Suunto TSS and training load, explained for cyclists

How the Suunto app scores a ride

Suunto picks the method for you. TSS can be based on threshold power, threshold heart rate or threshold pace, and the app chooses by activity type and the data available: power for cycling, pace for running and swimming, heart rate in most other cases [Suunto TSS]. With no sensor data at all, it falls back to a rough MET estimate.

The power version is the classic formula. Suunto publishes it as duration in seconds, multiplied by Normalized Power and Intensity Factor, divided by FTP times 3,600, times 100 [Suunto hrTSS]. One hour ridden exactly at threshold scores 100. A three-hour easy ride can land on the same 100, which is the point of the metric: it puts a short hard session and a long easy one on one scale.

Without a power meter the watch computes heart-rate TSS instead. Suunto describes hrTSS as based on time in heart-rate zones derived from your lactate threshold heart rate, with an estimate of how much TSS an hour at each level of exertion accumulates [Suunto hrTSS]. The MET fallback is cruder still: it considers only duration and activity type, and Suunto itself calls it a very rough approximation [Suunto TSS].

The app tells you which method it used. Each TSS value carries a tag: p for power, hr for heart rate, r for running pace, s for swimming pace and met for the MET estimate, and a minus sign marks a value someone edited by hand [Suunto TSS]. Check the tag before you compare two rides. A p-tagged 80 and an hr-tagged 80 came from different inputs and different thresholds.

The one setting that moves every Suunto TSS

The Zone 4 to Zone 5 boundary. Suunto states that on its watches the HR Zone 4 to Zone 5 limit equals the lactate threshold, and hrTSS uses that level to calculate the score [Suunto hrTSS]. For power, Suunto tells riders to make the Zone 4 to Zone 5 limit the same as their FTP in TrainingPeaks [Suunto Power Metrics].

So the boundary is your threshold, as far as the scoring is concerned. Move it up by 5% and every hard ride scores lower; move it down and the same ride scores higher. Power TSS rises with the square of Intensity Factor, so an FTP set 5% low inflates a threshold-heavy session by roughly 10%, and that error carries into every Fitness and Fatigue point downstream.

The default is where most riders go wrong. Suunto's user guide for the Race says max HR is calculated by default with the standard equation of 220 minus your age, and the default heart-rate zones are percentages of it [Suunto Intensity Zones]. A population formula is a poor anchor for one person: the large meta-analysis behind the modern equation found individual maximal heart rates scattered widely around any age-predicted line [Tanaka et al. 2001]. A rider whose real max sits 10 beats above the formula has a Zone 5 limit that is too low, and every hrTSS they record runs hot.

The fix lives in the watch, under Training, then Intensity zones, then Advanced zones, where cycling gets its own heart-rate, pace and power zones [Suunto Intensity Zones]. Set the cycling heart-rate Zone 4 to Zone 5 limit to your measured threshold heart rate, and the power Zone 4 to Zone 5 limit to your FTP. No FTP yet? A Suunto FTP can come from a test or from an estimate built on the rides you already have.

Fitness, Fatigue and Form in the Progress view

Fitness is Suunto's name for CTL, a 42-day weighted average of your TSS. Fatigue is ATL, a 7-day weighted average. Form, the Training Stress Balance, is the difference between the two, and the app splits its graph into four areas: losing fitness, maintaining fitness, productive training and going too hard [Suunto Training Load].

These are the impulse-response ideas Banister formalized: training produces a slow fitness effect and a fast fatigue effect, and performance is the gap between them [Banister 1990]. The 42-day and 7-day windows are the time constants that make Fitness slow and Fatigue fast. Raise your weekly load and Fatigue climbs faster than Fitness, so Form drops. Suunto says plainly that a short spell in the losing-fitness area is useful, for example before a race [Suunto Training Load].

One detail matters to a cyclist. The Progress view is built from the TSS of every workout, and Suunto scores runs and swims too, from threshold pace [Suunto TSS]. A run counts toward Fitness and Fatigue alongside your rides, which is right for your whole body and confusing if you read the line as cycling fitness alone.

How high Fitness should go, and how fast it should rise, is a separate question. Weekly TSS targets and safe ramp rates for a self-coached amateur apply to Suunto's Fitness line exactly as they apply to any other, once its thresholds are set correctly.

Why Suunto TSS disagrees with TrainingPeaks, and with us

Because each platform scores the ride against its own threshold. Suunto reads the Zone 4 to Zone 5 limit on your watch; TrainingPeaks reads the FTP and threshold heart rate stored in TrainingPeaks [Suunto Power Metrics]. When those two settings differ, the same ride gets two scores. The formula is the same; the input is not.

The method can differ too. Suunto offers hrTSS as the alternative for riding without power [Suunto hrTSS]. If the watch scored a ride from heart rate and TrainingPeaks scores the same file from power, you are comparing hrTSS with power TSS. The two track each other loosely at steady efforts and drift apart on intervals, because heart rate lags a 30-second surge that power records instantly. A hand-edited value, marked with the minus sign, breaks the comparison entirely.

AdaptCycling will disagree with the Suunto app for the same reasons, and by design. We read the recorded file second by second and score power rides against an FTP estimated from your own power curve, not the zone limit on the watch. If your Suunto Zone 5 limit is set 20 watts below that estimate, our TSS for a threshold ride will sit below Suunto's. Neither number is broken; they answer the question against different thresholds.

There is one structural difference to know before training with a Suunto and a cycling plan side by side. We read cycling workouts only. Runs, hikes and swims are skipped before any file is fetched, so our fitness and fatigue picture is cycling load, while Suunto's Progress view includes every sport. A triathlete will see a higher Fitness in the Suunto app than in our chart, and the gap is the running and swimming.

Quick answers

Sources cited in this guide

1. 01
   Suunto TSS. Training Stress Score in Suunto app. Suunto.
2. 02
   Suunto hrTSS. Training with TSS and hrTSS. Suunto.
3. 03
   Suunto Power Metrics. View TSS and other power-based cycling metrics in real time. Suunto.
4. 04
   Suunto Training Load. Understand and manage your training load with Suunto. Suunto.
5. 05
   Suunto Intensity Zones. Suunto Race user guide: Intensity zones. Suunto Support.
6. 06
   Banister 1990. Modeling human performance in running. Journal of Applied Physiology.
7. 07
   Tanaka et al. 2001. Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology.

More inside Training with a Suunto

Other articles in this series

1. 01

   Suunto cycling training plan: your options in 2026

   Suunto Coach, the app's workout builder, ten SuuntoPlus guide partners, or a plan built from your rides. Who decides next week, and what it reads.

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2. 02

   Structured cycling workouts on a Suunto watch

   Every structured ride on a Suunto arrives as a SuuntoPlus Guide. How to build or receive one, why power steps need a meter, and why it won't sync.

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3. 03

   Suunto watch and a bike computer: one training record

   A Karoo syncs rides to the Suunto app. Why one ride recorded on watch and head unit counts twice in Suunto's load, and how to keep one record.

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4. 04

   Suunto FTP: test it, or estimate it from your rides

   Suunto's FTP test is 20 minutes, and power zones are edited on the watch. How to set FTP, keep it current, and train the HR-only route.

   →