Here’s a topic that’s commonly debated: Is it better to run on tired legs throughout your week’s workouts, or to run on fresh legs? That is, is it better to do most of your runs as transition runs, or as separate workouts the same day. Like many topics in triathlon there is no one answer to this question. Let’s take a look at the two scenarios:
1) Running on tired legs typically results in a peripheral system limited workout. That is, if maximum effort is applied, speed would be capped by peripheral system fatigue vs. core limiters (oxygen uptake/delivery, etc). For triathletes that focus on Ironman racing a peripheral type of limiter (in training) is exactly what you are looking for, since for 99 percent of the folks their race is limited by the lack of peripheral system durability. Based on that, this type of training is very race specific and provides an opportunity to train the most common limiter hit on race day. Another way of saying this is that most people don’t meet their speed potential curve (run what their 5k times suggest they should) while racing IM due to lack of durability. Running on tired legs improves durability which is the exact issue most face in IM.
2) Running on fresh legs allows a core system limited workout, which improves speed potential. For athletes racing Olympic distance where durability typically isn’t an issue (most athletes meet their speed potential curve) the limiter is speed potential, which is improved by pushing core system limiters during workouts. Based on that, for the short course racer, it makes more sense to run on fresh legs most of the time so that the runs have “better quality”.
Given these two explanations, it is easy to see that for Ironman athletes it makes sense to run on tired legs since durability is the major limiter on race day. Conversely, it makes more sense for Olympic distance athletes to run on fresh legs so that core system limiters can be pushed in training as they are on race day. Of course in IM training there are days when you run on fresh legs to improve speed potential (core targeted workout) and of course there are transition runs involved with Olympic distance training. However, at the end of the week, the IM program should have more T runs than fresh runs and the Olympic program should have more fresh runs than T runs. As I have said before, proper training programs are all about race specificity in training.