Fitness, fatigue and form, explained

It starts with one number per workout

All three lines are built from the same thing: a training load score for each workout. It combines how long you went with how hard you went, measured against your threshold, which is the effort you could hold for about an hour.

An hour at threshold scores 100. An easy hour scores around 50. A three-hour easy ride can score more than a hard hour, which is fair: it costs you a lot too. If you’ve seen the term TSS, this is that number.

Because the score is measured against your own threshold, a run, a ride and a swim can be added together. Shape scores each workout from the best data it has: power on the bike, pace for runs and swims, and heart rate for everything else. You can try each formula with the TSS calculator (power), the rTSS calculator (running pace) and the heart rate calculator.

Fitness: what you’ve built (CTL)

Fitness is your average daily load over the last 42 days, about six weeks. Recent days count a little more than older ones, so the line moves smoothly instead of jumping when an old week falls out of the window. Train an average of 60 a day and your fitness settles at 60.

It moves slowly on purpose. A workout that scores 100 puts fitness about 2.4 points higher than skipping it would have. One big week barely shows. Six steady weeks show a lot.

Fitness here means how much training your body is used to handling. It’s not your race pace or your VO2 max. A rising line means you’re doing more, which usually makes you faster, but the chart can’t promise that.

Fatigue: what it cost you (ATL)

Fatigue is the same kind of average, over the last 7 days instead of 42. The short window makes it react fast. That same 100 workout adds about 13.3 points to fatigue on the day, more than five times what it adds to fitness. Most of it’s gone again within a week.

Here is the same idea from the other side: how much of one workout each line still counts as the days go by. Fatigue forgets fast. Fitness remembers for weeks.

Form: what is left to spend (TSB)

Form is fitness minus fatigue.

When your last week was harder than your six-week average, fatigue sits above fitness and form is negative. That’s normal. It’s what training more than you’re used to looks like, and it’s how fitness goes up.

When you ease off, fatigue falls much faster than fitness, so form turns positive and you feel fresh. That’s the whole idea behind a taper: give up a little fitness to lose a lot of fatigue.

Over the easy week, fitness dropped 5 points and form went up 26. That trade is why you rest before a race.

What the form numbers mean

There’s no official scale, but these are the rough ranges many coaches use. The examples from this guide are marked on it.

Read them with your fitness in mind. A form of -20 is a deep hole when your fitness is 30, and an ordinary week when it’s 100. The load ratio below takes care of that for you.

Load ratio: is this week too much?

Shape also shows a load ratio, which is fatigue divided by fitness. It answers one question: is this week out of step with the last six? Because it’s a ratio, it reads the same at any fitness level.

The hard week above ends at a ratio of 1.22, still balanced.

What happens when you stop

Here is someone training 60 a day who takes two weeks completely off. Fatigue is nearly gone after the first week. Fitness falls more slowly: 51 after one week, 43 after two.

So a week off with a cold costs about 15% of your fitness on the chart, and two weeks about 28%. Form shoots up while you’re off. That’s not a sign you’re ready to race, only that the tiredness is gone.

Try it: plan a taper

A taper is the easy stretch before a race. Too short and you race tired. Too long and you lose fitness. Move the sliders and watch form on race day.

Racing straight off this build, form is about -10. A one-week taper at half the training takes it to +11, and fitness only drops from 70 to 65.

How Shape draws the chart

In Shape the chart lives on the Today page. Solid lines are what you have done. Dashed lines show what happens if you do the workouts on your calendar, up to eight weeks ahead, so you can see what your plan does to your form on race day before you start it.

Only finished workouts move today’s numbers. Planned ones only draw the dashed line, and a planned workout you skip drops out.

Things that throw the numbers off

• Older workouts that aren’t in yet. Fitness is a six-week average, so it reads truest with your whole training history behind it. Wahoo brings all of it in when you connect. Garmin sends the last 30 days, so upload your Garmin export once and fitness counts every workout from day one.
• Thresholds that are out of date. Every score is measured against your threshold. If your FTP is set too low, every ride looks harder than it was, and fitness and fatigue both read high. Update your thresholds when your fitness changes.
• Workouts with no data. A workout with no heart rate, power or pace has no load unless you add one, so a strength session without a heart rate strap doesn’t show up on the chart.
• Life outside training. The chart only sees workouts. It doesn’t know about a bad night’s sleep, a stressful week at work or a cold. That’s what readiness is for: it reads your sleep, resting heart rate and HRV.

Questions

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More about training load in Shape