What is Training Stress Score?

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

To access these metrics, you’ll need one of the best power meters. The best bike computers and best multisport watches will display these data as you ride. For additional information on how your body responds to your power output, consider wearing one of the best heart rate monitors too.

Key power meter metrics explained

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 average powe which is simply the average wattage you produce including zeroes.

Then we have Normalized 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. Normalized Power is better estimation of the physiological “cost” of a ride than average power.

The relationship of average power to Normalized Power gives us another metric 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 Variability Index of 1.05 or less but for a crit, with multiple sprints and accelerations, you may see a Variability Index of 1.2 or higher.

What is Training Stress Score in cycling?

Training Stress Score (TSS) combines all of these metrics – FTP, average power, Normalized Power and Variability Index – with ride duration into one simple number. It 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 or zone 2 – 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 Normalized Power rather than average power, 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 average power might be similar.

How do you use Training Stress Score 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!

Using Training Stress Score to estimate fatigue, fitness and form

In TrainingPeaks, you’ll see your Performance Management Chart (PMC). By extrapolating from your daily Training Stress Score, this quantifies your fatigue, fitness and form.

Fitness: Acute Training Load (ATL)

Acute Training Load takes a weighted average of the daily TSS for the past seven days to estimate your fatigue. After a hard ride, you’ll see your ATL score spike and decline following easier days. 

Fitness: Chronic Training Load (CTL)

Chronic Training Load is produced from a weighted average of your daily TSS for the last 42 days. It also considers the past 12 weeks of training. By training consistently you’ll see your fitness and fatigue rise gradually. 

Form: Training Stress Balance (TSB)

Form is the sum of yesterday’s Fitness minus yesterday’s Fatigue. This is claimed to reflect that heavy training load doesn’t equate to good form. As you taper for an event, your Training Stress Balance will go from negative to zero or turn slightly positive. In theory, this indicates you are fit and fresh to race. 

What Training Stress Score 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. These include experience, target event, time of year etc etc.

Also, for triathletes, it’s not just about the bike. 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.

Therefore, a good triathlon training plan will not just be based on TSS. It will factor in subjective measures of effort and fatigue, like Rate of Perceived Exertion.

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