24 April 2026
Fit, but for what?
One month down a sports-science rabbit hole — and how it changed the way we track fitness, fatigue and form.
All models are wrong
"All models are wrong, but some of them are useful." Weather forecasting doesn't simulate every molecule of the atmosphere — but it's useful enough to tell you whether to bring a jacket. The question isn't whether a training model is correct. It isn't. It's whether it's useful enough to help you make better decisions than guessing.
The popular models
Most training models descend from Banister's 1975 impulse-response paper — a training-load number feeding a fast rolling average (fatigue) and a slow one (fitness). Coggan's Performance Management Chart (TSS/CTL/ATL/TSB) is the dominant implementation. But all TSS is treated as equal: a 5-hour easy ride and a 90-minute interval session can score the same load while training completely different systems.
So we built a two-dimensional model
BEINA breaks every session's stress across two dimensions: Power (everything above the second lactate threshold) and Endurance (everything below it). The split is grounded in Kontro et al.'s three-dimensional energy-system model, simplified to what's actually useful. You're not just 'fit' — you're fit for something.
Same stress, different stimulus
A 1.5hr VO2max session and a 4.9hr endurance ride can both score 120 TSS — but one is 84% Power, the other 86% Endurance. Power trends on a 28-day average (fast to build, fast to fade); Endurance on 42 days (slow to build, sticks around). That's the whole point.
Durability, power curves and fatigue
Hour 20 of a Strathpuffer lap rotation is nothing like hour two. We surface durability via aerobic decoupling — how your HR-to-power holds up late in long efforts — plus a rolling 90-day power curve we actively celebrate when you set a new peak. And there's no single fatigue score: the Coach pieces it together from training, HRV, sleep, RPE and your journal, in plain language.
Slightly less wrong, slightly more useful
Still a model. Just a more useful one. It stays grounded in published science, but surfaces what actually helps you make decisions — and feeds straight into a conversation with the Coach, not a festival of graphs.