Training intensity: understanding your VO2max will enable you to produce a more effective training programme

Training intensity - To find out which level of intensity is right for you, first work out your intensity 'domains'

If you are serious about training at the right intensity, or find that the sessions set for you are too easy or too difficult, then perhaps you aren't 'normalising' your intensity correctly.

When training programmes for endurance athletes are set, a certain amount of 'normalisation' has to take place. What this means is that rather than expressing an intensity as an absolute value, it is expressed as a relative value. In other words, rather than asking an athlete to run for 30 minutes at six-minute miles, we ask him or her to run at THEIR OWN 10K pace. This is because different people have different abilities.

A common way to normalise exercise intensity is to express it as a percentage of VO2max. This means that the Olympic champion and the Sunday fun-runner can both take some meaning from a session when they are asked to run at 80% of their VO2max. It would obviously be silly to ask them both to run at six-minute miles and expect them both to get the same benefit from the session.

Defining the 'domains'
However, while certainly an improvement, this VO2max normalisation may not be the best or even a good way to describe exercise intensities. Rather than just measuring VO2max and taking a percentage of it, what we have to do is carefully define exercise intensity 'domains' for each athlete, where each domain represents a range of work-rates (or running speeds) which results in specific unique and defined physiological responses.

The best way to explore this concept is to consider a series of constant-load bouts of exercise (that is, exercise where the work-rate profile is as shown in Figure 1). To begin with, we can take exercise from rest to moderate intensity. This is exercise below lactate threshold, and so someone exercising in this 'moderate intensity' will not be producing any net lactate, and while their VO2 will be greater than at rest, it will reach a steady state (that is, unchanging) within about three minutes (see box a of Figure 2). An athlete will be able to keep going at this intensity for a very long time - a number of hours - provided he or she takes in enough fluid and carbohydrate and stays sufficiently cool.

Stepping up the intensity
Now let's get our athlete to exercise a bit harder - this time at a level where there is a net increase in lactate production. He/she is now exercising just above lactate threshold, in the 'heavy' domain. Blood acidity and lactate concentration are higher than at rest (and higher than in the 'moderate' domain) but again can reach a steady state. However, VO2, while it may reach a steady state, takes longer to do so than at exercise below lactate threshold (see box b of Figure 2). This is because of a phenomenon called 'the slow component of oxygen uptake'. In Figure 2 you can see the simple shape of VO2 in the moderate domain. In the heavy domain, for the first couple of minutes, VO2 follows the shape for the moderate domain, but there is this second 'slow' component superimposed on this simple shape.

Not only does VO2 take a longer time to reach a steady state, but it then reaches a level above that which might be predicted from the response in the moderate domain. Below lactate threshold, for a 10-Watt increase in work rate, VO2 increases typically about 100ml/min. But above the threshold, for the same 10W increase in work rate, VO2 may increase 120ml/min - oxygen efficiency has decreased. The cause of this delayed decrease in efficiency is currently under considerable debate in the scientific community, and the person who gives the definitive answer will become very famous in the field! The potential contributing factors to this decrease in efficiency include increased muscle temperature, lactate or stress hormone concentration, or a delayed recruitment of the less oxygen-efficient fast-twitch muscle fibres. The answer is likely to lie in a combination of these. In this domain, exercise can last in the order of tens of minutes because VO2 and lactate do not reach their limiting maximal values.

Going up to 'critical power'
If we increase the work rate again, the next threshold we reach is the 'fatigue threshold' or 'critical power'. This threshold is at the highest work-rate at which lactate concentration and VO2 can reach a steady state. Therefore, just above this threshold, in the 'severe' domain, lactate concentration and VO2 can't reach a steady state but instead continue to rise throughout the bout. Fatigue will occur within about 30 minutes and may coincide with the attainment of VO2max (see box c, Figure 2) and lactate concentration max.

So what does this mean for you, as a coach or an athlete? The problem now is to work out your own thresholds, in particular the fatigue threshold. Lactate threshold can be measured from a standard incremental exercise test, which many labs and gyms can perform as a matter of course. The work-rate at the fatigue threshold can be worked out by performing a series of constant load bouts and plotting a 'speed-duration' or 'power-duration curve' as described in my article in PP 127, January 2000.

The lactate and fatigue thresholds do not occur at the same %VO2max for every person, which is why each individual should find out at what %VO2max their thresholds lie. Someone who is very fit may have lactate and fatigue threshold very close to their VO2max. Thus a training session set at 80% VO2max may be in the heavy domain for a high-quality athlete, who can therefore maintain this pace for more than 30 minutes, but in the severe domain for someone with a lower fatigue threshold, and so they will fatigue in a matter of minutes. Indeed, if '80% VO2max' is in the severe domain, then the athlete will, after a few minutes, reach 100% VO2max. Therefore, asking him to exercise at 80% VO2max makes no sense, because he'll be performing most of that 80% VO2max bout at a level GREATER than 80%.< p> Although it is more complicated, training intensities should therefore be set as 'run for 20 minutes in your heavy domain' or 'run at 50% of the difference between your lactate and fatigue thresholds'.< p> Stephen Garland

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