Response

Dear Editor-in-Chief: We are pleased to expand our discussion about the "central governor model." Nowhere does our paper suggest that an incremental exercise test "always" produces the highest V˙O2max, and we do not argue that point. Contrary to Dr. Noakes' contention, Figure 2 shows each of the 156 individual measurements we made, along with the mean and 95% confidence limits. The individual lines are difficult to distinguish because the two measurements overlap. The typical error of measurement (6) for the incremental versus supramaximal test was 4.1%, well within the variability reported by others (9,11). We are not sure what Dr. Noakes means by "the same" V˙O2max for the two techniques. However, using a definition of "the same" as within the typical error of the test, then about half of the test pairs were the same. Of the half that were not, about three fourths had the incremental test value greater than the supramaximal, and about one fourth had the supramaximal test greater than the incremental test. Dr. Noakes espouses that a "central governor" shuts down the body to avoid a "disturbance of homeostasis," and during incremental exercise, the highest V˙O2 achieved does not reflect true maximal oxygen transport ability. Our study unequivocally validates the V˙O2max concept; that is, that a finite rate of oxygen uptake exists, beyond which increasing work rate does not increase oxygen uptake. This actual finite point can be challenging to demonstrate. However, in our study, the subjects sustained a very large amount of extra work-more than 30% greater than that required to support such work rates oxidatively; yet, V˙O2max was rarely higher, and never substantively so, than on the incremental test. Thus, during the incremental test, no "central governor" stopped the test before V˙O2max to avoid ischemia or homeostatic disturbance, because on a separate effort, subjects were capable of exercising at much higher external work rate without ischemia. Others have reported similar data (2). Moreover, patients with ischemic heart disease regularly are stressed beyond myocardial ischemia (3), but they continue to exercise-their "central governors" must fail all the time! Dr. Noakes seems to confuse "performance" with maximal oxygen uptake. V˙O2max is a characteristic bounded by the Fick equation: (left ventricular end diastolic volume − left ventricular end systolic volume) × heart rate × arteriovenous oxygen difference. It can be acutely altered by removing pericardial constraint (12), slowing heart rate (7), or adding or removing blood (5). An athlete with a V˙O2max of 80 mL·kg−1·min−1 can run 5000 m faster than a athlete with a V˙O2max of 40 mL·kg−1·min−1, so this characteristic is clearly tied to endurance performance (4). Of course, this is not the only characteristic influencing how fast athletes run. Sport-specific economy, anaerobic capacity, fuel utilization, and the speed or oxygen uptake at the maximal steady state all contribute to running performance (8). Athletes routinely generate power outputs above those eliciting V˙O2max by activation of more motor units. All models of exercise performance accept and embrace this fact (1). Some confusion about our paper may be due to the definition of "supramaximal." In this context, we mean a power output and energy expenditure greater than what can be sustained via oxidative ATP production as described by Medbo et al. (10). That our athletes could generate more force, but not more oxygen uptake, is strong evidence for a maximal oxygen uptake that is not exceeded by greater voluntary motor recruitment. Ultimately, the key message of our study is that despite great effort and additional motor unit recruitment and force production, our athletes did not take up more oxygen on the supramaximal test than on the incremental test. Benjamine D. Levine Institute for Exercise and Environmental Medicine Presbyterian Hospital of Dallas University of Texas Southwestern Medical Center Dallas, TX Megan Hawkins Department of Integrative Physiology University of North Texas Health Science Center Fort Worth, TX James Stray-Gundersen Institute for Exercise and Environmental Medicine Presbyterian Hospital of Dallas University of Texas Southwestern Medical Center Dallas, TX Peter B.