Before drilling into what Training Stress Score (TSS) is and how it relates to cycling, there are a few other metrics to understand.

　　FTP (functional threshold power) was defined as the maximum power that you could hold for 60 minutes. However it’s better viewed as your sustainable power so, for an untrained cyclist, it could be what they could manage for 30 minutes.

　　Another way of thinking about it is your cycling 'red-line' that, if you push above it for too long, you’ll run out of gas.

　　Next is AP (average power) which is simply the average wattage you produce including zeroes.

　　Then we have NP (normalised power) which takes into account the fact that no ride, even a perfectly paced flat time-trial, is perfectly steady.

　　It uses an algorithm to factor in sprints, surges, hard efforts and other variables and says what power you would have produced had your effort been perfectly steady. NP is better estimation of the physiological “cost” of a ride than AP.

　　The relationship of AP to NP gives us another metric, VI (variability index) which is calculated by dividing your normalized power by your average power.

　　An evenly-paced bike leg on a flat course should have a VI of 1.05 or less but for a crit, with multiple sprints and accelerations, you may see a VI of 1.2 or higher.

　　What is TSS in cycling?

　　Training Stress Score (TSS) combines all of these metrics – FTP, AP, NP and VI – with ride duration into one simple number, and objectively describes how hard any ride was and the physiological impact of it.

　　So if you were to ride for an hour consistently holding your FTP, that would give you a TSS of 100. Half an hour at this pace would give a TSS of 50. An hour riding at of 65% of FTP – endurance pace – would result in a TSS of 42.

　　Riding for four hours at this intensity, would yield a TSS of 168. So, either ride intensity or duration can increase the TSS of a workout.

　　However, as TSS uses NP rather than AP, it also takes into account variability of power output during a ride.

　　This is because constantly spiking, either intentionally during an interval workout or unintentionally through poor pacing, places a higher stress on your body than steady riding.

　　So an hour of intervals will produce a higher TSS than an hour of steady state, even though AP might be similar.

　　How do you use TSS for cycling?

　　If you’re looking to track and monitor training load, it’s a far better metric than time or distance ridden as it’s a direct measure what you’re putting your body through.

　　You can plan workouts and know exactly beforehand what training load they’ll create.

　　If you use TrainingPeaks or a similar platform, you can plan training blocks in advance, know the TSS of the sessions you will do and use this information to build towards and even plan effective tapers to key events.

　　Finally, if you do a session of under an hour and get a TSS of more than 100, good news, you’re probably due an FTP test!

　　What TSS should I aim for?

　　That’s a bit of a how long is a piece of string question as the answer will vary on a number of factors including experience, target event, time of year etc etc.

　　Also, for triathletes, it’s not just about the bike, as both running and swimming will create their own training load and therefore need to be accounted for.

　　It is possible to derive TSS scores for these activities, but they won’t be as accurate as those derived using a cycling power meter.

　　For reference though, a typical Cat 1/2 cyclist would probably have a weekly TSS of 770-960 and a long-course triathlete 850-1700 – including running and swimming.