Exercise physiologists and coaches describe endurance performance using three major variables (which you’ve heard about multiple times if you’re a regular reader of this newsletter… so bear with me).
- VO₂ max, or the upper limit of aerobic energy production
- Running economy, or the oxygen cost of holding a given pace
- Fractional utilization, or the percentage of VO₂ max that can be sustained at lactate threshold or race intensity
The third variable is perhaps most appealing from a performance perspective.
Two runners might have the same VO₂ max, but if one can sustain 90% of it while the other can sustain only 80%, the first runner should be able to hold a higher aerobic output. Assuming their running economy is identical, that should translate into a faster pace.
Mathematically, it works.
But does fractional utilization actually separate faster athletes from slower ones? And can runners reliably improve it through training?
A recent narrative review examined those questions across running, cycling, and cross-country skiing. Its conclusion challenges one of the most familiar models in endurance physiology:
Fractional utilization may be part of the mathematical description of performance, but it is not a particularly strong or independent predictor.
Why The Percentage Is Misleading
Fractional utilization is usually calculated by dividing oxygen consumption at a physiological landmark—often lactate threshold—by VO₂ max.
If a runner reaches their lactate threshold at an oxygen consumption of 45 milliliters per kilogram per minute and has a VO₂ max of 60, fractional utilization is 75%.
That seems straightforward. But a percentage can change because the numerator changes, because the denominator changes, or because both change together.
The paper illustrates this with two theoretical runners:
- Runner A has a VO₂ max of 60 mL·kg⁻¹·min⁻¹ and reaches threshold at 75% of VO₂ max.
- Runner B has a VO₂ max of 50 mL·kg⁻¹·min⁻¹ and reaches threshold at 90% of VO₂ max.
Both runners reach threshold at the same absolute oxygen consumption—45 mL·kg⁻¹·min⁻¹.
If they also have the same running economy, say 200 milliliters of oxygen per kilogram per kilometer, both produce the same threshold speed of 8.5 miles per hour/13.5 kilometers per hour.
Runner B has a much higher fractional utilization, but is not faster. Their higher percentage merely compensates for a lower aerobic ceiling.
This is the first reason to be careful with fractional utilization: it is not an absolute measure of how much aerobic power an athlete can produce. It describes where one physiological value sits relative to another.
A runner using 85% of a VO₂ max of 70 is producing far more aerobic energy than someone using 90% of a VO₂ max of 50. The percentage alone cannot tell you who is faster.
The Data: Faster Runners Don’t Use a Higher Percentage
The authors examined 35 studies assessing the relationship between fractional utilization and endurance performance.
They considered two versions of the measure:
- fractional utilization at a physiological threshold, such as lactate threshold
- fractional utilization at a race-specific intensity or during a time trial
In most studies, the percentage did not clearly distinguish athletes of different abilities.
Consider one study of 97 recreational marathon runners.
Runners finishing between 2.5 and 3 hours reached their second lactate threshold at approximately 84% of their VO₂ max. Runners taking more than 4.5 hours also reached it at approximately 84%. Their marathon performances differed by more than 90 minutes, even though their fractional utilization was the same.
Another study reported values of 83%, 82%, and 85% in elite, national-level, and recreational runners, respectively. Again, faster athletes did not operate at a systematically higher fraction of VO₂ max.
The largest analysis included 888 athletes—495 runners and 393 cyclists—ranging from recreational participants to world-class competitors. Fractional utilization explained no more than 4% of the variation in speed or power at lactate threshold and no more than 1% at the lactate turn point.
The competition-specific evidence was just as underwhelming…
Fractional utilization did not correlate with performance in the reviewed 5-kilometer running, 20- and 60-minute cycling, or 600-meter cross-country skiing time trials. In one comparison, African runners competed at a higher fraction of VO₂ max than caucasian runners—92.2% versus 86.0%—but their average 10-kilometer times were similar: 32.8 versus 32.0 minutes. In this case, more utilization did not produce better performance.
Why Fractional Utilization Falls As Races Get Longer
Fractional utilization is not a stable characteristic like height. It depends heavily on how long you’re going to sustain an effort.
You can operate near VO₂ max for several minutes. You cannot do so for several hours.
In one study of 30 male marathon and ultramarathon runners, the estimated sustainable fraction fell progressively as race distance increased:
- 10 kilometers: 85.8%
- Half marathon: 80.7%
- Marathon: 74.3%
- 56 kilometers: 67.2%
- 90 kilometers: 56.8%
Physiologically, this makes sense. As an event gets longer, the sustainable fraction of your maximal aerobic capacity generally falls.
But it complicates the interpretation of studies reporting a relationship between fractional utilization and finish time, because athletes finish sooner. Because sustainable fractional utilization partly depends on exercise duration, a faster athlete may register a higher percentage simply because they exercised for less time. The fraction may therefore reflect the performance rather than explain it.
When A Higher Percentage Means You’re Getting Weaker
Perhaps the clearest warning comes from what happens under fatigue.
The review highlighted a study in which well-trained male runners completed prolonged runs lasting 90 and 120 minutes. Afterward, lactate-threshold performance, VO₂ max, and running economy deteriorated (i.e., they lost durability). Yet fractional utilization at lactate threshold increased.
The runners were not fitter, obviously. Their aerobic ceiling had fallen enough that the threshold represented a larger percentage of what remained. That’s not a good thing, in this particular instance.
The same principle helps explain some striking values in older athletes.
Masters world-record holders have sometimes sustained more than 90% of their VO₂ max at marathon pace. That is physiologically impressive… but it may also compensate for an age-related reduction in VO₂ max. A higher fraction allows the athlete to preserve more absolute aerobic output despite a lower ceiling.
So a higher percentage can mean that the sustainable output rose. It can also mean that the ceiling fell. You need both numbers to know the difference.
Can fractional utilization be trained?
Several studies reported improvements in VO₂ max, economy, or endurance performance without corresponding increases in fractional utilization. Other interventions did report changes.
In one three-week cycling study, fractional utilization increased from 82.1% to 85.1% with short intervals but declined from 83.5% to 80.6% with longer intervals. An eight-week high-intensity intervention in elite junior cross-country skiers also increased fractional utilization at the ventilatory threshold relative to a control group.
These findings are interesting, but they do not establish a reliable prescription for improving the percentage of your maximal aerobic capacity you can sustain.
And the strength-training literature was no clearer. Five reviewed studies added lower-body strength training to normal endurance training. All improved maximal strength, and some improved economy or endurance performance, but fractional utilization generally did not change consistently with strength training.
Plausible mechanisms include increased absolute oxygen consumption or power produced at threshold. Greater mitochondrial content might distribute the metabolic demand across more machinery. Improved capillarization might support oxygen delivery. Changes in muscle recruitment could spread force production across more fibers, reducing fatigue development.
Those adaptations matter. But changes in the underlying physiology do not guarantee that the ratio between threshold oxygen consumption and VO₂ max will increase—especially if VO₂ max is improving at the same time.
A runner could raise their threshold from 45 to 50 mL·kg⁻¹·min⁻¹ while increasing VO₂ max from 60 to 70. Absolute threshold capacity improved substantially, but fractional utilization fell slightly, from 75% to about 71%.
Calling that a negative adaptation would be absurd.
What this means for runners
Ultimately, runners should focus on training the performance system rather than chasing a percentage.
Improving VO₂ max raises your aerobic ceiling. Improving running economy reduces the cost of a given pace. Raising absolute threshold speed or power expands what you can sustain. Improving durability helps preserve those qualities after 90 minutes, two hours, or late in a marathon.
A successful training block might improve all of these while leaving fractional utilization unchanged—or even lowering it in some cases (i.e., if your ceiling rises).
That is not a training failure. It is a failure of the percentage to capture the entire adaptation.
Fractional utilization has an important place in endurance performance equations. But it should not be mistaken for a standalone fitness score or a prediction of who’s the better runner.
The numerator matters. The denominator matters. Economy also matters. And over long races, the ability to keep all three from deteriorating—durability—may matter most of all.
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