RPE and session-RPE: the science behind the simplest load metric

Few things in sport science are as cheap, as simple, and as underrated as one question after training: "How hard was that session, on a scale of 0–10?" From that single answer — multiplied by the session's duration — you can quantify each player's internal training load, track its accumulation, detect monotony in the load, and gauge the risk of overtraining and injury. This method is called session-RPE, and it is today one of the most researched and most widely used load metrics in world sport (Haddad et al., 2017). This article covers where the method comes from, how well it holds up to scientific scrutiny, how it relates to the "internal vs external load" framework, how monotony and strain are calculated, and — most importantly — how to collect and use RPE so that the numbers actually mean something.

The concept of perceived exertion (rating of perceived exertion, RPE) originates in the work of the Swedish psychologist Gunnar Borg. His foundational paper, Psychophysical bases of perceived exertion (Borg, 1982), advanced the idea that the subjective sense of effort can be measured in a reliable, quantitative way — that the body is, in effect, a single integrated sensor taking in signals from the muscles, heart, lungs and central nervous system and returning one overall number. Borg developed two scales: The Borg 6–20 scale — designed so that the number multiplied by 10 gives a rough estimate of heart rate (RPE 13 ≈ 130 bpm in a young person). The 6–20 range mirrors resting-to-maximal heart rate. The Category-Ratio 10 scale (CR10) — 0 to 10 (sometimes with an anchor above 10), which behaves like a ratio scale and is better suited to describing rising discomfort and breathlessness. It is the CR10 (or modified CR-10) that the session-RPE method uses. The validity of the Borg scale for both endurance and resistance exercise has repeatedly been shown to be high, though later analyses note that the accuracy depends on the conditions and on how the scale is presented (Borg, 1982; Haddad et al., 2017).

The leap from moment-to-moment RPE to a usable load metric for whole sessions came from Carl Foster and colleagues. In their paper A new approach to monitoring exercise training (Foster et al., 2001), they proposed a simple multiplication: session-RPE load (AU) = RPE (CR10, 0–10) × session duration (minutes) The number is expressed in arbitrary units (AU). A 60-minute session a player rates 7 gives 420 AU; a light 30-minute recovery rated 3 gives 90 AU. This is elegant because load is always both intensity and volume — a short hard session and a long easy one can produce the same load, and the method captures both in a single figure. Key point: RPE is taken for the session as a whole, not at any single instant, and ideally some time after it finishes (see section 6). Foster et al. (2001) showed that session-RPE tracked objective heart-rate measures well in both steady-state/interval exercise and basketball practice, even though the absolute numbers were not identical.

To understand what session-RPE measures, you need the distinction between external load and internal load (Impellizzeri, Marcora & Coutts, 2019): External load is the work the athlete performs — distance, high-speed running, number of sprints, lifts and kilos, accelerations/decelerations. This is measured by GPS, accelerometers and training logs. Internal load is the biological and psychological response to that work — heart rate, lactate, perceived exertion, fatigue. The same external load can produce very different internal loads in two players, or in the same player on a fresh day versus a tired one. session-RPE is a measure of internal load. That is its strength: it captures how the body actually experienced the session, not just what was on paper. Impellizzeri et al. (2019) stress that the best picture comes from reading internal and external load together — the external load tells you what was done, the internal load tells you how the player coped with it.

The method is not just convenient — it stands up to scientific scrutiny unusually well for such a simple tool. In soccer. Impellizzeri et al. (2004), in what became a landmark study, collected internal load from 479 training sessions and compared session-RPE against three heart-rate-based load methods. Every individual correlation was statistically significant, ranging from r = 0.50 to r = 0.85 (P < 0.01). The conclusion: session-RPE is a good indicator of the global internal load of soccer training — and it requires neither equipment nor cost. The review. In their systematic review, Haddad et al. (2017) surveyed the literature from 2001–2016. They found that 950 studies had cited the original Foster paper and that 36 studies had tested the validity and reliability of the method using the modified CR-10 scale. The verdict confirmed good validity, reliability and internal consistency of session-RPE across many sports and in both men and women. In resistance training the method also works (Day et al., 2004), with the caveat that here RPE reflects local muscular effort and discomfort more than central cardiorespiratory load — so numbers from the weight room must be interpreted in their own context.

A year before the session-RPE paper, Foster (1998) had already noted that the total volume of load alone does not tell the whole story — how the load is distributed across the week matters too. From daily session-RPE load he defined two quantities: Monotony = mean daily load ÷ standard deviation of daily load (across the week). High monotony means every day is similar — no real rest days, no contrast between hard and easy. Strain = weekly load (the sum) × monotony. This penalises the combination of high load and low variety. Foster linked high monotony and strain to increased rates of illness, injury and overtraining symptoms. The message is simple and powerful: it is not just how much you train, but how you distribute it — a week of high but monotonous load is riskier than a week of the same total load with clear hard/easy swings. More recent work supports dose–response relationships between these markers and fitness: in professional soccer players, changes in weekly load, monotony and strain were significantly associated with changes in maximal oxygen uptake, maximal aerobic speed (MAS) and strength over a 10-week period (Clemente et al. / Nikolaidis et al., 2019).

The method is only as good as the data going into it. Practical rules from the literature: 1. Use the CR10 scale (0–10) with standardised verbal anchors (0 = rest, 10 = maximal effort), and always use the same scale. 2. Ask ~30 minutes after the session. Studies show that RPE taken immediately (5–10 min) can be coloured too heavily by the last — especially a hard or easy — portion of the session; about 30 minutes later the number better reflects the session as a whole (Uchida et al.; Singh et al.). In some contexts (e.g. boxing) 10 min suffices, but 30 min is the safe standard. 3. Collect it privately. Players should not hear each other's answers — otherwise the numbers cluster together (social influence). 4. Multiply by actual duration of active training in minutes, not total elapsed time including long breaks. 5. Be consistent. The value lies in the trend over weeks and months, so the method must be identical every time.

RPE is subjective, and that is both its strength and its weakness. Haddad et al. (2017) summarise factors that can change the number without any change in external load: Exercise mode — interval and resistance training vs continuous endurance can give different RPE for the same internal load. Wellbeing, sleep and stress — a tired, poorly slept or stressed player rates the same session harder. (This is not "error" — it is real information about internal load.) Environment — heat, humidity and altitude raise RPE. Experience and personality — more experienced athletes and different personality types scale effort differently. Timing and wording of the question — see section 6. This underlines that RPE should be read against each player's own norm, not as a fixed between-person threshold.

Weekly load and trend. Accumulated session-RPE load per week shows whether a player is building, maintaining or accumulating fatigue. Acute:Chronic Workload Ratio (ACWR). session-RPE-based load is a common basis for the ACWR — the ratio of acute (e.g. 7-day) to chronic (e.g. 28-day) load — which has been linked to injury risk when it rises too fast (Gabbett, 2016; Hulin et al., 2016). It should be used as a reference, not a prediction. Subjective vs objective. The review by Saw et al. (2016) of 56 studies found that subjective self-reports (including RPE and wellness) reflected acute and chronic load with greater sensitivity and consistency than many objective tools — and that they carry independent information. session-RPE is therefore not "second-class" data when GPS is missing; it is a valid metric in its own right.

RPE is a single number and cannot separate where the fatigue comes from (central vs peripheral, metabolic vs mechanical). It is best read alongside objective load and wellness. In resistance training it reflects local effort more than cardiorespiratory load. It depends on the athlete's honesty and understanding — the scale must be taught and embedded in the routine. Cultural/linguistic differences in the scale's wording can matter; use a standardised translation.

session-RPE is a rare example of a tool that is at once cheap, simple and scientifically well established. From the Borg scale (Borg, 1982) and Foster's multiplication (Foster et al., 2001) you get a measure of internal load that tracks heart-rate-based methods well (Impellizzeri et al., 2004), survives systematic review (Haddad et al., 2017), and — through monotony and strain (Foster, 1998) — gives early warning of monotonous, risky load. The key to making the numbers mean something is discipline: the right scale, ~30 min after the session, privately, consistently, and always read against the player's own norm. Practical takeaways in brief: use CR10 × minutes; ask 30 min post-session and privately; track weekly load, monotony (keep it low with clear hard/easy days) and strain; and read RPE as a personal signal — not a fixed threshold.

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