In the last post, through the use of two very simple examples, it was explained why TSS should not be used as an effort budget and it was shown the reason why TSS-constant curves appear to yield IF’s that translate into correctly paced Ironman bike legs.
The post generated quite a bit of discussion, and I feel that some of it was useful. Among the useful bits was the evidence on the exact shape of the Power vs Duration curve. Figure 1 shows the Power vs Duration curves for constant percentage drop-off for double the duration, i.e., log(t) curves for 11, 9 and 7% drop-offs, respectively. To those curves, within which the vast majority of the individual Power vs Duration curves falls, the TSS=300 curve was added.
As we can see yet again in this example, the TSS-constant curves “cut” across the possible intensity levels, with an obvious "disregard" for the Power vs Duration curve. Even for the durations that served as basis for the development of the “method”, the 5-6h range, the “TSS method” gives IF ranges for the cap or the starting point or whatever anyone wants to call it that are simply too big for such a small variation in duration (0.71 < IF < 0.77).
So how did TSS get involved in pacing strategies? Likely, just because it was there (Another "metric" that happens to be there is VI - another metric that is nothing more than a curiosity). Those that think that looking at numbers is the same as producing relevant data noticed that TSS fell into a nice range for triathletes that were subjectively considered to have “ran well”. While doing that, they forgot that TSS is used to quantify training load in the rather simplified impulse-response model of training adaptation, used to model training-induced changes in performance. This obviously has nothing to do with pacing strategies for a triathlon.
It should be self-evident to all that when we are talking about pacing strategies for triathlon, the only overall metric that makes sense to use is IF. On top of being an overall measure of exercise intensity, only IF is directly related to the Power vs Duration curve.
The use of IF is also very intuitive. In order to be able to run well off the bike, a triathlete reduces the intensity, IF, of her effort on the bike from the level of effort she could do in a maximal effort, given by the Power vs Duration curve. Only the Power vs Duration curve is “real”, in the sense that it relates to a maximal effort for a given duration. A triathlete uses a fraction of that effort in order to be able to access her running fitness, with the ultimate goal of minimizing the overall race time.