Fezrun,
Welcome to 80/20 Training! This can be a tough question to answer quickly. In fact, we dedicate much of the first 3 full chapters of our books 80/20 Triathlon and 80/20 Running to address this question.
It sounds like you’re onboard with the general 80/20 concept: 80% easy and 20% moderate to hard intensity. The question is: why do we avoid Zone X when distributing the intensity of the 20%?
Zone X is not a dead zone, but it’s the least valuable zone to build general fitness. It’s uncomfortable enough to not want to go any harder, but just hard enough that you feel you’re getting a good workout. We call it the “moderate intensity rut” because if you give an athlete the ability to freely choose their intensity, Zone X is what they’ll fall in to.
Unfortunately, Zone X requires just about as much recovery as Zone 3+, but Zone 3+ provides significantly more fitness payoff. So, the return on investment for Zone 3+ is much greater than Zone X. You just can’t build VO2 and LT at 90-95% of LT. We therefore polarize general training to avoid Zone X in favor of higher intensities.
But, when you say that elite athletes spend a lot of time in Zone X, this is true! Zone X ends up being a sweet spot for marathon, half, Ultra, and full Ironman racing (see our guide https://8020endurance.com/race-pacing/). During the specific phase of training (last ~10 weeks before an event), athletes at these distances should in fact incorporate a significant amount of Zone X if that is the zone they expect to race in. During the specific phase, we are no longer trying to build general fitness, we are trying to build specific fitness and for some distances and athletes this requires Zone X. It does not change the 80/20 ratios, time in Zone X still counts towards the 20%, we just replace Zone 3+ with Zone X for some distances in the specific phase of training.
Our plans are very specific in that we expressly give the athletes permission or explicitly schedule the use of Zone X for key workouts for those longer events. But, you won’t see Zone X in any of our 5K, 10K, Sprint or Oly plans and you won’t see it in the first 10 weeks of almost any 80/20 plan when we are trying to build general fitness.
David
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