The coach that shows its work.

When your 7-day heart-rate-variability trend drops, timing hard sessions by HRV beats sticking to a fixed plan.

When your recent training load spikes well above your longer-term average, injury risk appears to rise — so the engine suggests backing off slightly.

A night under about six hours measurably dents next-day endurance — so a hard session after short sleep costs more than it builds.

A resting heart rate well above your own baseline can be an early sign of accumulated fatigue or illness — but one reading proves nothing on its own.

Midsole foam stiffens and loses energy return as miles accumulate, raising impact forces — but how fast depends on the shoe, so a universal 500-mile rule is only a rough guide.

Runners who rotate two or more pairs had about 39% lower injury risk in a 264-runner cohort — likely because foam recovers with rest and different shoes vary the load on your legs.