The off-season should be just that. A break.

When the season returns, so does the hard work. Intervals. Long sessions. Races. It all sounds exciting, until you are overtrained, burnt out and carrying an injury.

Most athletes look closely at each session: pace, power, splits, heart rate. Very few look at the total load they are building week after week. That is where problems start.

One number helps you see it: Training Stress Score, or TSS.

What is Training Stress Score (TSS)?

TSS measures how much stress a session puts on your body. It combines two things: how long you trained and how hard you trained.

The reference point is simple. One hour at your threshold equals 100 TSS. Your threshold is the highest effort you can hold for roughly one hour. Cyclists measure it with power (FTP), runners with pace.

A longer session or a harder session means a higher score. A higher score means more recovery is needed.

How TSS is calculated

The formula is hours × Intensity Factor² × 100, where Intensity Factor is your effort relative to threshold. Because intensity is squared, it counts for far more than duration. 30 minutes at threshold scores 50 TSS, while an hour of easy riding at 65% of threshold scores around 42.

What do TSS numbers feel like?

As a rough guide for a single session:

•     Under 50 — light and easy; you could train again later the same day

•     50 to 100 — a solid session; legs aware, but fine

•     100 to 150 — hard; recovery is required

•     150 and above — a long, demanding day; plan the days after carefully

Why running TSS costs more than cycling TSS

Running loads muscles, tendons and bones far more than cycling. The same score on foot costs more recovery than on the bike, so runners work with lower numbers and build more carefully. Triathletes should track each discipline separately as well as the combined weekly total.

What is a good weekly TSS?

Forget kilometres. Your weekly TSS shows how much training you are actually absorbing.

There is no universal target. The right number depends on your training history, age, discipline and life outside sport. The right weekly TSS is the one you can repeat week after week while still sleeping well, recovering and hitting your key sessions.

How much should you increase weekly TSS?

Raise it by more than about 10% in a week and watch for the warning signs:

•     Lower HRV — your nervous system is under more strain than usual

•     Higher resting heart rate — a few beats above your normal for several mornings

•     Heavy legs — easy sessions feel harder than they should

•     Poor sleep — difficulty falling or staying asleep despite being tired

•     Irritability and cravings — mood dips and a constant pull towards sugar

•     Small niggles that do not go away — early signs that tissues are not keeping up

This is your body asking you to ease off.

Fitness, fatigue and form: how TSS builds over time

Most training platforms turn your daily TSS into three trend lines:

•     Fitness (CTL) — your average daily TSS over roughly six weeks; the load you have built

•     Fatigue (ATL) — your average daily TSS over about seven days; how tired you are now

•     Form (TSB) — fitness minus fatigue; negative during a build, positive when you are fresh to race

A simple way to plan your training load

Build for 3 weeks with gradually rising load. Then take 1 lighter week, around 30 to 50% less, so your body can absorb the work. Follow your hardest days with an easy one.

For example, a four-week block might look like this:

•     Week 1 — 400 TSS

•     Week 2 — 440 TSS

•     Week 3 — 480 TSS

•     Week 4 — 250 to 340 TSS, the recovery week

This is how structured training cycles work, at every level.

Why add HRV to TSS?

TSS measures stress from training. HRV reflects stress from everything else: work, sleep, travel, family life.

Together they tell you when to push, when to back off, and when to fuel more. If a high-TSS day is planned but your HRV has sat well below your normal range for several days, swap the intensity for an easy session.

How to fuel around TSS

More load means more fuel. As volume and intensity rise, so does the amount of carbohydrate your muscles burn. Your fuelling should rise and fall with your TSS, not stay the same every day. Sports scientists call this fuelling for the work required (Impey et al., 2018).

Why fuelling matters more as TSS rises

Carbohydrate is the main fuel for hard and threshold-level efforts. The higher the intensity, the more a session relies on muscle glycogen, and long, high-TSS sessions can heavily deplete those stores.

When energy intake fails to match training load over weeks, the risk is low energy availability. The International Olympic Committee's 2023 consensus statement on Relative Energy Deficiency in Sport (REDs) links it to impaired training adaptation, reduced performance, poorer bone health and higher injury risk (Mountjoy et al., 2023). Several early signs overlap with the overload warnings above.

Daily carbohydrate: match it to your training load

The joint position stand from the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada sets daily carbohydrate targets by training load (Thomas et al., 2016; Burke et al., 2011):

•     Light training — 3 to 5 g of carbohydrate per kg of body weight per day for low-intensity or skill-based sessions

•     Moderate training — 5 to 7 g/kg per day for around 1 hour of training per day

•     High training load — 6 to 10 g/kg per day for 1 to 3 hours of moderate to high-intensity training per day

•     Very high training load — 8 to 12 g/kg per day for 4 to 5 hours or more per day

In practice, a 70 kg athlete could need 420 to 700 g of carbohydrate on high-load days during a build, and closer to 210 to 350 g on light days in a recovery week.

Fuelling by session TSS

A practical guide that maps the research onto the session ranges above:

•     Under 50 TSS — normal meals and water are usually enough; no need to fuel during sessions under an hour

•     50 to 100 TSS — a carbohydrate-based meal 1 to 4 hours before; 30 to 60 g of carbohydrate per hour during sessions of 1 to 2.5 hours

•     100 to 150 TSS — arrive fuelled with 1 to 4 g/kg of carbohydrate in the 1 to 4 hours before; around 60 g per hour during; recovery fuelling straight after

•     150 TSS and above — up to 90 g of carbohydrate per hour for sessions beyond 2.5 hours, using glucose and fructose blends; plan fluid and sodium; refuel quickly afterwards

Why 90 g per hour needs two types of carbohydrate

The gut absorbs glucose at around 60 g per hour through a single transporter. Combining glucose or maltodextrin with fructose uses a second transporter, allowing up to around 90 g per hour to be absorbed and used (Jeukendrup, 2014). Train your gut in high-TSS sessions before relying on it on race day. Carbohydrate-electrolyte solutions contribute to the maintenance of endurance performance during prolonged endurance exercise.

Recovery fuelling after high-TSS sessions

•     Carbohydrate — when your next hard session is less than 8 hours away, aim for 1.0 to 1.2 g/kg per hour for the first 4 hours (Burke et al., 2011; Thomas et al., 2016)

•     Protein — around 0.3 g/kg, roughly 20 to 25 g of high-quality protein, after training (Thomas et al., 2016; Moore et al., 2009)

•     Fluid and sodium — around 1.25 to 1.5 litres for every kg of body weight lost, with sodium to help you retain it (Thomas et al., 2016)

Carbohydrates contribute to the recovery of normal muscle function (contraction) after highly intensive and/or long-lasting physical exercise leading to muscle fatigue and the depletion of glycogen stores in skeletal muscle. Protein contributes to the maintenance of muscle mass.

Fuel the week, not just the session

•     Build weeks — fuel towards the upper end of your daily range on high-TSS days

•     Easy days after hard days — prioritise refuelling; this is when glycogen is restored

•     Recovery weeks — reduce carbohydrate in line with the lower load, but avoid cutting energy hard; this is when adaptation happens

Which apps calculate TSS?

You do not need to calculate it yourself. TrainingPeaks and intervals.icu calculate TSS for you. Garmin, Coros, Polar and Wahoo use their own training load scores that work in a similar way. Stick with one platform, because the scores are not directly comparable. Your job is to track the number and learn your limits.

The goal

Find the weekly range where you feel strongest. Then protect it.

That number is your personal blueprint for getting faster without burning out. And remember: recovery starts with what you eat and drink around your hardest sessions.

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Frequently asked questions

What does TSS stand for?

TSS stands for Training Stress Score. It is a single number that combines the duration and intensity of a session to estimate how much stress it puts on your body.

Is 100 TSS a lot?

100 TSS equals one hour at your threshold effort. For most amateur athletes, that is a hard session that needs a recovery day or an easy session to follow.

How much should I increase my weekly TSS?

As a guide, no more than around 10% per week, with a lighter week of 30 to 50% less load every 3 to 4 weeks.

Does TSS work for running?

Yes. Running TSS is based on pace relative to your threshold pace. The same score costs more recovery than on the bike because running places more impact on muscles, tendons and bones.

How many grams of carbohydrate per hour should I take on a high-TSS session?

Research supports 30 to 60 g per hour for sessions of 1 to 2.5 hours and up to 90 g per hour beyond 2.5 hours.

Should I eat less in a recovery week?

Eat less carbohydrate in line with the lower load, but do not under-fuel. Recovery weeks are when your body adapts to the training, and chronic under-fuelling is linked to reduced performance and higher injury risk.