Fractional Utilization / Sustainable Percent VO2 max
The Sustainable percent VO2 max (also known as fractional utilization) refers to the percentage of VO2max that athletes can sustain during an endurance competition. In essence, it’s a measure of your ability to sustain high work rates using primarily aerobic metabolism.
It’s a known predictor of success in endurance events that last longer than 30 minutes. Becoming increasingly important as race duration increases.
While it’s influenced by genetic factors, it’s also highly trainable – athletes often achieve significant improvements following specific endurance training.
This is where sustained race pace efforts are so important.
Fractional Utilization and race distance
Your ability to sustain a high percentage of VO2max decreases with increasing race duration.
For example, very highly trained runners can sustain close to 100% VO2max during a 5km race (~95-98% VO2max). This drops to around ~92 (typically 90-95% VO2max) during a 10km and around ~78% (typically 75-85% VO2max) during a marathon.
This decreases significantly during ultra-distance events. For example, research looking at performance determinants during an 80.5km treadmill ultramarathon found the average % VO2max sustained was 53% (Howe et al., 2021).
These percentages will be significantly lower in less well-trained athletes.
Why is Fractional Utilization important?
To optimize performance, endurance athletes need to sustain a pace or intensity as close to VO2max as possible.
Research suggests this is important when race duration is longer than 30 minutes. Becoming increasingly important when race duration increases (Ghosh, 2004; Lucia et al., 2001; Fallowfield and Wilkinson, 1999; Coetzer et al., 1993; Coyle et al., 1995; Costil et al., 1973).
Less important for shorter duration races
It is less important during shorter races (e.g. 1500m-5km).
For example, research by Støa et al. (2010) found no correlation between %VO2max sustained and 5km race performance.
This shouldn’t be surprising, given that highly trained athletes can sustain an intensity that is already very close to VO2max during 5km races. So, there is only limited potential for improvement in this percentage during these events.
Here, athletes can achieve greater improvements by increasing the velocity at VO2max and efficiency at race pace.
Increasingly important as race duration increases
As race duration increases, it becomes increasingly challenging to sustain intensities near maximal oxygen consumption. Thus, the ability to maintain work rates at high percentages of VO2max becomes more crucial.
If we consider Marathon running, even a small increase in %VO2max sustained (e.g. 1-2%) will have a big impact on overall race performance. This becomes even more important for ultra distance events – like ultra marathons and long distance triathlons – where minor improvements will have a significant impact on overall race performance and placings.
So, what factors influence our ability to sustain high percentages of VO2 max?
Factors influencing your ability to sustain high percentages of VO2max
The ability to sustain high percentages of VO2max depends on various factors. These include:
- The aerobic capacity of active muscles and the proportion of type I muscle fibers (Coyle et al., 1991 & 1995).
- Muscle capillary density is also important (Coyle et al., 1988) as is mitochondrial density and efficiency.
- It’s also strongly linked to the lactate threshold (Coetzer et al., 1993) and the VO2 at the lactate threshold (Coyle et al., 1988).
Given this, it’s reasonable to expect that training that raises the lactate threshold will also improve the capacity to maintain a high percentage of VO2max.
Individual Variation in Fractional Utilization
The ability to sustain high percentages of VO2max varies widely between individuals.
This helps to explain individual variation in race performance across different distances.
For example, if two runners have the same VO2max and velocity at VO2max (vVO2max), then the runner that sustains a higher percent of VO2max will be faster in races of over 30 minutes duration.
It’s important to consider that fractional utilization decreases as race distance increases, and (importantly) the rate of this decline varies between athletes.
For example, 2 athletes may sustain similar percentages of VO2max during a 10k race. But one athlete can sustain a higher percentage during a marathon than the other athlete.
This explains why some athletes are more competitive over certain race distances.
For instance, marathon specialists can sustain higher percentages during the marathon. In contrast, a 10km specialist may sustain a higher percentage of VO2 max over the 10km than the marathon specialist.
Assessing the sustainable percent VO2max
There are a few ways to assess the percentage of VO2max.
In a lab, sports scientists can assess this by measuring oxygen uptake at race pace (running), average power output (cycling, rowing, etc.), or during a time trial on a treadmill or cycle ergometer.
Here, the athlete would run at speeds maintained during races, or cycle at power outputs recorded during time trials, etc. Or, if completing a lab-based time trial, they would maintain the highest work rate (speed/power) across the duration of the time trial. In each case, researchers record oxygen uptake (VO2), heart rate, and speed/power.
They can then convert this to a percentage of VO2max from a recent VO2max test.
They can also calculate the percent VO2 max by comparing average speed/power outputs, with laboratory recorded VO2 values from a recent incremental test. Since this requires interpolation, it’s considered less accurate than direct VO2 measurement at the corresponding race speeds/work rates.
A few points to consider:
- Factors such as the VO2 slow component and cardiac drift, will influence the effectiveness of direct comparison between performance during competition and values from lab-based VO2 testing. This is particularly true with recreational athletes.
- Differences in environment and conditions (temperature, humidity, wind speed, undulating terrain etc.) will also affect the comparison.
Using heart rate to assess sustainable effort
Heart rate and VO2 follow a similar linear relationship to changes in speed and power.
So, If like most athletes, you don’t have access to a sports science lab, there is a relatively simple and cost effective alternative.
All you need is to record heart rate during competition – ideally across different race distances/durations. You then calculate the percent of heart rate max (HRmax) sustained, which can be used to estimate the equivalent percent of VO2max (%VO2max).
This can then be used to estimate percent VO2 Max.
By comparing this across different race distances, you can gain insights into your endurance capabilities.
How to do this…
First, you need heart rate data from different race distances.
Second, take the average heart rate for each distance and divide that by your maximum heart rate. You, then multiply that figure by 100, to find the %HRmax you sustained for that race distance.
Third, convert your %HR max to an “estimated” %VO2max using the following formula:
%VO2max = (%HRmax – 37) / 0.64
Or, simply use the table below to see your estimated %VO2max.
| % HRmax | Estimated % VO2max |
| 100 | 98.4 |
| 99 | 96.9 |
| 98 | 95.3 |
| 97 | 93.8 |
| 96 | 92.2 |
| 95 | 90.6 |
| 94 | 89.1 |
| 93 | 87.5 |
| 92 | 85.9 |
| 91 | 84.4 |
| 90 | 82.8 |
| 89 | 81.2 |
| 88 | 79.7 |
| 87 | 78.1 |
| 86 | 76.6 |
| 85 | 75.0 |
| 84 | 73.4 |
| 83 | 71.9 |
| 82 | 70.3 |
| 81 | 68.8 |
| 80 | 67.2 |
| 79 | 65.6 |
| 78 | 64.1 |
| 77 | 62.5 |
| 76 | 60.9 |
| 75 | 59.4 |
| 74 | 57.8 |
| 73 | 56.2 |
| 72 | 54.7 |
| 71 | 53.1 |
| 70 | 51.6 |
The following gives an approximate idea of what would be excellent/very good %VO2max results by race distance…
For 5K
- ~95-98% VO2max = Excellent
- ~90-95% VO2max = Very good
For 10k
- ~92-95% VO2max = Excellent
- ~87-92% VO2max = Very Good
For 10 Mile
- ~87-92% VO2max = Excellent
- ~82-87% VO2max = Very Good
For Half Marathon
- ~85-90% VO2max = Excellent
- ~80-85% VO2max = Very Good
For 20 Mile
- ~82-87% VO2max = Excellent
- ~77-82% VO2max = Very Good
For Marathon
- ~80-85% VO2max = Excellent
- ~75-80% VO2max = Very Good
From my recent race performances I can sustain the following % HR Max and estimated %VO2max…
5k = 169bpm (98% HRmax, 92.2% VO2max)
10k = 167bpm (94.9% HRmax, 90.5% VO2max)
Half Marathon = 162bpm (92% HRmax, 85.9% VO2max)
20 Mile = 158bpm (89.8% HRmax, 82.5% VO2max)
This suggests that my strengths lie more from Half Marathon upwards. Not, overly surprising given my training is normally focussed around this.
Training to improve sustainable percent VO2max
To improve the sustainable percent of VO2max, training should prioritise enhancing the muscle aerobic capacity and lactate threshold.
As such, it’s important that training volume is sufficient to maximize the muscles oxidative capacity. In addition, you should include an adequate volume of lactate threshold training to enhance muscle aerobic capacity, lift the lactate threshold and ultimately increase your ability to sustain higher percentages of VO2max.
It’s also important to complete a proportion of training time at race speeds/intensities.
For example, a 10k runner should include sufficient training at 10km pace, a marathon runner needs to devote time to training for sustained periods at marathon pace, and a 40k time trial cyclist should train at the power outputs used during 40k time trials.
It is important to remember there’s less scope for improvement during shorter duration events (e.g. 5-10km running), compared with longer races like half or full marathons. So the time devoted to improving the sustainable percent VO2max should depend on race duration – with more time devoted when the focus is longer duration events.
Summary of sustainable percent VO2max
- The ability to sustain high percentages VO2max is a crucial factor in endurance events of greater than 30 minutes.
- While less important in shorter events, it becomes increasingly important as race duration increases.
- It’s influenced by several factors, including the muscle capillary density, percent of type I muscle fibers, muscle capillary density, aerobic capacity of the muscle, the LT and the LTVO2.
- There’s wide variation between individuals, with some athletes suited to longer races (e.g. half-marathon, marathons, long distance triathlons etc), and others suited to shorter duration races (e.g. 5km).
- We can be improve this through specific training that maximizes the fatigue resistance and aerobic capacity of muscle fibers (e.g. adequate volume of moderate intensity training, tempo/threshold training).
- The scope for increasing the sustainable percent of VO2max is much greater in longer duration events (marathon etc) than during shorter duration events (3k, 5k etc).
- When targeting longer duration races, athletes should look to increasing the percent VO2max at race pace/intensity (power).
References
Coetzer, P., Noakes, T.D., Sanders, B., Lambert, M.I., Bosch, A.N., Wiggins, T. and Dennis, S.C. (1993). Superior fatigue resistance of elite black South African distance runners. Journal of Applied Physiology, 75, 1822-1827.
Coyle EF. (1995) Integration of the physiological factors determining endurance performance ability. Exerc Sport Sci Rev. 1995;23:25-63. Review.
Coyle EF, Coggan AR, Hopper MK, Walters TJ. (1988) Determinants of endurance in well-trained cyclists. J Appl Physiol. 1988 Jun;64(6):2622-30.
Coyle, E.F., Feltner, M.E., Kautz, S., Hamilton, M.T., Montain, S.J., Baylor, A.M., Abraham, L.D. and Petrek, G.W. (1991). Physiological and biochemical factors associated with elite endurance cycling performance. Medicine and Science in Sports and Exercise, 23, 93-107.
Coyle EF, Sidossis LS, Horowitz JF, Beltz JD. (1992) Cycling efficiency is related to the percentage of type I muscle fibers. Med Sci Sports Exerc. 1992 Jul;24(7):782-8.
Costill DL, Thomason H, Roberts E. (1973) Fractional utilization of the aerobic capacity during distance running. Med Sci Sports. 1973 Winter;5(4):248-52.
Fallowfield, J.L. and Wilkinson, J.L. (1999). Improving sports performance in Middle and Long-Distance Running. Chichester: John Wiley and Sons, LTD.
Ghosh AK. (2004) Anaerobic threshold: its concept and role in endurance sport. Malays J Med Sci. 2004 Jan;11(1):24-36.
Howe CCF, Swann N, Spendiff O, Kosciuk A, Pummell EKL, Moir HJ. (2021) Performance determinants, running energetics and spatiotemporal gait parameters during a treadmill ultramarathon. Eur J Appl Physiol. Jun;121(6):1759-1771. doi: 10.1007/s00421-021-04643-2. Epub 2021 Mar 11. PMID: 33704547; PMCID: PMC8144128.
Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. J Physiol. 2008 Jan 1;586(1):35-44. doi: 10.1113/jphysiol.2007.143834. Epub 2007 Sep 27. PMID: 17901124; PMCID: PMC2375555.
Larsen HB. (2003) Kenyan dominance in distance running. Comp Biochem Physiol A Mol Integr Physiol. 2003 Sep;136(1):161-70.
Lucia A, Hoyos J, Chicharro JL. (2001) Physiology of professional road cycling. Sports Med. 2001;31(5):325-37. Review.
Støa EM, Støren Ø, Enoksen E, Ingjer F. (2010) Percent utilization of VO2 max at 5-km competition velocity does not determine time performance at 5 km among elite distance runners. J Strength Cond Res. 2010 May;24(5):1340-5.