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J Appl Physiol 104: 278-279, 2008; doi:10.1152/japplphysiol.00595.2007b
8750-7587/08 $8.00
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POINT-COUNTERPOINT

Rebuttal from Dr. González-Alonso

Plato's Myth of the Cave came to mind when reading my opponents' argument: "... the SV of endurance-trained athletes is maintained and generally increased throughout incremental to maximal exercise." Have my esteemed opponents been looking at "the shadow" or "the true" cardiac response to maximal exercise? In the context of this debate, Dr. Warburton and Dr. Gledhill argue that maximal SV and thus Q is achieved in healthy individuals at the point of fatigue based primarily on their data showing a continuous increase in SV from ~50 ml/beat at a resting heart rate of ~60 beats/min to ~150 ml/beat (range ~100–200 ml/beat) at a heart rate of ~190 beats/min (Fig. 1; 4, 5, 8, 10). A thorough examination of the experimental protocol is warranted to determine the validity of these findings.

My opponents repeatedly determined cardiac function during the last ~2 min of each ~4–6 min exercise stage in incremental exercise tests using protocols that matched subjects for heart rate, rather than VO2 (4, 5, 8, 10). During the maximal workload, however, Q was determined early in exercise to avoid fatigue, limiting the ability to complete the acetylene-rebreathing maneuver. The lower maximal a-vO2 difference in their studies (5, 8, 10) compared with others (Refs. 1, 6, 7, 9; 138–155 vs. 170–180 ml/l, respectively) and the 3–4 l/min higher Q in trained compared with untrained subjects at the untrained VO2max (5) suggest that Q was indeed measured before fatigue and/or Qmax was overestimated. Thus a major limitation of these and other studies in the literature to answer the debated question is that Q is not measured in the few seconds before exhaustion when found to be blunted (6, 7, 9).

Another explanation for the ever-increasing SV during incremental exercise is that the heart rate-matched protocol elevates heart rate values for a given VO2 (3). Incidentally, the heart rate and SV data up to 50% VO2max of Gledhill et al. (5) agree closely with the findings of Mortensen et al. (9), who used a protocol including a warm-up period that elevated the initial heart rate to ~90 beats/min. Hence, the pattern of the SV response during incremental exercise is greatly dependent on the level of heart rate (2, 3). In summary, my opponents' argument that "the human heart is able to maintain and even increase SV during maximal exercise" lacks "the true" maximal measures of SV and is confounded by the elevated heart rate for a given VO2.

REFERENCES

  1. Åstrand PO, Cuddy TE, Saltin B, Stenberg J. Cardiac output during submaximal and maximal work. J Appl Physiol 19: 268–274, 1964.[Abstract/Free Full Text]
  2. Boushel R, Calbet JAL, Rådegran G, Søndergaard H, Wagner PD, Saltin B. Parasympathetic neural activity accounts for the lowering of exercise heart rate at high altitude. Circulation 104: 1785–1791, 2001.[Abstract/Free Full Text]
  3. Ekblom B, Ekblom O. Stroke volume and the endurance athlete. Scand J Med Sci Sports 16: 70–71, 2006.[CrossRef][Web of Science][Medline]
  4. Ferguson S, Gledhill N, Jammik VK, Wiebe C, Payne N. Cardiac performance in endurance-trained and moderately active young women. Med Sci Sports Exerc 33: 1114–1119, 2001.
  5. Gledhill N, Cox D, Jammik R. Endurance athlete's stroke volume does not plateau: major advantage is diastolic function. Med Sci Sports Exerc 26: 1116–1121, 1994.
  6. González-Alonso J, Calbet JA. Reductions in systemic and skeletal muscle blood flow and oxygen delivery limit maximal aerobic capacity in humans. Circulation 107: 824–830, 2003.[Abstract/Free Full Text]
  7. González-Alonso J, Dalsgaard MK, Osada T, Volianitis S, Dawson EA, Yoshiga CC, Secher NH. Brain and central haemodynamics and oxygenation during maximal exercise in humans. J Physiol 557: 331–342, 2004.[Abstract/Free Full Text]
  8. Krip B, Gledhill N, Jammik V, Warburton D. Effect of alterations in blood volume on cardiac function during maximal exercise. Med Sci Sports Exerc 29: 1469–1364, 2004.
  9. Mortensen SP, Dawson EA, Yoshiga CC, Dalsgaard MK, Damsgaard R, Secher NH, González-Alonso J. Limitations to systemic and locomotor limb muscle oxygen delivery and uptake during maximal exercise in humans. J Physiol 566: 273–285, 2005.[Abstract/Free Full Text]
  10. Warburton DER, Gledhill N. Counterpoint: Stroke volume does not decline during exercise at maximal effort in healthy individuals. J Appl Physiol; doi:10.1152/japplphysiol.00595.2007a.




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