Fatigue can mislead you — it arrives with a delay. A week in which you overdo it may feel excellent; the bill can arrive later, when your legs feel heavy and the desire to train fades. Training-load metrics are not there to give you another graph, but to help you see sooner where your training is heading.
The three numbers you need
Fitness (CTL · chronic training load)
Describes your longer-term training base. It responds slowly, building across weeks and months and declining at a similar pace. Higher long-term fitness usually means you are better adapted to regular training load.
Fatigue (ATL · acute training load)
Describes shorter-term load from the latest days. It responds quickly: harder training days raise it, while several easier days gradually bring it down.
Form (TSB · training stress balance)
Is the difference between fitness and fatigue: TSB = CTL − ATL. Slightly negative form can be normal during a training block; more positive form usually means greater freshness. One specific number is not universally good or bad.
Training nudges form slightly negative so fitness can grow, then you ease back before a race to let form rise.
These three concepts are enough to understand the basic logic of training load.
How training load is calculated
In simple terms, load rises with both training duration and intensity. An hour in easy Z2 affects the body differently from an hour of intervals, even though both last sixty minutes. Distance alone is therefore not enough: the same mileage can represent two very different weeks.
PaceroLoad is not simply two numbers multiplied together. Its deterministic calculation uses the available duration and intensity data to create a consistent input for the development of fitness, fatigue and form.
If zones or LTHR are still unfamiliar, start with the guide to heart rate training.
Typical situations and what to do
| What you see | What it may mean | What to do |
|---|---|---|
| Form is slightly negative and fitness is rising | A common state during an appropriate training block. | Continue and follow the trend alongside how you feel. |
| Form stays strongly negative for several weeks | It may reflect a longer period of higher load. | Consider reducing volume or intensity and add an easier week. |
| Fitness has plateaued for months and fatigue is low | Your current training may no longer provide a new stimulus. | Add gradually — more volume or one appropriate quality session. |
| Fatigue jumped after one hard weekend | A short-term response to a larger training dose. | Take several easier days and watch for a return towards your usual range. |
| Two weeks until race day | It may be time to begin reducing volume. | Ease volume so fatigue can fall and freshness can gradually rise. |
Why more is not always better
Training works through supercompensation: load creates a stimulus and the body adapts to it during recovery. Without easier days and weeks, fatigue may accumulate faster than fitness.
The goal is therefore not to keep load as high as possible, but to alternate an appropriate stimulus with room to adapt. A longer-term trend matters more than one record week.
Signals that you may be overdoing it
Numbers are only one part of the picture. Pay attention when several signals move away from your own history at the same time:
- Resting heart rate is clearly above your usual range for several mornings.
- Sleep has worsened; poorer sleep can be one signal that recovery is falling behind.
- Your legs feel heavy even after a rest day and easy runs no longer feel easy.
- Your desire to train is fading — a lasting loss of enthusiasm for running is useful training information too.
- Performance at a similar intensity is falling compared with your usual range.
How Pacero works with training load
In Pacero, fitness, fatigue and form are calculated deterministically from your training. They are not AI estimates, but metrics calculated from real training data. When creating the next week, Pacero considers training load alongside what was planned and what you actually completed. If load is rising too quickly, safety rules may lead to a more conservative next week or a recommendation to ease the current session.
Training load also contributes to today’s training decision. The Morning recommendation can combine it with sleep and resting heart rate, translating those signals into a simple answer: follow the plan, keep some reserve, choose an easier option or recover.