Relative Effort (Strava)

Relative Effort is Strava's training-load score for a single activity — a heart-rate-based number that estimates how hard a session taxed you from how much time you spent in each HR zone. It's the rebranded successor to Strava's old 'Suffer Score,' and it rolls up into Strava's Fitness & Freshness chart (their take on CTL/ATL). Because it's built from heart rate, it is not directly comparable to power-based TSS.

Relative Effort ≈ time-in-HR-zone weighted by zone intensity (a TRIMP-style score)
    higher HR zones are weighted more heavily
    HR-based — NOT interchangeable with power-based TSS

Relative Effort is a TRIMP-style metric (training impulse): it weights the time you spend in each heart-rate zone, counting minutes in higher zones more heavily, to produce one load number per activity. Strava introduced it as the more approachable, heart-rate-driven replacement for the Suffer Score, and it feeds their Fitness (a chronic average) and Freshness (form) curves — Strava's own rendering of the CTL/ATL/TSB model.

The reason it isn't comparable to TSS is that they measure different things. TSS is computed from power relative to FTP; Relative Effort is computed from heart rate relative to your HR zones. Heart rate lags effort, drifts with heat, fatigue, caffeine, and sleep, and can't see very short high-power surges — so the same ride can produce quite different Relative Effort and TSS values. Summing the two, or comparing your Relative Effort to a friend's TSS target, is an apples-to-oranges error: each is internally consistent, but they're different currencies.

Relative Effort earns its keep when you don't have a power meter — it's a legitimate, heart-rate-grounded way to quantify and trend training load, and for many riders it's the only load metric they have. Its limits are heart rate's limits: it under-rates short, punchy efforts and gets thrown off by anything that moves HR independently of effort. As a within-athlete trend it's genuinely useful; as a cross-athlete or cross-metric absolute, it isn't.