Your body isn’t failing you – it’s just working through its energy systems!
Have you ever felt completely wiped out after just a few seconds of jump squats or burpees? Even if you are fit, your body has limits when it comes to high-intensity exercise. Find out why – and what you can do to improve it!
You think you are fit. You’ve been training consistently, focusing on strength training and going on regular 5k runs. Squats feel easy, you can plank for a good minute or two, you can manage some decent push ups (with knees off the floor!) and you have even gained this long awaited muscle tone.
Then you try a high-intensity workout everyone is talking about or your trainer throws in a few intense exercises to your regular circuit. Jump squats, burpees, mountain climbers – and within 20 seconds you feel like you are going to die! Your legs are on fire, your lungs scream for air and you can’t believe how quickly you’re wiped out. How?
Does it sound familiar? Well, you are not alone here. Even well-trained individuals struggle with this. And no, it is not a sign that you are out of shape – it’s simply how your body’s energy systems work. Let’s break the science behind it.
The Science Behind That Sudden Exhaustion
When you perform short, intense bursts of exercise, your body taps into its anaerobic energy system.
Anaerobic = without oxygen
It’s a fantastic system that can provide your body with a huge amount of energy in an incredibly short space of time. You can sprint, power lift, jump high, but – only for a few seconds.
Unlike “easy” steady-state cardio (think slow jog or cycle), which needs oxygen to produce energy, anaerobic exercise asks for instant power. This is how it works:
1. Stored ATP:
Your muscles have a small reserve of adenosine triphosphate (ATP), the body’s energy currency. This lasts for about 2–3 seconds of all-out effort.
Yes, you read it right. Two to three seconds.
2. Creatine Phosphate (CP) System:
To extend that burst, your body taps into creatine phosphate, another quick energy source.
This buys you another 5–10 seconds.
3. Glycolysis Kicks In:
Once immediate energy stores are depleted, your body rapidly breaks down glucose (from stored glycogen) to keep you moving.
This process produces lactate and other metabolites as a byproduct. They can’t be cleared effectively from your muscles without oxygen.
4. Lactate Build-Up:
When lactate and other metabolites accumulate faster than your body can clear it, you feel that intense muscle burn and fatigue. This is what forces you to stop after just 20–30 seconds.
If you are a world’s top athlete, you might last up to 90 seconds 😉
Here is my very simplified and schematic illustration to show you how quickly you run out of energy that you need to power these dreaded burpees. 🙂
Picture: Anaerobic energy systems over time during high intensity exercise
Let’s sum it up:
1. The Turbo Boost System – ATP and CP (first 0-30 seconds)
- Powers very short, explosive movements (think sprints, jumps or heavy lifts).
- Uses stored ATP and creatine phosphate – but only lasts 10-30 seconds.
- This is why you feel like dying after a few jump squats.
- Even fit people struggle with high-intensity bursts if they are not trained for it.
2. The Quick Energy System – Glycolysis (at 30-90 seconds)
- Kicks in after 30 seconds of intense effort and lasts up to 90 seconds.
- Breaks down glucose without oxygen and produces lactate and other metabolites like hydrogen.
- The critical point when your body can’t get rid of lactate efficiently is called “lactate threshold” and the more you train, the longer it takes to reach that point.
- Once you are at “lactate threshold”, the intense burn in the muscles begins and you can’t keep exercising at such high intensity any longer.
- Do you know now why you feel like your legs turn to jelly after back-to-back burpees?
Why This Doesn’t Mean You Are Unfit
People often mistake this burning breathless exhaustion for a lack of fitness. I hear this all the time!
But the truth is that even elite athletes hit their anaerobic limits quickly. Yet they last a tiny bit longer. So what makes the difference and why can they last longer than you?
The answer is they have trained their bodies to clear up the build-up of lactate more efficiently.
If you’re new to high-intensity training, your body hasn’t yet adapted. Your muscles, lungs and energy systems need time to adjust.
But adaptation is not only possible, but happens surprisingly quickly!
With consistent high intensity training you can build up your anaerobic capacity and be able to push harder for longer.
Real-Life Analogy: Your Smartphone Battery
Still too complicated? Let’s try to simplify things even more.
Think of your anaerobic energy system like your smartphone’s battery. You start with a full charge (stored ATP). Now, if you open multiple apps and turn the brightness right up (high-intensity exercise), the battery drains fast.
You can use a power bank (creatine phosphate – CP), but once that’s gone, your phone switches to battery-saver mode (glycolysis) slowing things down.
And if you keep pushing a lot (scrolling, videos, maps) without recharging, your phone overheats and shuts down (lactate build-up).
Frequently Asked Questions
Q: Can I improve my fitness to last longer during high-intensity exercise?
Absolutely! Regular HIIT (high-intensity interval training) can help your body clear lactate more efficiently and increase anaerobic endurance.
Q: Why do my legs feel like they are on fire?
You’ve reached lactate threshold. It happens when your body breaks down glucose for energy without oxygen and can’t clear lactate efficiently any longer.
The burning sensation is actually a result of increased acidity levels in your muscles due to lactate, but we wanted to keep things simple, right? The main point here is: at lactate threshold it hurts!
Q: Will this get easier over time?
Sure. The more you train, the better your body becomes at managing energy stores and the clearing up of lactate.
Some people even enjoy burpees (that’s me! But I must admit that mountain climbers still kill me every time 😉 )
Q: Does this mean my endurance training is useless?
Not at all. Endurance training develops your aerobic system, which is crucial for long-duration activities. We will talk about this in the next article (subscribe to my newsletter to make sure you don’t miss out!)
But adding some anaerobic work helps build explosive power and resilience.
The Takeaway
Let’s make it clear. Feeling exhausted after 20 seconds of intense exercise is not a sign of poor fitness. It’s just your body working through its natural energy systems and switching the way it generates power to the muscles.
Keep training, because that’s the key! And I am sure that over time you will increase your capacity to push through those limits.
This article is Part #1 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: Why the First 10 Minutes of Cardio Feel So Hard (And Why It Gets Easier)
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