SPECIAL COMMUNICATIONS: Letters to the Editor-in-Chief
In their recent article, Boone et al. (1) studied trained versus untrained cyclists and expanded the dimensions of ramp exercise and V˙O2 kinetics research by adding iEMG analysis to their protocol. This is an interesting and welcome approach. However, it is difficult to accept the authors' interpretation that higher iEMG and V˙O2 responses mean that "…trained cyclists demonstrate reduced mechanical efficiency in the ramp protocol…" (abstract, p. 402).
Intuitively, the notion that long-term endurance training would lead to inferior rather than superior muscle efficiency is hard to swallow. More importantly, the authors' interpretation seems to have overlooked key information stemming from their own and previously cited data.
Why is high ΔV˙O2/ΔW ratio in ramp testing associated with low mechanical efficiency? Endurance athletes have higher capillary density and cardiovascular and muscle oxidative capacities, and their V˙O2 response should, therefore, be expected to be faster than that of untrained controls (5). If at all, this is positively related to mechanical efficiency and cycling economy. At identical workloads (Fig. 2, p. 405), the representative athlete's V˙O2 was ≥10% lower than that of the nonathlete.
The exact reason for the cyclists' higher iEMG response in the ramped but not in the step protocol can be debated. However, because becoming energetically more efficient is a known outcome of endurance training (2), it defies common sense to offer "low efficiency" as a sole explanation. Relying on evidence referenced by the authors themselves (3,4), there is a more plausible alternative. The authors associate the cyclists' higher iEMG response with the V˙O2 overshoot at load transitions, previously documented in trained athletes (3,4). These data reveal two significant observations:
1) V˙O2, V˙CO2, V˙E, and HR responses are much faster in trained athletes (4).
2) A V˙O2 overshoot, when present, is associated with an HR overshoot as well (3,4).
These findings suggest that, in endurance-trained athletes, early augmentation of ventilation and cardiac output accelerates V˙O2 and V˙CO2 responses to workload changes, thus minimizing O2 deficit and mitigating the subsequent build-up of acidosis. Clearly, this is an advantage the athletes gain through training. The HR overshoot seems to be an augmented early adaptation of the cardiovascular system resulting in a V˙O2 overshoot. Thus, elevated muscle activation (iEMG) would not be the cause of a disproportionate increase in V˙O2, or a V˙O2 overshoot, but rather a covariate thereof, as could be expected in this general scheme of augmented transitory response. The elevated iEMG does seem to reflect an expanded spectrum of motor-unit activation, but it does not necessarily mean that extra oxygen is needed or consumed. Possibly, during the adaptation phase, the workload is carried out by more motor units. It is suggested that a V˙O2 overshoot, when present, does not reflect the muscle's oxygen uptake but rather the disproportionate increase in cardiac output due to the HR overshoot and the transitory surge in venous return. By extension, this can be likened to the "cardiodynamic" phenomenon responsible for phase 1 in V˙O2 kinetics (3,4,6).
Raffy Dotan, MSc
Faculty of Applied Health Sciences
St Catharines, Ontario, Canada
1. Boone J, Koppo K, Barstow TJ, Bouckaert J. Aerobic fitness, muscle efficiency, and motor unit recruitment during ramp exercise. Med Sci Sports Exerc
2. Conley DL, Krahenbuhl GS. Running economy and distance running performance of highly trained athletes. Med Sci Sports Exerc
3. Kilding AE, Jones AM. V˙O2
"overshoot" during moderate-intensity exercise in endurance-trained athletes: the influence of exercise modality. Respir Physiol Neurobiol
4. Koppo K, Whipp BJ, Jones AM, Ayeles D, Bouckaert J. Overshoot in V˙O2
following the onset of moderate-intensity cycle exercise in trained cyclists. Eur J Appl Physiol
5. Phillips SM, Green HJ, MacDonald MJ, Hughson RL. Progressive effect of endurance training on V˙O2
kinetics at the onset of submaximal exercise. J Appl Physiol
6. Wasserman K. New concepts in assessing cardiovascular function. Circulation