Why 85% of Max Heart Rate Doesn't Work for Anyone in Particular
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For decades, the standard way to prescribe exercise intensity has been to give the athlete a percentage. A percentage of maximum heart rate. A percentage of VO₂max. A percentage of heart rate reserve. The numbers feel scientific, they are easy to apply, and they are baked into nearly every fitness watch, treadmill display, and training plan you can buy.
The problem is that they do not actually describe what is happening inside the athlete.
When two people are told to run at "85% of max heart rate," it is reasonable to assume they will be doing something physiologically similar. They will not. One athlete might be coasting along well below their threshold, working aerobically and able to sustain the effort for hours. Another, at the same prescription, might be deep into the heavy intensity domain, accumulating lactate and unable to hold the pace for more than twenty minutes.
Same number. Completely different bodies.
What the Research Actually Shows
This is not a theoretical concern. It has been documented in peer-reviewed work for more than twenty-five years.
In a study by Meyer and colleagues, 36 trained endurance athletes were tested in a laboratory. Each athlete had their individual anaerobic threshold measured, and then the researchers calculated where each person actually landed at common percentage-based prescriptions.
The results were striking.
At 85% of max heart rate, athletes were placed anywhere from 87% to 116% of their individual threshold. Some were comfortably below their threshold, sitting in an aerobic zone. Others were 16% above it, in a domain where lactate accumulates rapidly and the work cannot be sustained.
Blood lactate values at the same prescription ranged from 1.25 to 4.93 mmol/L. That is the full range from "easy aerobic" all the way to "above the second lactate threshold."
At 70% of max heart rate, the variation was even larger. Athletes landed anywhere from 53% to 85% of their threshold.
The conclusion in the paper is direct. Percentages of HRmax and VO₂max do not describe individual physiology accurately enough to be used for intensity prescription.
Why the Variation Exists
The reason is straightforward.
Maximum heart rate is influenced by genetics, age, and a number of other factors that have very little to do with how hard the body is actually working at submaximal intensities. Two athletes with identical max heart rates can have very different thresholds. One might transition from moderate to heavy intensity at 75% of their max. Another might transition at 90%.
The same prescription, applied to both, produces very different physiological states. One athlete is being asked to work just below threshold. The other is being asked to work well above it.
This is not a small effect. It is the difference between a session that builds aerobic capacity and one that accumulates fatigue rapidly.
The variation also widens as fitness improves. As athletes adapt to training, their thresholds shift upward, but their max heart rate barely moves. The ratio between the two changes. A percentage that worked early in the season can be off by a wide margin a few months later.
What This Looks Like in Practice
The downstream effect of this variability shows up in training outcomes.
In a randomized controlled trial by Wolpern and colleagues, two groups followed identical training programs for twelve weeks. Same duration. Same frequency. Same total volume. The only difference was how intensity was prescribed.
One group used the standard percentage of heart rate reserve approach. The other group had their intensities set relative to their individually measured ventilatory thresholds.
The threshold-based group improved their VO₂max more than twice as much on average. Every single athlete in that group responded to the program. In the percentage-based group, only 42% responded. The remaining 58% were classified as non-responders.
These results were not explained by age, baseline fitness, sex, or adherence. The athletes in both groups followed the program. The difference was the prescription method itself.
This pattern has been replicated. A 2025 review in Sports Medicine pooled fifteen cross-sectional studies, three randomized trials, and a meta-analysis of forty-two trials covering more than seven thousand participants. The result was consistent. Threshold-based prescription produced zero non-responders in randomized trials, while percentage-of-peak prescriptions produced 31–58% non-responders. Pooled VO₂peak gains were more than double under threshold-based prescription.
A Different Anchor
The alternative is to prescribe intensity relative to physiological landmarks that exist in each individual athlete.
These landmarks are the ventilatory thresholds, sometimes called VT1 and VT2. They mark the boundaries between intensity domains:
- below VT1, the body works primarily aerobically and can sustain the effort for long durations
- between VT1 and VT2, the body is in a heavy domain where fatigue develops more quickly
- above VT2, the body is in a severe domain where the effort cannot be sustained for long
These boundaries are different for every athlete. They shift with training, with fatigue, and with the conditions of the day.
When intensity is prescribed relative to these thresholds, the same prescription produces a similar physiological state across different athletes. "Just below VT1" means the same thing for everyone. "85% of max heart rate" does not.
Why This Has Not Been the Default
The barrier has always been practical.
Measuring individual thresholds traditionally required a laboratory. An athlete would have to perform a graded exercise test with a metabolic cart and blood lactate sampling. The result might be valid for a few weeks before the body adapts and the thresholds shift again.
For most athletes, this is not realistic. So they default to what is available, which is a percentage of max heart rate, often estimated from age rather than measured directly.
The result is a system where the prescribed intensity rarely matches the intended one, and where a substantial fraction of athletes do not respond to programs that should, on paper, work for them.
What Changes When Thresholds Are Measured in the Field
When thresholds can be identified outside of a laboratory, the entire framework for prescribing intensity changes.
Each athlete has their own anchors. The numbers are theirs, not borrowed from a population average. Easy days are actually easy. Threshold work happens at the threshold. Hard sessions land in the right domain.
When the thresholds shift over time, as they do with adaptation and fatigue, the system can recalibrate. The prescription stays accurate as the athlete changes.
This is the difference between training based on what the body is doing and training based on what a percentage suggests it should be doing.
Final Thoughts
The percentage-based approach has been the standard for so long that it can be easy to assume it must be doing something useful. At the group level, the means line up with thresholds reasonably well. The error only appears when you look at individuals.
That is exactly the level at which training is actually performed.
When intensity is prescribed as a percentage of max heart rate, the prescription describes the population, not the person. For some athletes it lands in roughly the right zone. For others it does not. The same number can mean different things in different bodies, and over the course of a training program the difference compounds into different outcomes.
The shift toward individualized, threshold-based prescription is not a small refinement. It is a move from describing intensity in terms that approximate physiology to describing it in terms that match it.
The athlete in the room is not the average. The training prescription should not be either.
References
- https://doi.org/10.1007/s40279-025-02272-9
- https://pubmed.ncbi.nlm.nih.gov/10487378/
- https://doi.org/10.1186/s13102-015-0011-z