Could Fractional Utilisation Be Limiting Your Performance?

9 May

Physiological Limiters #3 - Fractional Utilisation

Of the three physiological factors that affect performance in endurance events, Fractional Utilisation is the least discussed but most trained! Most athletes and coaches wouldn't refer to it as Fractional Utilisation or Sustainable Percentage, but a lot of training is dedicated to improving the intensity an athlete can sustain over a race distance.

What is Fractional Utilisation?

Along with running economy and VO2max, FU determines endurance performance.

The sustainable percentage of your VO2max (commonly referred to as fractional utilisation, or FU) is the percentage of VO2max that can be sustained during an event, it doesn't matter whether it's bike, run, swim or skipping for that matter.

As race distance increases, the percentage of your VO2max that can be sustained decreases. Typically well-trained athletes can run at close to 100% VO2max during a 3-5km race. For a 10km this is closer to 92% and for a marathon, 78%.

Remember this is well-trained athletes, so the time they can run is important. If you run a 50min 10km you are not going to be able to sustain 92%. This is nearly double the duration of an elite runner, so you will be sustaining a lower percentage.

To maximise your performance, you are aiming to sustain the highest percentage of VO2max that you can.

The longer the race the more significant this is. In events lasting 10 – 15min athletes are able to sustain close to 100%, leaving little scope to improve this. In these events VO2max itself and Economy/efficiency are more relevant. However, as the distance increases it is harder to hold an intensity close to VO2max and small increases in sustainable percentage make meaningful changes to performance.

Factors Influencing Fractional Utilisation

The %VO2max sustained seems to be dependent on a number of factors including:

the aerobic capacity of the active muscles
the percentage of type I muscle fibres
muscle capillary density

Most of these factors are highly trainable, and this is why this aspect of endurance performance is an important one to consider when planning training.

Individual variation in the %VO2max that can be sustained

Fractional Utilisation can vary widely between individuals and can help to explain differences in race performance over different distances. Generally, if two athletes have equal VO2max and economy (link to articles) then the athlete with the highest sustainable % VO2max will win in a race situation of > 20min duration.

However, it must be remembered that the sustainable % VO2max decreases as race duration increases and will vary from athlete to athlete. Specific training will win out in athletes with similar VO2max and economy. The athlete who has prepared for a marathon will inevitably win the marathon, and the athlete prepared to run at 90%+ for a 10km will win over that distance, because they have optimised their ability to sustain the highest percentage of VO2max.

Assessing Fractional Utilisation

The %VO2max sustained can be evaluated under laboratory conditions, by measuring oxygen uptake at average race pace, at average power output, or, during a treadmill/cycle ergometer time trial. The oxygen uptake (VO2) sustained is then converted to a percentage of VO2max from a recent VO2max test. The %VO2 can also be established by comparing race pace/power outputs with laboratory recorded VO2 values from a recent incremental exercise.

Modern software and devices also make it possible to make some educated estimates from field only trials.

For instance in WKO5 (software analysis tool) there means of calculating an estimated VO2max from training data and field tests, and then looking at race and training data to identify the sustained percentage of VO2max. This works best when used with a power meter, either for running or cycling.

Improving Fractional Utilisation

Since, the sustainable %VO2max appears to be strongly linked to the muscle’s aerobic capacity, muscle capillary density and the VO2 at the lactate threshold, training should focus primarily on enhancing the muscle’s aerobic capacity. It is therefore important that training volume is sufficient.

In addition, specificity of training will be important. For a 30 – 60min event Lactate Threshold 2/Critical Power/FTP will be significant.

For an event lasting multiple hours, Lactate Threshold 1 / Fat Max / Aerobic Threshold will be more relevant.

It should be noted that it is nearly impossible to completely isolate an area of physiology in training. There will be some interplay between VO2max, economy and FU with training sessions.

For instance, a high volume of aerobic training will have an impact on improving VO2max, but it will also have an impact on FU. When combined with a polarised approach it will potentially impact all areas. However, to maximise the effects you will need to be specific in your training the closer you get to your event.

If your target event is an hour or so long, your training should progressively focus more on the specific endurance required for that event.

If you are targeting a multiple hour event, the focus should be more on developing the specific endurance required. See more on Periodisation here.

Practical Application

In the real world, we do not have easy access to a method of assessing VO2max on a regular basis or unless using specialist tools (WKO5), so we tend to substitute VO2max for Functional Threshold Power and use this as our reference point.

In triathlon, the majority of events will be performed at an average intensity which is lower than FTP, or in a short race such as a sprint for an elite athlete which might be close to 100%FTP.

Below are tables with the range of FUs related to FTP (NOT VO2max) for bike (left) and run (right) that we can expect to see in different disciplines. Remember, though the distance might be the same, the duration can be very different, hence the large range. Also, remember the level of conditioning an athlete has will affect their FU.

Above: Guidelines for pacing triathlons using power in the run and bike. These are based on an athlete racing without issues and being consistent, from AG to elite, but probably not at a novice level. These are figures we see in regular performers.

You can use this information to help influence your training and race preparation, and in reviewing races you have completed. If you are performing in races at the very higher end of the scale, then to go faster you are going to have to lift your FTP, it's not possible to do a 70.3 at your FTP! If you are at the lower end of the range then you should look at your fatigue resistance and endurance to raise your FU. Trying to increase FU whilst maintaining very low percentages of it, will not help your overall race performance.

You should also consider both running and cycling in the triathlon together. It's pointless being able to sustain a high percentage on the bike if it compromises your speed on the run. As a triathlete, you need to optimise your overall performance, not just the separate disciplines.