Reference · train by feel
RPE Scale: Train Without a Device
Rate of Perceived Exertion (RPE) gauges intensity by feel. Borg’s original scale runs 6–20 and loosely tracks heart rate ×10. CR10 runs 0–10. CR10 3–4 (Borg 10–12) is easy Zone 2. CR10 7–8 (Borg 15–16) is threshold Zone 4.
| 5-Zone | CR10 | Borg 6–20 | How it feels | Polarized |
|---|---|---|---|---|
| Z1 · Recovery | 1–2 | 6–9 | Very easy, full conversation | Zone 1 (easy) |
| Z2 · Endurance | 3–4 | 10–12 | Easy, can talk in sentences | Zone 1 (easy) |
| Z3 · Tempo | 5–6 | 13–14 | Moderate, short sentences | Zone 2 (gray) |
| Z4 · Threshold | 7–8 | 15–16 | Hard, a few words only | Zone 3 (hard) |
| Z5 · VO₂max+ | 9–10 | 17–20 | Very hard to maximal, no talking | Zone 3 (hard) |
Borg 6–20 vs CR10
Borg’s original scale runs 6 to 20 so that multiplying by ten roughly gives heart rate for a healthy adult (RPE 13 ≈ 130 bpm). The later CR10 scale runs 0–10 and suits non-linear sensations like breathlessness and muscle burn.
Why RPE still matters with a device
Heart rate lags and drifts in heat. Power and pace do not show today's fatigue. RPE fills that gap. Strength training and CrossFit have no heart-rate anchor, so RPE and velocity-based training are the primary gauges there.