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Rating of perceived exertion as a predictor of the duration of exercise that remains until exhaustion
  1. T D Noakes
  1. T D Noakes, Research Unit for Exercise Science and Sports Medicine, University of Cape Town, Sports Science of South Africa, South Africa; tdnoakes{at}

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In an earlier letter in Journal of Applied Physiology,1 I showed that in subjects exercising at a fixed work rate while either carbohydrate-replete or partially carbohydrate-depleted,2 the rating of perceived exertion (RPE) rises as a linear function of the duration of exercise that remains. At the time, I assumed that this was an entirely novel observation.

However, while searching the publication record of Dr Allen Cymerman, currently of the US Army Research Institute of Environmental Medicine, my attention was drawn to a paper coauthored by Dr Cymerman and published in 1979, entitled “Perception of effort during constant work to self-imposed exhaustion”.3

In that study, subjects walked or ran at 80% of maximum oxygen consumption (VO2max) to “self-imposed exhaustion”. Exhaustion occurred at the same RPE during both prolonged submaximal exercise and during maximum exercise (for measurement of VO2max). During prolonged submaximal exercise to exhaustion, RPE rose as a linear function of the percentage of total exercise duration3 (fig 1 of that paper, page 1117). In addition, the linear increases in RPE “occurring early during work were a sensitive predictor of exhaustion time”. Thus, the authors concluded that: “Psychophysical judgements made early during work were reasonably accurate predictors of exhaustion time”.

Compare this 1979 conclusion with my 2004 conclusion: “Hence, the rate at which the RPE increases could indeed serve as a marker of the time left to exhaustion during exercise at a constant workload” (page 1571) and that “These findings are compatible with a number of hypotheses, including the following. At the onset of exercise, on the basis of some carbohydrate signal that is increased after a carbohydrate-enriched diet, the subconscious brain calculates the anticipated duration of the exercise that can be safely sustained without causing whole body energy depletion. Anticipating the maximal RPE that it (or the individual) will tolerate, the brain centre responsible for the generation of the RPE then increases the RPE as a function of the percentage of the total exercise time that has been completed (or the percentage of time that remains)”2 (pages 1571–1572).

Horstman et al.3 also refer to an even earlier paper by Morgan and Borg4 in which RPE was measured during progressive maximum exercise to exhaustion for measurement of VO2max. The authors reported that: “the rating of perceived exertion was found to increase in a linear fashion as work intensity increased . The linear function permits extrapolation to a theoretical end point, which in turn makes possible the prediction of maximal work capacity” (page 129). Horstman et al.3 therefore concluded that Morgan and Borg4 had already established in 1976 that the RPE is “a good predictor of the point at which an individual will discontinue work” (page 1112).

None of the quoted authors is aware of this letter and none asked that it be written. However, it is important that the appropriate priority for this idea should be properly acknowledged.

The importance of these findings is that they are compatible with the theory that there is an “anticipatory” (feed-forward) control of exercise performance that is activated early in exercise,5 as probably first conceived by Ulmer.6 The presence of this anticipatory component is not often considered in the modern teachings of exercise physiology, even in those models that acknowledge that the neuromuscular system plays an important role in the regulation/limitation of human exercise performance.7

Interestingly, Morgan and Borg4 perhaps overlooked the importance of their finding, concluding rather that RPE “represent a Gestalt or configuration of total bodily inputs” so that on the basis of “factors such as heart rate, oxygen uptake, ventilatory minute volume, carbon dioxide production, lactate accumulation, cathecolamine production, body temperature and aches and pains in the working muscles and joints”, “normal individuals are capable of accurately rating the intensity of work being performed” (page 127). Which, if any, of these variables determines the RPE remains uncertain.8

More to the point, the finding that the RPE rises even when exercise continues at the same exercise intensity and during which many of these variables remain relatively constant2 does not offer much hope that any or all determine the RPE. On the other hand, exercise at higher intensities can be sustained for shorter durations. Hence, according to the theory that the RPE is a measure of the duration of exercise that remains (or that can be sustained at that exercise intensity), it will be higher during exercise at higher intensities and for which the possible exercise duration will be shorter.

Thus the original finding of Horstman et al,3 confirmed by the data of Baldwin et al2 and several recent studies,913 all of which show that the RPE increases as a linear function of the duration of exercise that remains, even when the exercise intensity is unchanged, suggests that humans have an exquisite capacity to predict accurately the duration of exercise they will be able to sustain at any exercise intensity. Alternatively, it may be that the brain chooses the exercise duration even before the exercise begins, and then uses the RPE to inform the body when the exercise will and must terminate.



  • Competing interests: None declared.

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