Have you ever wondered why the first 10 minutes of your run, bike ride or swim feel so awful?
Picture this – you’re in the the perfect running mindset, put on your shoes, music in your ears, start jogging and – within moments your legs feel heavy, you are out of breath and you’re questioning all your life choices. Every time you begin to jog, it starts with this misery.
If you already know the feeling, of course you persevere. Because suddenly – maybe around the 5 to 10-minute mark – it’s like someone flipped a switch. Your body relaxes, the breathing becomes steadier and everything starts to feel easier.
Well, it doesn’t happen just to you. We all go through this every single time. I often hear “I must be totally out of shape because I struggle so much as soon as I start to run.” But the truth is it has absolutely nothing to do with your fitness level. It’s all about how your body transitions from anaerobic to aerobic energy production. To put it simply: it is determined by exercise physiology at your cellular level and has got nothing to do with your motivation or strength.
This article is Part #2 of my Understanding Energy Systems In Exercise series. I try to explain here, in simple words, what’s happening inside your body. I believe that if you understand the process, you can train smarter and have more willpower to push through tough moments.
So let’s break down why this happens, what is going on inside your body and how to make those first few minutes feel less like torture.
What Is Cardio Exercise, Actually?
Do you actually know what we mean by cardio exercise?
Cardio (short for cardiovascular exercise) is any movement that you can sustain over a longer period of time and that increases your heart and breathing rate. We call it steady-state cardio or aerobic (with oxygen) exercise.
You will find in this category for example:
- Running (outdoors or on a treadmill)
- Cycling (indoor or outdoor)
- Swimming
- Rowing
- Hiking. And so on.
Different to short bursts of high-intensity exercise (think burpees), that primarily rely on the quick energy source (anaerobic = oxygen free), steady-state cardio needs your body to generate energy efficiently over a longer period. For that you need oxygen, hence the name aerobic exercise.
Why Is Cardio Exercise Important?
Although strength training and HIIT workouts get most of the attention and publicity, we all need cardio exercise. Why?
Let’s take a closer look at the benefits of steady-state cardio:
- Improves cardiovascular health by strengthening your heart and lungs. Think of it as training for your heart, arteries and lungs, that in longer term can decrease resting heart rate and lower blood pressure.
- Increases the HDL (good) cholesterol levels (and this helps prevent atherosclerosis)
- It increases bone mass and decreases the risk of osteoporosis.
- Boosts endurance – so you can do lot’s of daily life activities without losing breath.
- Improves fat metabolism by training your body to use fat for energy more efficiently. If you’re trying to lose some weight, that’s the one for you!
- It’s crucial for your mental health, because it reduces stress and anxiety through the release of endorphins. This is why going for a walk or a run clears your mind and helps get rid of stressors.
But let’s return to our opening question: why is the first few minutes of cardio so exhausting?
Essentially, to reap all those fantastic benefits mentioned above, your body needs to switch into its most efficient energy system first.
And that’s where the first 10 minutes come into play.
What Happens in the First Few Minutes of Cardio?
We already discussed this in Part #1 of this series, but just to recap:
At the very beginning of cardio exercise your body first focuses on anaerobic pathways first (oxygen free). These are the energy systems that can deliver energy quickly but won’t last very long.
Here’s a breakdown of what happens:
0–30 Seconds: The ATP-PC System
Your muscles use their stored energy in the form of adenosine triphosphate (ATP) and creatine phosphate (CP) to power movement. This energy system is incredibly fast but only lasts about 30 seconds.
30-90 Seconds: Anaerobic Glycolysis
Once ATP and CP run out, your body switches to breaking down glucose (from stored glycogen) to create energy. This process, called anaerobic glycolysis, doesn’t require oxygen, but produces lactate as a byproduct.
The lactate accumulation leads very soon to that familiar unbearable burning sensation in your muscles. From here, if you want to continue with exercise, your body has to switch to using a more sustainable energy source.
This is why the first few minutes feel tough. Your body is working hard to produce energy, lactate overload makes you feel the burn in your quads and your lungs struggle for air. Your body simply hasn’t fully engaged your most efficient fuel system yet.
Picture: Anaerobic energy systems over time during high intensity exercise
The Shift to Aerobic Energy
At around 5-10 minutes your body shifts from anaerobic to aerobic metabolism. This is the most efficient fuel system for your muscles that can keep you going for a very long time.
Your run or pedalling on the bicycle suddenly feels much easier!
A Bit of Science: How Aerobic Metabolism Works (The Krebs Cycle)
Your heart and lungs have now ramped up to deliver enough oxygen to your muscles. Now you can switch to the aerobic system, also known as oxidative phosphorylation.
Finally your body is ready to:
- Use oxygen to efficiently break down carbohydrates and fat for energy. You know that well – your breathing rate increases, but is stable and manageable. The steady delivery of oxygen helps to produce energy in a smooth and reliable way.
- Produce far more ATP than anaerobic pathways. Imagine ATP as energy bricks that your muscles need in order to move. During Krebs cycle you steadily produce an awful lot of ATP, energy bricks, and you can keep going like that for a really long time.
- The lactate buildup reduces, so your muscles stop burning. A reminder – lactate builds up during anaerobic energy production, without oxygen. Once you’ve switched to aerobic (with oxygen) Krebs cycle, the lactate buildup doesn’t cause you problems anymore.
The whole process occurs in the mitochondria, which are called the powerhouse of your cells for a good reason. So the (aerobic) Krebs cycle is the way to go – energy gets extracted from glucose and fatty acids in a much more sustainable way than in anaerobic glycolysis.
What Happens at 20, 30 and 90 Minutes of Cardio?
20-Minute Mark: Finding Your Flow
If you made it through the first 20 minutes of steady exercise, your aerobic system is fully engaged and you’re in the zone. You are breathing rhythmically and your muscles feel warm and good. And you? You feel like you could keep going forever!
30-Minute Mark: Fat Metabolism Increases
After about 30 minutes your body starts breaking down more fat for fuel. From now fats are your primary energy source to spare glycogen stores (glycogen = stored glucose). This is why moderate-intensity cardio is great for endurance training and efficient use of stored fats.
90-Minute Mark: Glycogen Depletion
If you exercise for longer than 90 minutes – like going on a long bike ride or running over 15-20km – your glycogen stores will eventually run low.
Glucose (or glucose stored as glycogen) is the “background fuel” that Krebs cycle needs to keep going. Even though Krebs cycle uses mostly fats to generate ATP (energy bricks), it cannot do it without a small “background” input of glucose. However our bodies are smart and prepared for such eventuality: if there is no more glucose available, they will start breaking down protein from muscle tissue and find glucose there.
To put it simply – if your body runs out of sugar, you wouldn’t be able to keep going. So if you keep pushing, it will find the sugar – in your muscles. Unfortunately at the expense of destroying your muscle tissue at the same time.
This is when endurance athletes need to replenish carbohydrates to keep going. If glycogen runs out completely you’ll hit the infamous wall – you simply crash and have zero energy to exercise anymore.
Carbs are absolutely crucial for endurance performance. If you’ve ever wondered why you see energy bars and energy drinks in the gyms and sport shops, now you have the answer.
We’ll talk about this more in the next article (don’t forget to subscribe to my newsletter if you’ve not done that yet!)
For now, just remember that eating something during prolonged exercise sessions makes a huge difference.
How to Make the First 10 Minutes Easier
If you struggle with the beginning of your workout, try these tips:
- Start Slow: There is no need to go all out immediately – ease into it gently.
- Warm Up First: Prepare your joints and muscles for the movement. They will thank you for it!
- Breathe Deeply: Focus on steady and controlled breathing to bring in more oxygen. There is no need to stop completely if you feel out of breath, but slow right down.
- Mentally Prepare: keep in mind that it will get easier if you push through.
The Bottom Line
Feeling sluggish at the start of your workout is completely normal. It’s just your body shifting from quick, but inefficient anaerobic energy to long-lasting aerobic energy. I believe that once you understand this process, you will be more patient with yourself and enjoy your workouts more.
Did you find this explanation helpful? I’d love to hear your feedback in the comments below!
And if you have any questions about exercise, nutrition or motivation – drop me a message and I’ll do my best to help!
What’s Next?
In the next post, we’ll talk about fueling during exercise. We’ll cover what to eat, whether you need energy bars and special drinks, when to eat and how to keep up energy levels for longer workouts.
Stay tuned!
This article is Part #2 of Understanding Energy Systems in Exercise series.
In this blog series I will break it all down for you – why short bursts of exercise feel unbearable and why cardio starts out rough but gets easier after 20 minutes.
We’ll also dive into the science of fuelling your body for different workouts without falling for fitness industry tricks.
Whether you’re a weekend jogger, a HIIT lover or just trying to move more, I hope this series will help you make sense of what’s happening inside your body.
Coming soon:How Your Body Fuels Long Workouts (And Why Carbs Matter)
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