Body Insights gives each valid Apple Watch workout a metabolic Training Stress Score (mTSS). Those session loads build 7-day Fatigue (ATL), 42-day Fitness (CTL), and Form (TSB).
The detailed iPhone view is called Muscle Load. Apple Watch uses Training Load. Both show how recent work compares with the load you built over time.
What the Muscle Load result shows
Muscle Load puts the slower 42-day trend beside the faster 7-day trend. The gap helps Body Insights set a training phase and give you a next step.
Fitness (CTL)
Your 42-day rolling training loadThe slower trend that represents the training base you have built.
Fatigue (ATL)
Your 7-day rolling training loadThe faster trend that rises after harder training and falls sooner with rest.
Form (TSB)
A positive or negative valueHow recent fatigue sits against longer-term fitness. Positive values mean more freshness. Negative values mean more recent fatigue.
Training phase and guidance
A phase label with a next actionHow your current load, form, and progression pace fit together.
Example using sample values
Suppose Fitness is 50, Fatigue is 58, and Form is -8. Recent training is above the longer trend. The result shows more fatigue than freshness.
After several easier days, Fatigue can fall faster than Fitness. Form then moves toward positive while the 42-day Fitness trend changes more slowly. Check the phase and guidance before you decide whether to add another hard session.
How one workout becomes training load
Body Insights keeps the session estimate and the rolling trends separate. Each step has its own job: session load first, rolling trends second.
- Apple Watch records the workout duration and heart rate. Apple Health stores the available workout record.
- Body Insights checks the heart rate data and maximum heart rate. Both are needed to estimate intensity.
- The app calculates mTSS for the session. When available, glucose and post-workout recovery can adjust that estimate.
- Daily session loads feed the 7-day ATL and 42-day CTL trends.
- Body Insights reads ATL, CTL, Form, and progression pace together. These values set the training phase and guidance.
How to use training load
Read Fitness, Fatigue, and Form together instead of chasing one ideal training load number.
Before another hard session
Check whether Fatigue has moved well above Fitness. Then read the phase and next action. A rising ATL can be part of a planned build, but several hard sessions can also outrun the slower CTL trend.
During an easier week
Fatigue reacts to rest sooner than Fitness. Watch ATL fall and Form move toward positive while CTL holds more of the training base you built.
While rebuilding consistency
Use progression pace and the 42-day Fitness trend. One large workout can lift Fatigue fast, but steady sessions are what move the longer trend.
When the result changes or has gaps
An unrecorded workout cannot add training load. If a workout lacks the heart rate data needed for its intensity estimate, Body Insights leaves it out. The app does not count it as an easy session.
A corrected workout can change current and past load. The app updates these values after a refresh. ATL reacts first because it uses seven days. CTL needs more history because it uses 42 days.
Recovery Readiness is a separate result. Apple Watch can show it beside Training Load when fresh data is available. It also uses overnight readiness and recent HRV, so it is not another name for ATL, CTL, or TSB.
Use Muscle Load to plan recorded training. Keep pain, symptoms, illness, and unrecorded activity as separate signals. See how the full Training Load system appears across Body Insights.
See your Muscle Load
Body Insights turns your Apple Watch workout history into Fitness, Fatigue, Form, and a clear training phase.
See my Muscle Load