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What is this threshold power, why the heck is it important and how should I measure it? 

By Hunter Allen

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Les Woodland's book Cycling's 50 Craziest Stories is available as an audiobook here. Or, for the print and Kindle eBook versions, just click on the Amazon link on the right.

Hunter Allen writes:

The million dollar question! It seems that everyone has their own definition of what your 'threshold' is. Some say that it is the point at which you begin to accumulate lactate in the blood, some say it is the point at which the lactate in your blood begins to accumulate exponentially, while others say threshold is the heart rate or wattage that you can maintain for one hour. All of these definitions have merit and we'll let the exercise physiologists debate the definition, but for our purposes in developing the correct training plan for you, we will define 'threshold' as the average wattage that you can sustain over a 20 minute test. This is often referred to as your 'functional threshold'.

Why do we need to understand this and know our threshold wattage? Well, exercise intensity at which lactate begins to accumulate in a person's blood—that is, their lactate threshold (LT)—is a powerful predictor of their endurance performance ability. And person's LT reflects the ability of their muscles to match energy supply to energy demand, which in turn determines the fuel 'mix' (i.e., carbohydrate vs. fat) used and the development of muscle fatigue.

Consequently, LT—especially when expressed as a power output, which also takes into account cycling efficiency—is the single most important physiological determinant of performance in events ranging from as short as a 3 km pursuit to as long as a 3 week stage race. Just as importantly, because the metabolic strain experienced when exercising at a given intensity is dependent upon the power output relative to power at LT, this parameter provides a physiologically sound basis around which to design any power meter-based training program.
Why 20 minutes? Well, in 20 minutes, this has given your body enough time to get your heart rate up the 'edge', and produce a high level of lactate in the blood, which is what most people refer to as the 'burn'. This is your 'edge' or threshold. If you start out too hard, you will go over your threshold and then have to back off of your effort in order to maintain it, so pacing is critical in a 20 minute test. A 20 minute test is not just a hard sprint up a short hill, it's a full on pacing effort in which you are trying to push the limit (but NOT go over it!) for a full 20 minutes.

Now, the average wattage that we get from a 20 minute threshold test will be about 3-5% higher than the average wattage you can maintain for an hour. That's o.k. When you get ready to do an hour time trial (40kilometers), then you can adjust your wattage goal down a bit.

Another excellent reason to use 20 minutes as your threshold wattage is because it will be the longest interval that you will do without stopping for a rest. Intervals are hard, and by breaking them up into 'bouts', you are more able to complete the effort. Shorter efforts are more mentally and physically 'do-able'.

Since one major goal of any training program is to increase power at threshold, your threshold power should be periodically reassessed to be certain it is still accurate. How often threshold power will change significantly will depend in part on an individual's training history and habits—for example, someone who is just beginning in and/or returning to cycling may see large and rapid changes in their threshold power, whereas an experienced rider who has been training for many years and/or an athlete who maintains a high level of conditioning year round will probably experience much less variation. In general, however, assessing threshold power a few times per year (e.g., near the start of training as a baseline, partway through the pre-competition period to track improvement, and during the season to determine peak fitness achieved) is probably sufficient.

Now, let's get to work finding your threshold and choosing the best plan for you!

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