feeling doesn’t last for long.

A few minutes later, you’ve moved beyond that initial shock (and temperature change) and float into a world where swimming freestyle is almost as comfortable as walking.

Water is nearly 800 times more dense than air, and as they say, a body in motion stays in motion! Beyond the simple inertia, what’s actually happening inside your body as you move through the water? Let’s dive in and find out!

Think of your body as a vessel. When you start your workout, every part of your body works together to move your vessel forward. Some body parts and energy systems work harder than others.

Your heart will begin to beat faster to pump blood to your muscles, while your stomach will slow down because digestion is no longer a priority.

Related: What is the Fastest 50 Free Time Possible?

When you’re exerting physical effort, your body tries to accomplish three main things:

• Increase Oxygen Flow
• Eliminate Metabolic Waste
• Eliminate Heat

Energy Systems 101

Making all those things happens requires your body to create Adenosine Triphosphate, or ATP. ATP is the basis of function of your body’s activity, and depending on the type of workout you do, your body will kick into one of three states:

• Phosphagen System
• Glycogen System
• Aerobic Respiration System

Check out our video on 10 Health Benefits of Swimming:

Phosphagen System

In this state, your cells have enough ATP to last 5-15 seconds. During short-term, intense activities, a large amount of power needs to be produced by the muscles, creating a high demand for ATP.

Example Swim Set: 4 x 15m Bursts @ 2:00

Glycogen System

Since 5-15 seconds of physical movement gets used up pretty quickly, your muscles also have a reserve called glycogen. Glycolysis is the predominant energy system used for all-out exercise lasting from 30 seconds to about 2 minutes and is the second-fastest way to resynthesize ATP.

Example Swim Set: 4 x 100s Best Average @ 3:00

If you want to learn how to swim faster, check out our tips in this video:

Aerobic Respiration System

After you’ve been swimming for about two minutes and your body realizes that you’re not stopping anytime soon, it goes into aerobic respiration and responds with oxygen. The aerobic system, which is dependent on oxygen, is the most complex of the three energy systems.

The metabolic reactions that take place in the presence of oxygen are responsible for most of the cellular energy produced by the body. However, aerobic metabolism is the slowest way to resynthesize ATP.

Example Swim Set: 10 x 200s Short Rest @ 2:30

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

Related: How I Lost 10kg Swimming with MySwimPro

Your heart beats faster, increasing blood flow to supply additional oxygen to your limbs.

As you warm up, your body tries its best to release heat. Your blood vessels dilate, bring heat toward 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 heat out. Some people’s faces (and bodies) turn red during a tough swim, 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.

Pairing your swimming with strength training will make a huge difference difference in your endurance. Try these free Dryland exercises, and check out the Dryland Training Plans in the MySwimPro app!

Related: MySwimPro – The #1 Dryland App for Swimmers

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 swim, 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.

Tracking your heart rate while you swim can be very beneficial to analyzing your performance in the water. With the MySwimPro app, you can analyze detailed reports on your heart rate for all your Swims with a compatible smartwatch. Learn more here.

Related: The 7 Most Beautiful Open Water Swimming Destinations in the World

Your brain loves swimming. 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. The same thing happens when you swim!

Inspiring Swimming Transformation

Meet Patty Deters, a MySwimPro Ambassador who has lost 75lbs, manages chronic illness and finds confidence through swimming. Watch her incredible story below!

For more tips like this, follow MySwimPro on YouTube!

Looking for swim workouts? Download the #1 app for swimmers – MySwimPro! Sign up for MySwimPro Coach to unlock unlimited Swim and Dryland Workouts, personalized Training Plans, Analytics and more!



  1. Alexandra Frerotte on

    Always loved swimming. Growing up in NYC, I would be in the subway by 5 am in order to get an hour swimming before school . When I was forced (by the economic situation in NYC)
    swimming was my therapy . Found W.L pool , back in the 1970″s was an Olympic size , but the rest was dingy.
    After 9 years with my husband and as I approached the BIG 40 , finally I was pregnant. Dr Wilkerson advise ” swimming ” the best exercise , my husband joined in the daily evening swim at Yorktown HS, I swam 80 laps the day before our daughter was born.
    I suffered a Traumatic Brain Injury in 2007 and was assigned a PAL who loves to swim and I went back to W.L ( Sandra Bullock went to W.L and gave the money to get a fantastic Aquatic Center ) Bailey (my PAL) and I were in the pool the day of the earthquake .
    Love to come in and smell that chlorine, it gets you going.

  2. The main reason swimming is better than all other sports is that it is low impact.
    That said it is repetitive, and mind-numbingly boring.
    This is what dealt takes the stamina to train.

  3. Swimming!
    Moving thru water in a state of breath and action not familiar to most land mammals.
    A task handled by commitment,discipline,timing,and pure joy of the breath a gentle balance to be experienced.
    Find that balance, it’s for ever giving and soothing !
    We take for granted so much in living
    Swimming defently brings me back to the basic needs of being !
    I love to swim:)

  4. I was made redundant and was not in a good place ,I started swimming early in the morning that same time I would be going to work . The change in me was amazing instead of dreading the day I felt alive ready for anything .,the best form of exercise for me !

  5. I have been getting tightness in my left calf and some numbness on foot . Not pain just a different feeling. Any ideas. I eat bananas and pickles. Was wearing a knee brace for the last 6 months and I am wondering if I have a nerve entrapment

    • Hi Kayla, thanks for the comment. I’m not sure what’s causing the tightness, but recommend you check it out with a qualified physician.