Introduction
The sled push is widely used across many sports: rugby, basketball, team sports, even general strength training. And for good reason. It develops force, acceleration and the ability to express strength horizontally. It also looks pretty impressive when you load it up.
But in HYROX, the sled push is not performed fresh. It comes after two running laps and the SkiErg, with most of the race still ahead of you. So being strong enough to move the sled is only the starting point. You also need to move it efficiently, maintain your output under fatigue and recover quickly enough to keep running well.
That is what the Sled Fatigue Index helps you assess.
What is the Sled Fatigue Index?
The Fatigue Index describes how much your performance declines between two comparable efforts. For this test, we compare:
fresh 50 m sled time → fatigued 50 m sled time
A higher Fatigue Index means that your sled performance dropped more after the run. A lower Fatigue Index means that you maintained your performance more consistently.
But lower is not automatically better. You can achieve a low Fatigue Index in two very different ways:
- by moving the sled quickly on both attempts;
- or by moving it slowly on both attempts.
That is why the percentage alone does not tell the whole story. To understand your result, you need to consider both: how fast you moved the sled and how well you maintained that speed. More on that point below.
How to Perform the Test
The protocol is simple:
- 50 m sled push
- 1 km run at HYROX race pace
- 50 m sled push
Include your normal Roxzone transition in the 1 km run time.
Record:
- your first sled time;
- your 1 km run time;
- your second sled time;
- your heart rate after both sled efforts.
Enter both sled times into the calculator above to calculate your performance drop.
Keep the conditions consistent
For a useful comparison, use the same sled, surface, load, footwear, warm-up, distance and timing method. Small changes in friction can have a surprisingly large effect on sled time.
Understanding Your Result
1. Slow + Consistent
Your two sled times are similar, but both are relatively slow.
What it may mean: your main concern should be getting strong enough. You probably lack general strength, sled-specific strength or technical efficiency.
Main training priority: build general strength and improve your ability to apply force into the sled.
2. Fast + Large Drop
Your fresh sled time is fast, but your performance drops considerably after the run. Your heart rate and perceived effort may also rise sharply.
What it may mean: you are strong enough. The problem is maintaining that output once fatigue starts accumulating. Your main limitations may include:
- poor pacing;
- insufficient sled-specific fatigue resistance;
- limited aerobic support;
- inefficient technique under fatigue.
Main training priority: develop repeatable submaximal sled output before adding more highly fatigued race simulations.
3. Fast + Consistent
Both sled efforts are fast, with only a small performance drop.
What it may mean: you can move the sled quickly and maintain most of that output after running. This is the outcome we are looking for. But the test is not quite finished: you still need to assess how quickly you can return to race pace after the second push.
Main training priority: maintain your sled qualities and gradually improve the transition back into running.
Heart Rate Adds Context
Heart rate does not determine your Fatigue Index, but it can help explain the result. Two athletes may have the same performance drop while showing very different physiological responses: one maintains a controlled heart rate, the other needs a much higher heart rate to produce the same result.
When retesting, look for the complete trend:
- faster or more consistent sled times;
- the same controlled run pace;
- lower or more stable heart rate;
- lower perceived effort;
- better running after the sled.
A lower heart rate alone does not prove that your aerobic system has improved. It becomes meaningful when the test conditions and performance remain comparable.
Retesting
Use the test before and after a training block focused on sled performance. We recommend retesting every 6–10 weeks, depending on the goal and structure of the block.
Avoid comparing results from different sleds or surfaces. The sled is already inconsistent enough without our help.
More importantly, do not just collect another number. Use the result to decide what your next training block actually needs.