Hit a steep climb hard and your legs start to burn. What is happening, and what can you do about it? It comes down to how your body makes energy.
Watch: You've Only Got 3 Seconds of Energy | Energy Systems Ep.1. More on our YouTube channel.
Three energy systems
Immediate (phosphocreatine). Stored energy in the muscle for all-out efforts of up to around 10 seconds. Think sprints and jumps out of corners.
Anaerobic glycolysis. Breaks down carbohydrate quickly without needing oxygen. Powers hard efforts from roughly 10 seconds to 2 minutes, like attacks and short climbs.
Aerobic. Uses oxygen to burn carbohydrate and fat. Slower to ramp up but almost unlimited. Powers everything longer than a couple of minutes, which is most of cycling.
They all work together. The mix shifts with how hard and how long you go.
What is the burn?
Lactate often gets the blame, but lactate is actually a useful fuel. The burning feeling comes from a build-up of hydrogen ions and other by-products when you are working hard. It is a signal, not damage, and it fades soon after you ease off.
Training each system
Aerobic: lots of easy riding, plus sweet spot training and threshold work. Build the engine first.
Anaerobic: hard efforts of 30 seconds to 2 minutes with plenty of recovery. See cycling intervals, types and zones.
Immediate: short, maximal sprints with full recovery. See how to sprint faster.
Want to see where your systems sit? Our lactate tools and lactate testing show your thresholds and how your body handles rising effort.
Why this matters for your training
Different events lean on different systems. A crit is repeated anaerobic bursts on an aerobic base. A time trial is almost all aerobic. A track sprint is mostly immediate and anaerobic. Knowing which your event needs tells you what to train.
Can you train your tolerance to the burn?
Yes. Hard intervals improve your ability to buffer and clear by-products, and your brain gets used to the feeling. Practice also teaches you that the burn is temporary. Ease off slightly and it fades within seconds.
Watch the series
Our Energy Systems video series goes deeper into each one, from the first few seconds of a sprint to why your heart is the bottleneck. The first episode is above.
Get it planned
Want each system trained in the right order? Prefer to go it alone? A Season Plan (£150) maps it out for you. Want weekly support? Coaching starts at £140 a month.
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