Foto: iStock.com/BartekSzewczyk

Have you ever noticed that the first repetition of your interval training feels harder than the ones that follow, even though you're running at the same pace? Here's why that happens and how you can work around it.

When you suddenly increase your pace, your oxygen demand spikes immediately. However, your body needs a moment to meet that increased demand. This creates a temporary imbalance between the oxygen your legs need and the oxygen your heart and lungs can supply.

This shortfall inevitably triggers signals that speed up your breathing and heart rate, dilate your blood vessels, and activate the oxygen-processing enzymes in your muscles. It usually takes about two to three minutes before your muscles are sufficiently supplied with oxygen again.

Why the First Few Minutes Matter

This temporary oxygen deficit has a crucial effect: your muscles start tapping into their valuable anaerobic energy reserves to compensate. This generates metabolic byproducts that cause your muscles to feel weak or fatigued. What's used up in these first few minutes can’t be replenished during the interval or race unless you slow down.

To avoid this initial oxygen shortage, it’s recommended to include a short, intense effort of 45 to 60 seconds at 5K race pace in your warm-up about 10 to 20 minutes before the first interval or race begins.

This burst activates the enzymes, widens the blood vessels, and raises your heart rate, helping your body adapt more quickly to the effort level of the upcoming interval or race.

To finish the warm-up, it is recommended to do three to five strides of 80 to 100 meters with 90 seconds of rest in between, in order to increase muscle tension. However, there should still be 5 to 10 minutes remaining before starting the first interval effort or the race itself.