In one sentence
A 1-to-10 answer to "how hard was that?", from 1, barely an effort, to 10, all out, which is the one measure of training that needs no device and that every session has.
Why it matters
Effort is the thing a heart-rate monitor approximates and a questionnaire can simply ask. Gunnar Borg built the scale so that a person's rating of exertion would track the physiological strain of work, and the CR10 version combines the simplicity of a category scale with ratio properties, so that a 6 is about twice a 3 (Borg 1982). Multiplied by the minutes, the rating gives the session-RPE load, and in Foster's validation the session-RPE load tracked a heart-rate-based load consistently across steady cycling, intervals and basketball practice, with the two regression lines from different groups nearly overlapping (Foster 2001). That is why VT uses it for everything a training load feeds: the TDEE engine's activity term, the load side of Capacity, and the training confounder for Symptom Radar.
Why CR10 and not 6 to 20
Borg's original scale runs from 6 to 20 and was built to track heart rate; the CR10 runs from 0 to 10 with verbal anchors and ratio spacing. They are different scales with different anchors, and mixing them breaks the arithmetic, so VT uses one: the CR10's range, asked as a whole number from 1 to 10. If a coach gives you a 6-to-20 number, do not type it in.
How to answer well
Rate the whole session, about half an hour after it, not the hardest minute; a session with an easy warm-up and a brutal last interval is rated as the whole. Use VT's anchors: 1 to 3 light, 4 to 6 moderate, 7 and 8 hard, 9 and 10 all out; a stroll where you could sing is about 2, comfortably hard is 5, and 8 is where full sentences stop. Rate the day as well when VT asks (the daily Borg), which captures the days that were hard without a session.
How Vitality Tracker uses it
Session-RPE load = rating × minutes, for any session, with or without a device; summed per day and across the week it is the training load behind the TDEE engine's activity curve, the Capacity load side and the Symptom Radar confounder, which is measured against your own training goal. Each session's rating is stored with the session, and a session marked as novel is read as a larger load than its number suggests.
How far to trust the number
Self-reported, and reliable within a person who uses the anchors consistently; it is the measure the research validated for exactly this use.
Sources
- Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exerc 1982;14:377–81 (PMID 7154893)
- Foster C, Florhaug JA, Franklin J, et al. A new approach to monitoring exercise training. J Strength Cond Res 2001;15:109–15 (PMID 11708692)
- Vitality Tracker, What VT measures (catalogue of 15 October 2026).