Maybe you’re doing cardio every day. You go for a walk, get on a treadmill, go for a run, or spend some time on an elliptical. You know cardio is good for you, so you make it a part of your daily routine.
But do you really know what's happening inside your body while you're doing it? Why does your heart rate go up? Why do you start breathing harder? Why do you feel hot and start sweating?
And why does the same workout sometimes feel easier after you've been doing it for a while? There’s actually quite a lot going on.
When you start exercising, your muscles need more energy and oxygen. Your heart starts pumping faster, your breathing changes, and your blood flow adjusts to keep up with your body’s increased demand. You can feel some of these changes happening. But there’s a lot more happening that you can’t feel at all.
So let's take a look at what really happens inside your body during cardio.
When You Start Exercising
When your muscles start working harder, they need more oxygen and fuel. Your heart responds by beating faster and pumping more blood. That's why your heart rate goes up during exercise. It's not just some number your fitness tracker is showing you. It's your body trying to keep up with the demand. The harder you exercise, the more blood your muscles generally need.
Then comes the breathing. This is probably the first thing you really feel during cardio. You start breathing faster. Maybe you can normally walk and talk without thinking about either one. But after you've picked up the pace, you might find yourself taking deeper breaths or pausing between sentences. No, it doesn't necessarily mean your body is struggling. It's your body responding to the increased demand.
Your working muscles are using more oxygen and producing more carbon dioxide. So your breathing increases to bring more oxygen into your body and help get rid of the extra carbon dioxide.
How Oxygen Reaches Your Muscles
What happens to that oxygen once you inhale it?
Your lungs take oxygen from the air you breathe. Your blood picks it up and carries it around your body, and your heart keeps the blood moving to the working muscles. The harder you work out, the higher the oxygen demand. That's also why your breathing and heart rate continue to rise as you increase the intensity.
Your body is trying to deliver enough oxygen to keep up with what your muscles are asking for. It’s not just about getting more oxygen into your body. Your body also needs to expel the carbon dioxide produced during exercise. So while you're breathing in more oxygen, you're also breathing out more carbon dioxide.
Why Do You Start Sweating?
You’ve probably noticed that you feel hot within just a few minutes into exercise. After a while, you start sweating. It’s your body’s way of keeping your temperature under control.
When your muscles are working, they produce heat. The harder and longer you exercise, the more heat your body has to deal with. Your body increases blood flow to the skin and starts producing sweat to help cool you down. As the sweat evaporates from your skin, it takes some of that heat away with it.
Let me ask you a question: have you ever wondered why the same workout can feel so different at different times of the day?
A lot of it has to do with the weather and the conditions you're exercising in. If you’re working out on a cool, dry morning, you may feel comfortable even after an intense session. Do the same workout on a hot, humid day, and you may feel much more tired and sweaty.
Honestly, many of us think that the more we sweat, the more effective our workout must be. But that's not necessarily true. Some people naturally sweat more than others. And temperature, humidity, what you're wearing, how hydrated you are, and how hard you're exercising can all affect how much you sweat.
Why Does Your Workout Start Feeling Harder?
You might start your workout feeling completely fine. But after 10-20 minutes, you notice that the same pace doesn't feel quite as easy anymore. Your breathing is heavier. Your muscles feel tired. You may even feel like slowing down.
What's happening?
As you continue exercising, your muscles keep demanding energy. The longer and harder you exercise, the more your body has to work to meet that demand. Your heart and breathing work harder to deliver oxygen, while your muscles rely more on other energy systems as intensity increases.
The Burning Feeling
There’s another thing you may notice when you push yourself during cardio: that burning feeling in your muscles. Maybe your legs start burning when you're running uphill. Or your thighs start to feel heavy when you're cycling hard. At some point, you may have heard someone say, “That’s lactic acid building up.” For a long time, lactic acid was blamed for that burning sensation and muscle fatigue during intense exercise.
So what does lactic acid really do? Lactic acid or lactate, to be more precise, is produced when your muscles break down glucose to help make energy. Your body produces more lactate when you're exercising at a high intensity and the demand for energy increases. But lactate does not stay in your muscles for long and gets cleared by your body within about an hour after you stop working out.
So what are you actually feeling? When you exercise at a high intensity, your muscles are producing energy very quickly. A number of changes happen inside the muscle, including changes in acidity and other metabolites, and these contribute to the burning and fatigue you feel.
If you’re feeling sore the next day, or even 24 to 48 hours after an intense or unfamiliar workout, that’s a different story. This is often called delayed-onset muscle soreness, or DOMS. When you put your muscles through exercise they're not used to, especially exercises that involve a lot of lengthening under tension, the muscle fibers can experience microscopic damage.
Your body responds to this with a repair and inflammatory response, which is part of why you feel sore and stiff later. So, if you wake up in the morning after a tough workout and suddenly feel it in your legs, that soreness isn't because lactic acid is still sitting in your muscles. It's your body's response to the exercise you did.
What happens when you do cardio regularly?
You may have a question in your mind: “Can I do cardio every day?” The short answer is yes. Your body is more adaptable than you think, and regular cardiovascular exercise can help improve both physical and mental health. That said, how your body responds depends on your routine, exercise intensity & duration, and recovery.
Here are some of the many benefits of doing cardio every day:
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Improves cardiovascular health by strengthening heart muscle, lowering blood pressure, and improving blood circulation.
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Helps regulate your mood by releasing endorphins, the feel-good chemicals in your brain.
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Helps you manage your weight
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Improves sleep quality and helps you fall asleep faster
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Increases your energy and stamina
By now, you might be thinking: if cardio is good for you, can you just do it every day? Well, it’s not that simple. Cardio itself isn't bad. But doing too much of it, especially without giving your body enough time to recover, can actually backfire.
For example, if you're doing high-intensity workouts every single day, your muscles and joints are constantly dealing with the stress of exercise. Without enough recovery, you may start feeling more tired, sore, or just not perform the way you normally do.
And nutrition is something you need to pay attention to. If you're doing a lot of cardio but not eating enough to support all that activity, your body may not get the energy and nutrients it needs to recover properly. Over time, concentrating only on exercising and not giving much importance to what you eat can also affect your hormonal balance.
Next comes the psychological aspect. If you're constantly pushing yourself to do more, run faster, burn more calories, or beat yesterday's numbers, exercise can slowly stop feeling enjoyable. You may feel drained, lose motivation, or simply start dreading your workouts. So, no, you don't necessarily need to go hard every day just because you're doing cardio.
EWOT: A New Face of Your Cardio
You’re doing cardio regularly. Your muscles demand more oxygen. So, what if you add supplemental oxygen while you exercise? Yes, Exercise With Oxygen Therapy, or EWOT, is pretty much what it sounds like. You do your cardio while breathing supplemental oxygen through a mask.
You’re still exercising. Your heart rate is still going up. Your muscles are still working. The difference is that you're adding supplemental oxygen while you exercise. Now you may naturally have a question: “What happens when I exercise with supplemental oxygen?”
Well, let’s look at it.
When you exercise normally, you breathe the air around you. Your lungs take in oxygen, and your blood carries it to the muscles that are working. When you add supplemental oxygen, the air you're breathing contains a much higher concentration of oxygen than normal room air. That changes the breathing aspect of your workout.
But it doesn't mean your body suddenly stops doing the work. Your heart still needs to pump blood. Your muscles still need energy. You're still exercising and creating the same demand that made your breathing increase. With EWOT, exercise is the main part of the session. The supplemental oxygen is added while you're moving and your body is already working harder.
The benefits? Improved circulation, increased energy levels, better performance, exercise tolerance, and a more manageable workout. All of these benefits make EWOT a good addition to your daily exercise routine. You’re not replacing exercise. You’re combining exercise with oxygen.
Related Reading: EWOT vs. Normal Cardio: Which is Better for Your Health?
Final Thoughts
The next time you’re halfway through a cardio workout, just take a moment to think about what’s happening. Your heart is working harder. Your breathing has changed. Your muscles are using more oxygen, and your body is working to keep up. Keep moving, stay consistent, and give your body time to recover.
