Sub-threshold training is one of the most misunderstood concepts in endurance sport. Athletes hear the word "sub" and assume it means easy. It is not. Sub-threshold work sits just below your lactate threshold - a range that is genuinely demanding, highly productive, and nothing like a Zone 2 shuffle.
What Sub-Threshold Actually Means
Your second ventilatory threshold marks the intensity at which lactate is being produced through glycolysis faster than it can be oxidised back through the Krebs cycle as fuel. Sub-threshold sits just below that point, typically around 85 to 92 percent of your maximum heart rate, depending on the individual.
This is not a comfortable intensity. It is the kind of effort where you can hold a conversation of a few words at most, your breathing is clearly elevated, and you are working. It sits well above Zone 2 and demands genuine physiological effort to sustain.
The confusion often comes from the way coaches talk about intensity distribution. When they say most of your training should be "below threshold," athletes interpret that as easy. But the space below threshold is enormous. There is a world of difference between recovery pace and 90 percent of max heart rate, and sub-threshold lives at the sharp end of that range.
Why It Delivers the Best Return on Investment
Sub-threshold training is the backbone of middle and long-distance endurance performance. It drives adaptation at and around your threshold without the recovery cost of training above it.
At sub-threshold intensity, you are accumulating significant time at a metabolically demanding workload. Lactate is being produced and recycled as fuel at a high rate, your cardiac output is elevated, and your oxidative system is working hard. But because you are not exceeding your threshold, the recovery demand is manageable. You can do a lot of this work without burying yourself.
This is why it is such a powerful training tool. You can accumulate far more volume at sub-threshold than you can at threshold or above, and that volume drives genuine aerobic adaptation. Ten repetitions of one kilometre at sub-threshold with short rest is a serious session - not because any single rep is maximal, but because the cumulative load is substantial. Over months, this is also how you build efficiency, the metric that ultimately determines how much of your ceiling you can use on race day.
The Zone 2 Misunderstanding
The current popularity of Zone 2 training has made the problem worse. Athletes now have two categories in their heads: easy Zone 2 work and hard threshold or VO2 max work. Sub-threshold does not fit neatly into either box, so it gets lumped in with easy training or ignored entirely.
This is a mistake. If your entire intensity diet consists of very easy and very hard, you are missing the range that is most specific to race performance for anything from Olympic distance upward. This is exactly why the 80/20 model falls short for age-group athletes. The ability to sustain a high percentage of your threshold for extended periods is exactly what determines your race pace - and sub-threshold training is how you build that capacity.
How to Execute Sub-Threshold Sessions Properly
The key to effective sub-threshold training is precision. You need to know where your threshold actually sits - not an estimate from a calculator or a number from a test you did six months ago. Your threshold changes as your fitness changes, and your sub-threshold zones need to move with it. This is exactly why the Norwegian method breaks down when athletes guess their intensity.
Sessions are typically structured as intervals with short rest periods. The rest is short because the intensity is sustainable - you are not recovering from a maximal effort, you are resetting briefly before accumulating more time at the target workload. Think 60 to 90 seconds of rest between efforts, not four minutes.
Heart rate, power, and pace all have a role in controlling these sessions, but heart rate is often the most reliable guide for athletes without laboratory testing. If your heart rate is sitting comfortably in the mid-80s as a percentage of maximum, you are probably too easy. If it is pushing into the low 90s and you are struggling to complete the session, you are probably too hard.
It Is Supposed to Be Uncomfortable
This is the part most athletes need to hear. Sub-threshold training is not supposed to feel relaxed. It is supposed to feel controlled but demanding. You should finish these sessions knowing you have done real work - not destroyed, but genuinely fatigued.
If your sub-threshold sessions feel easy, either your threshold estimate is wrong or you are not executing them at the right intensity. Both are fixable, but neither will fix itself.
Sub-threshold is not the space below hard. It is the space just below your limit - and training there consistently is what separates fit athletes from fast ones.