Linear relationship between the perception of effort and the duration of constant load exercise that remains
2004; American Physiological Society; Volume: 96; Issue: 4 Linguagem: Inglês
10.1152/japplphysiol.01124.2003
ISSN8750-7587
Autores Tópico(s)Cardiovascular and exercise physiology
ResumoLETTERS TO THE EDITORLinear relationship between the perception of effort and the duration of constant load exercise that remainsTimothy David NoakesTimothy David NoakesDiscovery Health Chair of Exercise and Sports Science and MRC/UCT Research Unit for Exercise Science and Sports Medicine University of Cape Town Sports Science Institute of South Africa Newlands 7700, South Africa E-mail: .Published Online:01 Apr 2004https://doi.org/10.1152/japplphysiol.01124.2003MoreSectionsPDF (62 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat To the Editor: Perhaps the study of Baldwin et al. (1) finally establishes that muscle glycogen depletion does not cause fatigue within the muscle by reducing the available energy below a critical amount. This study also proves the logical error in the statement of Conlee (2): “When glycogen runs out... the muscle fails from lack of aerobic adenosine triphosphate (ATP) production. However, plausible and attractive this theory is, it is unproven.... What is clear is that, in glycogen-depleted muscle, ATP is being used up faster than it can be manufactured, and so force output is diminished.” If this logic were correct, then muscle glycogen depletion would lead not to fatigue but to muscle rigor. That is, if the rate of ATP production falls below the rate of ATP use, then there must be a progressive reduction in muscle ATP concentrations leading to skeletal muscle rigor if no other safety mechanism exists.This logical error, in what has been termed the “energy depletion model of exercise fatigue,” has been emphasized previously (5). The essential contribution of Baldwin et al. (1) is to show that some other control mechanism must be present to ensure ATP and total adenine nucleotide pool (TAN) homeostasis during prolonged exercise, as shown by a number of other studies to which the authors refer. Left unanswered is the nature of that control process. However, the authors' data do provide an enticing, novel hint of what that control might be.Baldwin et al. (1) included data for ratings of perceived exertion (RPE) reported by the subjects during two exercise bouts in which subjects began in either the carbohydrate-replete or carbohydrate-depleted state. The authors do not fully discuss why they chose either to measure or to report these data; they only note that the RPE values at fatigue were identical, despite the exercise lasting for 34% longer (52 min) in the carbohydrate-replete state. It should be noted that, if fatigue is regulated purely by a peripheral control mechanism in the exercising muscles, regardless of its chemical or other nature, it makes no sense for the brain to be informed and hence to perceive that the exercise is becoming progressively more difficult (8). The brain is singularly unable to influence the outcome because it cannot, according to the usual model, overcome the effects of these peripherally acting metabolic events (8).In contrast, if performance during prolonged exercise is ultimately regulated centrally in the brain (3-9), specifically to ensure that muscle energy homeostasis is maintained and that a critical energy depletion does not occur (5), then the RPE may play some role in that homeostatic process. For example, the rate of increase in RPE may reflect the rate at which crucial fuel reserves are being depleted and hence the duration of the exercise bout that can still be sustained at that particular exercise intensity without threatening homeostasis.To evaluate this possibility, I plotted the data for RPE reported by Baldwin et al. (1) against 1) the duration of exercise in the two different conditions (Fig. 1A) and 2) the percentage of time that had been completed, again in both exercise conditions (Fig. 1B). The hypothesis is that, if the terminal RPE is the same, then perhaps the rate at which the RPE rises may provide information of how close to the end of exercise the athlete is at any time during the exercise bout. Because manipulation of the size of the starting body carbohydrate stores produced a 34% difference in exercise duration, it seemed probable that a sufficient range of data would be available to evaluate this hypothesis when all data were expressed relative to the percentage of the total exercise bout that had been completed (or remained). Fig. 1.A: plot of ratings of perceived exertion (RPE) against absolute exercise duration (minutes) shows that, regardless of whether exercise begins with low or high muscle glycogen content, there is a linear increase in RPE with exercise duration. However, the rate of rise of RPE is faster during exercise that begins with low muscle glycogen content (carbohydrate-deplete exercise). Note also that exercise terminates at the same RPE in both exercise conditions. B: when the RPE for both exercise conditions is plotted against the % of total exercise time (completed or remaining), the data fall along the same line. These data suggest that 1) the maximal RPE that can be sustained determines the time at which the decision is made to terminate exercise and 2) the rate at which the RPE increases may determine the duration of exercise specifically to ensure that total energy depletion does not occur in the active muscles during more prolonged exercise. [Data plotted from the relevant information in Table 1 of Baldwin et al. (1).]Download figureDownload PowerPoint Accordingly, Fig. 1A shows that there is a linear relationship between the RPE and the duration of exercise in carbohydrate-depleted and carbohydrate-replete conditions. 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.Figure 1B confirms this possibility by showing that the linear relationship between RPE and the duration of exercise overlaps for both conditions when the exercise duration is expressed as a percentage of the total exercise time. To my knowledge, this relationship may not have been noticed previously.One possible interpretation is that the RPE is the real determinant of the fatigue point in this model of exercise, that is, prolonged submaximal exercise at a fixed work rate. This is in keeping with the theory first proposed in the 1930s when it was found that amphetamines, which reduce or abolish the sensations of fatigue, enhance exercise performance (3). Thus it was proposed that exercise terminates when the feelings of discomfort overwhelm the potential rewards of continuing to exercise (3, 4). According to this interpretation, in the study of Baldwin et al. (1), the motivation to continue exercise stopped when a mean RPE of 18.1 units was reached in both conditions.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 absolute whole body energy depletion. Anticipating the maximal RPE that it (or the individual) will tolerate, the brain center responsible for the generation of the RPE then increases that RPE as a function of the percentage of that total exercise time that has been completed (or the percentage of time that remains). Alternatively, some signaling energy substrate, increased by a high-carbohydrate diet, may decline as a linear function of exercise duration, resulting in a linear increase in RPE. The maximum RPE is then reached before complete depletion of that energy substrate.Because similar RPE ratings at exhaustion occurred even though muscle glycogen contents were significantly different and were higher in the carbohydrate-replete trial, muscle glycogen content could not have been the exclusive determinant of the RPE in this study. Other sensory variables that also clearly did not contribute to this linear increase in RPE with increasing exercise duration were heart rate and oxygen consumption, which were little changed during exercise.The pivotal importance of the study by Baldwin et al. (1) is that it allows us finally to close the chapter on the concept that a state of absolute energy depletion is ever reached during prolonged exercise, just as it also does not occur during any other form of voluntary exercise (5). The challenge now is to understand how the body anticipates the total duration of the exercise bout that is to be performed and how the increase in the perception of effort is regulated to ensure that skeletal muscle energy homeostasis is not sufficiently disturbed to produce local tissue damage, in particular, the development of skeletal muscle rigor.GRANTSThe author's research relevant to this topic is supported by the University of Cape Town, the Medical Research Council of South Africa, Discovery Health Pty Ltd, and the National Research Foundation of South Africa through the Technology and Human Resources for Industry Programme initiative. References 1 Baldwin J, Snow RJ, Gibala MJ, Garnham A, Howarth K, and Febbraio MA. Glycogen availability does not affect the TCA cycle or TAN pools during prolonged, fatiguing exercise. J Appl Physiol 94: 2181-2187, 2003.Link | ISI | Google Scholar2 Conlee RK. Muscle glycogen and exercise endurance: a twenty-year perspective. Exerc Sport Sci Rev 15: 1-28, 1987.Crossref | PubMed | ISI | Google Scholar3 Lehmann G, Straub H, and Szakall A. Pervitin als leistungssteigerndes Mittel [Pervitin as an ergogenic aid during exercise]. Arbeitsphysiol 10: 680-691, 1939.Google Scholar4 Ikai M and Steinhaus AH. Some factors modifying the expression of human strength. J Appl Physiol 16: 157-163, 1961.Link | ISI | Google Scholar5 Noakes TD. Physiological models to understand exercise fatigue and the adaptations that predict or enhance athletic performance. Scand J Med Sci Sports 10: 123-145, 2000.Crossref | PubMed | ISI | Google Scholar6 Noakes TD. Commentary to accompany: training and bioenergetic characteristics in elite male and female Kenyan runners. Med Sci Sports Exerc 35: 305-306, 2003.Crossref | Google Scholar7 Noakes TD, Peltonen JE, and Rusko HK. Evidence that a central governor regulates exercise performance during acute hypoxia and hyperoxia. J Exp Biol 204: 3325-3234, 2001.Google Scholar8 St Clair Gibson A, Baden DA, Lambert MI, Lambert EV, Harley YXR, Hampson D, Russell V, and Noakes TD. The conscious perception of the sensation of fatigue. Sports Med 33: 167-176, 2003.Crossref | ISI | Google Scholar9 St Clair Gibson A, Schabort EJ, and Noakes TD. Reduced neuromuscular activity and force generation during prolonged cycling. Am J Physiol Regul Integr Comp Physiol 281: R187-R196, 2001.Link | ISI | Google ScholarjapJ Appl PhysiolJournal of Applied PhysiologyJ Appl Physiol8750-75871522-1601American Physiological SocietyREPLYRodney J. Snow, and School of Health Sciences Deakin University Burwood 3125, Victoria, AustraliaMark A. FebbraioSkeletal Muscle Research Laboratory School of Medical Sciences RMIT University Bundoora 3083, Victoria, Australia E-mail: [email protected]42004To the Editor: Our paper (1) focused on the effect of muscle glycogen availability on skeletal muscle energy metabolism and fatigue. As Dr. Noakes indicated, our data do not support the theory that fatigue during prolonged cycling exercise at a constant work rate was caused by an energy crisis within the contracting muscle. However, it should be noted, which we also stated in the paper, that, “We were unable to determine the precise cause(s) of fatigue during prolonged exercise, but this was not the aim of the study.” In particular, our measurement of muscle ATP content only provides information on the average mixed-muscle ATP content but does not elucidate the ATP levels at critical cellular sites within the various fiber types that may be critical in the fatigue process (6). Indeed, in a previous study (4), our group also demonstrated no reduction in the total adenine nucleotide pool during prolonged fatiguing exercise but showed a strong correlation between exercise duration and glycogen use, albeit in a limited number of subjects. We argued, therefore, that the relationship between glycogen content and exercise duration suggested that the maintenance of contractile force is possibly dependent on glycogen availability. Studies in both animals (3, 7) and humans (2) have suggested a link between sarcoplasmic reticulum Ca2+ uptake and release and glycogen availability. In addition, topographical localization of glycogen within human skeletal muscle has been observed (5). Therefore, depletion of glycogen in the sarcoplasmic reticulum may possibly lead to a failure of contractile force, although further research in this area is warranted. Hence, based on our present knowledge and techniques, we cannot definitively eliminate the possibility that the fatigue during prolonged exercise is due to a muscle energy crisis, although we agree that this is unlikely. In support of Dr. Noakes' theory on the cause of fatigue, we speculated that the fatigue may have a nervous system component, noting that fatigue was also associated with hypoglycemia. The theory that the onset of fatigue is associated with the perception of effort is extremely interesting and is entirely consistent with our experimental data, as reanalyzed by Dr. Noakes. We eagerly await further research designed to test this hypothesis. 1. Baldwin J, Snow RJ, Gibala MJ, Garnham A, Howarth K, and Febbraio MA. Glycogen availability does not affect the TCA cycle or TAN pools during prolonged, fatiguing exercise. J Appl Physiol 94: 2181-2187, 2003. Link | ISI | Google Scholar2. Booth J, McKenna MJ, Ruell PA, Gwinn TH, Davis GM, Thompson MW, Harmer AR, Hunter SK, and Sutton JR. Impaired calcium pump function does not slow relaxation in human skeletal muscle following prolonged exercise. J Appl Physiol 83: 511-521, 1997. Link | ISI | Google Scholar3. Chin ER and Allen DG. Effects of reduced muscle glycogen concentration on force, Ca2+ release and contractile protein function in intact mouse skeletal muscle. J Physiol 498: 17-29, 1997. Crossref | PubMed | ISI | Google Scholar4. Febbraio MA and Dancey J. Skeletal muscle energy metabolism during prolonged, fatiguing exercise. J Appl Physiol 87: 2341-2347, 2003. Google Scholar5. Friden J, Sege J, and Eckblom B. Topographical localization of muscle glycogen: an ultrahistochemical study in the human vastus lateralis. Acta Physiol Scand 135: 381-391, 1989. Crossref | PubMed | Google Scholar6. Korge P and Campbell KB. The importance of ATPase microenvironment in muscle fatigue: a hypothesis. Int J Sports Med 16: 172-179, 1995. Crossref | PubMed | ISI | Google Scholar7. Stephenson DG, Nguyen LT, and Stephenson GM. Glycogen content and excitation-contraction coupling in mechanically skinned muscle fibres of the cane toad. J Physiol 519: 177-187, 1999. 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