I’ve previously described “polarized training” studies showing that doing workouts above the anaerobic threshold is more beneficial for performance when compared with a similar amount of time spent training between the aerobic (AeT) and anaerobic (AnT) thresholds. But what would happen to performance if the above-AnT time stayed the same while the remaining training time was shifted toward either very low intensity (less than AeT) or the more moderate-effort range between the two thresholds (AeT to AnT)?
The latter is what self-coached athletes commonly do, especially if they have a limited amount of time to train. It seems to makes sense: Spending more of one’s remaining workout time at a moderately high intensity rather than just slogging along at a slow speed would seem to be the way to go. If it feels harder then it must be better for performance. After all, you can’t make all of your training very hard. So it makes sense to use your remaining available training time doing moderately hard workouts. Right?
This way of thinking means that if you train 10 hours per week with a total of 2 hours at or above AnT then you should get as many of the remaining 8 hours as possible at least close to AnT. The higher the average intensity of training, the faster you become and the better the race performance. So that translates to a lot of zone 3 time (between the thresholds). Makes sense.
Interestingly, some research on this way of thinking shows just the opposite. The more of the remaining training time (aside from the >AnT time) that is spent at a very easy intensity—below AeT—the greater the performance gains (Esteve-Lanao 2007, Munoz 2014). Unfortunately, there isn’t a lot research on this specific topic so there’s certainly room for a counter argument here.
Assuming that we accept this train-easy-rather-than-moderate concept, there may be several reasons that it’s more beneficial. An obvious one is that you are likely to be more rested when doing a subsequent hard (>AnT) session. There may also be benefits of going slow such as improving fat metabolism and capillary density in the slow twitch muscles. There could even be psychological benefits having to do with burnout.
The bottom line of this research is that training should be either hard or easy—most of the time. As soon as we start looking at training in terms of an entire season, periodization must be considered. Depending on the type of event for which you are training, there are times in the year when it may be appropriate to do a considerable amount of training at a moderate effort between the thresholds. For example, if you are training for a long-distance event lasting several hours that will be raced in zone 3, then a good deal of time in the last few weeks before the race should be spent training in zone 3. Also, very early in the season, such as in Base 1 and 2, it’s probably beneficial for most athletes following a classic, linear periodization program to do a considerable amount of training between the thresholds with very little time above AnT.
The bottom line here is that it is often beneficial to do very easy workouts rather than entirely avoiding them in favor of a moderate effort as many do. In my opinion, perhaps the biggest mistake athletes make in training is to make their easy workouts too hard.