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What is the Training Stress Score metric and why does it matter to triathletes?

2016 Ironman World Championship

— By Michael Liberzon

Train smart

Smart, purposeful training must constantly challenge the body. Week-to-week training load should increase gradually to promote adaptation that ultimately results in a stronger, faster athlete. This means either increasing intensity or increasing duration.

The big question is, of course by how much? Increase the load too quickly and you risk injury. Increase it too slowly – or not at all! – and you may not see progress. Enter Training Stress Score (TSS), a metric developed by Peaksware who produces the widely-used training database Training Peaks. TSS is a training metric that aims to combine intensity and duration and assign that combination a numerical value to be used when designing training that is both safe and productive.

What is TSS?

Take workout intensity and square it. Now multiply that by the duration (in hours) and you have TSS.

TSS = IF2 (intensity factor) x Duration (in hours)

What you get is a dimensionless measure of the metabolic cost of a workout. The longer the workout or the more intense, the greater the cost. The reason that intensity is squared is that it contributes much more to the “cost” in terms of glycogen consumption than duration. You know the feeling: a 10k all-out race will wipe you out in a way that an easy 20k run never will!

Want to learn more about measuring intensity? Check back on TMC for a piece about that soon.

How to use TSS in training

The greatest utility of tracking training stress score is managing training load. It should be used to design build phases that ensure that the training stimulus increases gradually over time. This helps maintain progression while mitigating the risks of overtraining /under-recovery. The typical week-to-week TSS increase is 5 to 10%, although this varies quite a bit from athlete to athlete and is dependent upon the goal of the give training phase.

Program recovery periods where TSS is 30 to 50% less than that of the preceding build period. Create pre-race taper protocols that reduce TSS while maintaining the appropriate level of intensity to suit race goals.

How to use TSS in racing

TSS can be effectively used to pace the bike leg in long course racing. One big challenge when competing in the half or full distance is to make the most of the bike while ensuring that there’s enough in the “fuel tank” to run well.

Enter TSS.

Recall that training stress score is a proxy for the glycogen consumed during a bout of exercise. As such, pacing the bike leg using TSS is a perfect way to strike that balance.
Here are the common recommendations for bike only TSS targets:
Full distance: 270-300
Half distance: 160-170

The reason for the range is to account for your running ability. If the run is a strength, you can afford to push a bit more power on the bike and aim for the higher end of the range. If the run is a weakness or if recovering from a run-related injury, play it safe and shoot for the lower end.
Another important point to consider here is this: the faster you are, the more you can afford to push the pace on the bike. Why? Remember that the formula for TSS involves both intensity and duration. So since a faster athlete will complete the bike leg faster, she can afford to maintain a somewhat higher power output for the duration. This, coincidentally, is the reason why pro athletes will sit at a significantly higher percentage of their threshold power in races.

It’s a good idea to review our pacing with power article found here.

Happy training!

Michael is an NCCP trained triathlon coach, certified personal trainer, and kettlebell instructor. His degree in mechanical engineering supports his evidence-based approach to coaching.

Michael is also the owner and head coach of the X3 Training Lab in Toronto.