Having compared my hrTSS and power-based TSS scores for indoor rides over a number of weeks, I had previously thought that hrTSS was a good enough estimation for use when power was not available (all outdoor rides for me). However, yesterday I did a 2 hr 40 min mountain bike ride and my calculated hrTSS was 233. This is clearly way too high and would have suggested that I was flat out for the entire ride (I wasn’t). Using the TR TSS estimator based on RPE gave a much lower figure of around 130, which I felt was a touch low but much more believable. Increasing the RPE by 1 gave me a value which I was happy with.
There are a number of reasons that I can think of that the hrTSS would be so high for mountain biking:
Most of the time I was coasting was on fast downhills and heart rate would have stayed high due to the adrenaline.
Periods of coasting were generally interspersed with short periods of sprinting, keeping heart rate elevated.
Heart rate would have often been higher for a given power output compared to road riding due to the amount of upper body work required on technical trails.
This last point in particular got me questioning how valuable TSS really is when undertaking different types of riding or different sports. If I had a power meter on my mountain bike, wouldn’t the TSS score be vastly underestimated as it would only take into account the stress of pedalling and not the stress of the upper body requirements? But then, does that matter if you’re using TSS as a way of planning weekly training load? If your upper body is tired but your legs are fresh then you should still have a similar capacity for indoor training than if your upper body was fine, right? Or will any stress impact your capacity for training, irrespective of the part of the body that is stressed? If the latter, then maybe there should be different calculations for estimating off road riding TSS and road riding TSS.