HYROX performance is often discussed using familiar endurance metrics. VO2 max. Lactate thresholds. Heart rate zones. These tools have value, but they were built for sports with far more stable demands.
HYROX is different.
In a single race, athletes alternate between running and repeated bouts of strength and strength-endurance work. Fatigue does not arrive from one system failing. It accumulates locally and systemically, often at different rates.
This challenge sits at the heart of a recent discussion between elite HYROX athletes and sports scientists Hidde Weersma and Thierry Willigenburg, ahead of their upcoming session at the HYROX Coaches Summit.
Their central question is simple but uncomfortable: What if we are measuring the wrong limits?
The Problem With System-Only Metrics
Most coaches rely on global markers such as heart rate, VO2 max, or blood lactate to assess performance capacity. These measures reflect how the whole system is responding.
But in HYROX, local muscular fatigue often becomes the limiting factor before the cardiovascular system does.
Hidde gives a clear example. During sled push efforts, his heart rate and breathing remain controlled. On paper, the effort looks manageable. Yet force output drops and the legs fail long before systemic markers suggest danger.
The problem is not aerobic capacity. The problem is local muscle oxygen delivery and utilisation. Heart rate cannot see that.
Local vs Systemic Fatigue in HYROX
HYROX fatigue rarely comes from a single energy system. It is the result of:
- Repeated transitions between running and loaded movements
- High local muscular demand under incomplete recovery
- Strength tasks that behave more like endurance events
This creates a mismatch between how an athlete feels and what traditional metrics show.
Hidde and Thierry describe this as the difference between systemic thresholds and local thresholds. An athlete may sit comfortably below aerobic limits while a specific muscle group is already operating beyond sustainable oxygen availability.
Once that local limit is crossed, performance drops fast.
Introducing Muscle Oxygenation
To better understand these limits, Hidde and Thierry focus on Near-Infrared Spectroscopy, or NIRS.
NIRS is a light-based technology that measures muscle oxygenation in real time. Instead of estimating effort indirectly, it shows how well a working muscle is being supplied with and using oxygen.
This matters because every HYROX performance pillar has a metabolic cost:
- Speed
- Endurance
- Strength
- Power
NIRS allows coaches to see how these demands interact at the muscle level, not just through heart rate or perceived effort.
Why This Matters
This approach potentially changes how coaches and athletes can think about both testing and training.
With muscle oxygenation data, coaches can:
- Identify which stations cause local muscular overload
- Distinguish between aerobic limitation and peripheral fatigue
- Track whether training improves oxygen utilisation at given speeds or loads
- Understand why an athlete “blows up” despite controlled heart rate
It also opens the door to sport-specific thresholds. Instead of one lactate or aerobic breakpoint, athletes may have different sustainable limits for running, sled work, or mixed-modality efforts.
According to Hidde and Thierry, knowing these thresholds allows training to become more precise, not more complex.
Measuring Performance During the Race
One major advantage of this technology is that it can be worn during training and competition.
You cannot run a HYROX race wearing a VO2 mask. You cannot stop for lactate sampling. Muscle oxygenation monitoring allows coaches to observe how fatigue develops across the entire race environment.
Hidde and Thierry plan to present elite-level race data during their HYROX Coaches Summit session, showing how oxygen availability changes across runs and stations in real competition. For coaches, this offers rare insight into what actually limits performance when it matters most.
A Shift in How We Interpret Performance
Neither speaker argues that VO2 max or lactate testing should be discarded. These remain valuable tools.
The argument is that HYROX demands more than one lens.
Performance is not dictated by one ceiling. It emerges from the interaction between global capacity and local tolerance. Until coaches can see both, key decisions about pacing, station strategy, and training load remain educated guesses.
As Hidde puts it, HYROX fatigue does not come from one energy system. No single metric can account for that complexity.
Coach Takeaways
- Stop assuming fatigue is aerobic
If an athlete fades during sleds, lunges, or wall balls while heart rate stays controlled, the limiter is likely local muscular oxygen availability, not VO2 max or conditioning. - Separate systemic and local limits
An athlete can be “fit enough” globally but still fail at specific stations. Treat running capacity and station tolerance as related but distinct qualities in your programming. - Train muscles, not just engines
Strength-endurance work should aim to improve how muscles use oxygen under load, not just increase reps or load. Short recoveries, sustained efforts, and mixed-modality sets matter. - Be cautious with single-number testing
VO2 max and lactate thresholds are useful, but they don’t explain station-by-station performance. Avoid basing HYROX pacing or training decisions on one global metric. - Look for where performance actually breaks
Ask: where does output drop despite “feeling okay”? That point often reveals the true performance constraint you should train. - Progress isn’t just faster or heavier
Improvement can mean holding the same pace or load with less local fatigue. Track quality of movement and sustainability, not just speed or watts.
What to Expect at the HYROX Coaches Summit
At the HYROX Coaches Summit, Hidde Weersma and Thierry Willigenburg (who is a co-founder at train.red who make these NIRS sensors) will expand on these ideas, introduce the HYROX Performance Pillars framework, and show how muscle oxygenation can be applied in practice. To find out more about, check out this article on the Summit.
Photo credit: Misha Haijtema (@mishahaijtema.photography)
