This is another one of those topics which you constantly hear people rumbling about and again it’s another one of those topics in which people look for ONE answer. The purpose of this writing is to clear up some of the confusion surrounding this discussion. The bottom line is that you don’t know if so called “Junk Miles” are actually junk miles unless you learn about an athlete’s race objectives (which help put a frame work around the need for extra miles). Here are the two scenarios:
1) An athlete who is training for the Olympic distance who is doing 15 hours of training per week. This athlete is just above critical volume for his event. With that, he has the durability required to meet his speed potential at that distance. If he now decided to go out and spend another 2-3 hours at a very low intensity just rolling around on his bike, those would be junk miles. See, in this situation the extra volume which is not required for durability really just impedes recovery from his quality workouts and dulls his speed potential development through increased catabolic hormones, soft tissue damage, etc. A better approach to adding stress for adaptation (improved speed potential) would be additional intensity work within the same volume range.
2) Next, we have an athlete who is training for the IM doing 20 hours per week. He is training at about 2/3 critical volume and therefore does not have the required to meet his speed potential at that distance. If he now decided to go out and spend another 2-3 hours at a very low intensity on his bike, that would be extremely valuable durability training. What we find is that the major limiter in IM racing (especially in the AG ranks) is the lack of durability. Here, every mile that gets you closer to critical volume will get you to the finish line faster. That is, these extra miles while training for the IM actually target the area where most athletes are limited…..durability. The addition of intensity in this situation although may improve speed potential, really just increases the risk of injury. Unfortunately, any increase in speed potential is very small relative to the a gain in durability which would come with extra miles……the intensity work is just not worth the risk. Long term progress is all about consistency; particularly in IM racing.
In summary, never train too far beyond critical volume since this will begin to impact speed potential development. A better path to added race speed is intensity work within the same volume (at critical). Conversely, since durability is a major limiter in longer distance racing where critical volume is typically not met, every mile is valuable so another 2-3 hours of low intensity work will help race speed. With these clarifications on a cloudy topic, it’s now easy to see what junk miles are and how they can impede progress if they are miles beyond what’s needed for your race distance.