The two most important metrics in regard to racing or training are A) Time, and B) Intensity. If you were to ask a runner this question you would usually get the answer time and distance but this is trail running, right? If we were to focus on time and distance in a week a 100 kilometres on trails around the Riverhead Forest in the mud is going to be very different from 100 kilometres around the mountains of the Southern Lakes of Wanaka and Queenstown, which again would be different from 100 kilometres on flat, manicured well drained soil. If we bear this in mind, time and distance tells us very little. We actually need to know about time and intensity and the same is true of racing.
If we look back 30 years, intensity was largely measured in terms of relative perceived exertion. Maybe if you were lucky in a road race you had distance markers so you had an idea of what your pace was for that race. We then moved to looking at our Heart Rate, which is still a method we use today. The issue with Heart Rate is it is variable considering mood, fatigue, hormone levels, caffeine intake and a multitude of other factors. The secondary factor is that heart rate is an output measure. That is, the person who maintains the highest heart rate isn’t necessarily the winner of the race if they move at a relatively slow pace whilst doing so.
In the last decade we entered the realm of GPS as a means of measuring our pace, speed, and distance on an ongoing manner. For a flat road run in good conditions (because GPS is subject to atmospheric interference) one of the methods that we can use to measure our intensity is our pace. We can can be reasonably specific in terms of the expected pace that is sustainable over a period of time with an understanding of basic physiology. But if we enter the realm of trail running, our heart rate is going to be relatively difficult to keep down if you are going uphill and relatively difficult to keep it up whilst running downhill. Pace is equally going to be difficult to maintain going uphill and very difficult to maintain going downhill, depending on the surface and technicality of the trail. We are on the cusp of being able to measure power (or wattage) which is the gold standard in cycling for measuring intensity, but we are not quite there yet in terms of this technology being readily available and used in the running world.
Without being able to measure our power output, we need to be able to use the methods I have detailed above to measure our intensity, that is - pace, relative heart rate and perceived exertion. We could also call upon our previous knowledge of any given race i.e. if we have run it before, and our breathing output.
In my coaching I talk about different intensities for both training and racing:
Absorption Intensity - This could be maintained for an extended period of time, i.e. four hours if they were conditioned to do so,
Aerobic Intensity - This could be maintained between 2- 4 hours for those who are conditioned to run longer distances,
Tempo Intensity - Slightly faster than aerobic, able to be maintained for just on or under 2 hours,
Sub Threshold Intensity - The border where the body is primarily using oxygen for energy and anaerobic respiration where the body is primarily is using stored fuel to produce energy. This intensity could be held for 45-60 minutes,
Super Threshold Intensity- Higher than Sub Threshold and could be held for 30-45 minutes,
Interval Intensity - This could be held for 20-30 minutes,
Repetition Intensity- The body is really working. This intensity could be held for up to 15 minutes.
There are several more levels of intensity past this however for the purposes of this article and our discussion around trail running we will not discuss those here, although several times I have seen people go above their repetition intensity at the start of trail races. This is an important point, because if you tap into that repetition intensity early you will quickly run out of energy and it is difficult to recover from that.
Lighting the Fire
The Matchbox analogy is well used by coaches and athletes the world over. Let’s say we are going into an event which will take us 2 hours. Our options are to maintain a Tempo Intensity if you wanted to push the envelope or perhaps to run in the Aerobic Intensity which is easier to maintain. You could also consider running the first three quarters of the race at Aerobic Intensity and the last quarter at Sub Threshold or Threshold Intensity if there is energy to do so (This is the classic start easy-finish strong strategy). The idea is that if I’m running at an Aerobic or Tempo Intensity then I’m burning my matches one at a time. The individual flames will not be as bright but they will last a lot longer. The hope is that one will have enough of these matches to get to the finish line before one runs out of energy.
If one were to, on the other hand, perhaps confuse ambition with ability, or have a limited understanding of intensity and run at too higher an intensity early on the challenge, this is like burning two matches at once. The flame will burn brighter, however you are halving the amount of time that the matches will last and may well run out of energy well before the finish of the event. Plainly speaking, this is the part where one ends up in a heap, hates the rest of the event and finishes at more like Absorption Intensity because of the amount of fatigue that too much energy expended too early has wrought on the body.