The gap in most apps
A running app only sees your running. A cycling app only sees your cycling. But if you run, ride, swim and lift, your fatigue is the sum of all of it — and no one is looking at the whole picture. The result: you look "fresh" in each app taken separately, while your legs are absorbing running and cycling.
Fatigue per muscle group
Baseload spreads each session's load across the muscle groups it actually loads. Every discipline has its own signature:
| Discipline | Mainly loads |
|---|---|
| Running, trail | The lower body (legs), and the core a little. |
| Cycling | The lower body, more moderately; the core a little. |
| Swimming | The upper body (pull, push) and the core. |
| Strength | The groups worked, depending on the session. |
As a result, two athletes at the same weekly volume can have completely different fatigue profiles. A triathlete who strings together swimming and cycling spares their legs for running; a pure runner pounds them every day.
Each group recovers at its own pace
Fatigue doesn't clear all at once: it decays over time, and not at the same speed for every group. Baseload models this decay with differentiated "half-lives":
| Muscle group | Recovers… |
|---|---|
| Legs (lower body) | Slowly (~72 h) — the long fibres absorb both concentric and eccentric work. |
| Upper body | At an intermediate pace (~48 h). |
| Core | Fast (~36 h). |
Because it sees fatigue per group, the coach can propose a genuine load shift: your legs are cooked but your upper body is fresh? It suggests a swim rather than an easy run. You keep training without piling more onto the group that's spent. That's what training smart in multi-sport means.
Every sport counts
Running, cycling, swimming, trail, triathlon, but also walking, hiking, strength — and even a sport HealthKit doesn't know about, one you declare yourself (climbing, wakeboard…). Baseload folds it into your overall load instead of ignoring it.
The split of load across muscle groups rests on expert coefficients, calibrated empirically — not on an individual measurement of your muscle activation. They give a reliable order of magnitude, not a truth down to the muscle. You can refine them by rating your sessions. See Our model, its limits.