---
title: "What the 15% rule can and can't tell you"
date: 2026-09-06
categories: ["Training"]
url: https://www.semiprocycling.com/digest/what-the-15-rule-can-and-can-t-tell-you
author: Damian Ruse (SEMIPRO Cycling)
---
# What the 15% rule can and can't tell you

![](https://www.semiprocycling.com/digest/images/a54dfe1908d8aadcb838abf9.webp)

Imagine you finish a training block, look at your numbers and find that your threshold power has improved. Your VO2max has improved too. Yet the percentage of VO2max you can hold at threshold has barely moved.

Did the training fail?

That percentage has been part of the standard endurance-performance model for decades. The idea is tidy: VO2max is your ceiling, threshold is the fraction you can use, and economy determines how much speed or power you get from it. In theory, if everything else stayed equal, a higher fraction should support better performance.

The problem is that the fraction is made from two numbers that can move together, move apart or even move in the wrong direction.

A new narrative review looked at 35 studies testing the relationship between fractional utilisation and endurance performance. Across runners, cyclists and cross-country skiers, only a few found a substantial relationship.

In the largest dataset, 495 runners and 393 cyclists from recreational to world-class level, fractional utilisation had only a trivial standalone relationship with speed or power at lactate threshold: no more than 4% of the variation. It still made a smaller contribution inside the complete model, so the defensible conclusion is not that the percentage is irrelevant. It is that the percentage is weak on its own for distinguishing higher from lower threshold speed or power.

The training literature produced a similarly mixed picture. Athletes often improved performance, VO2max or economy while the percentage stayed much the same. The review found limited and conflicting evidence that already-trained athletes can systematically train the percentage itself.

The clearest warning came from one small study of well-trained male marathon runners. Seventeen started, 14 completed the study and 13 contributed to the key fatigued comparisons. After 90 and 120 minutes of running, speed at threshold, VO2max and running economy declined, yet fractional utilisation went up because VO2max fell faster than the oxygen use at threshold. The percentage rose while the underlying performance measures deteriorated.

That is a useful example, not proof that the ratio always behaves this way. This review used an unsystematic search, not a meta-analysis, and the included studies used different threshold definitions, tests and athlete groups. Fractional utilisation was often a secondary outcome. The authors are arguing that the evidence is inconclusive, not that the variable has been disproved.

That does not make threshold useless. It means you need to look at the two sides of the ratio before deciding what happened.

The review does not directly test my 15% rule. My check compares threshold power with five-minute power. It is a field heuristic built from coaching riders, not a direct measurement of oxygen use at threshold divided by VO2max. The ideas overlap, but the two ratios are not interchangeable.

What the review changes is how firmly I would use the result. If threshold power is less than 85% of five-minute power, I still want to investigate why the gap is there. But the gap is a flag, not a diagnosis. It does not prove the aerobic base is too weak or automatically tell us what to train next. Sustained aerobic power may be underdeveloped. Five-minute power may simply be unusually strong. One of the estimates may be wrong. Or the shape may be exactly right for that rider and their event.

The review neither validates nor invalidates “raise the floor,” because it asks a different question. When I use that phrase, I mean the fitness you can reliably access on an ordinary day, when you're tired, or four hours into a ride. The paper mentions durability and includes one prolonged-exercise study, but it does not test whether improving fatigued output or repeatability raises that floor.

What it does challenge is the tidy idea that fractional utilisation is an independent switch we can deliberately turn up. Training may change VO2max, threshold power, economy, durability or several of them together. The ratio alone cannot tell us which adaptation we created.

What you do with this depends on which rider you are.

- **If you're time-capped:** don't spend a block trying to make one ratio look better. Track threshold power and five-minute power separately. If the gap opens, use it as a prompt to investigate before automatically adding more base or more intensity.
- **If you're over 50:** don't celebrate a rising fraction until you have looked at both sides of it. The review says age-specific longitudinal evidence is still missing. Watch the absolute numbers and what remains available late in the ride rather than assuming the percentage describes how well you are ageing.
- **If you're chasing a result, or it's a new event for you:** the review cannot tell you exactly what to train. Start with the demands of the event, measure the relevant qualities directly and check what remains under fatigue. Do not chase the ratio as the result.

Use a ratio to ask a better question, not to pronounce a verdict on the rider.

**Source.** Sondermann YM, Schwalm L, Kock H, Appelhans D, Skovereng K, Noordhof DA. Fractional utilization of VO2max—a key performance determinant in endurance sports? *International Journal of Sports Physiology and Performance*. Published online September 2, 2026. https://doi.org/10.1123/ijspp.2026-0039

## On my mind: Maintenance is still training

I had a call this week with an athlete who's moving into a different life phase. His child is getting older and needs more of him. His body feels a little older too. So we're shifting from pure cycling towards a mix of strength, running and riding.

That is completely fine.

Maintenance can sound like retreat, but it isn't. It is keeping enough of what you've built that the next opportunity starts from a platform, not from zero.

Different parts of the athlete can have different jobs. Strength might be building while cycling is simply being held. If a window opens in a week, there is room to do more without digging a hole. If life opens up again next year, there isn't a full rebuild waiting.

The mistake is expecting every phase of life to look like peak cycling ambition. A good plan should fit the phase you're in, not keep reminding you of the one you've left.

This week, ask yourself three questions: what can I build, what only needs maintaining, and what am I not willing to lose?

— Damian