SPECIAL COMMUNICATIONS: Letters to the Editor-in-Chief
It is generally accepted that training-induced adaptations both at the cardiovascular and muscle level should enhance the mechanical efficiency and economy of cycling. A higher efficiency in trained subjects has indeed been observed in previous studies (e.g., Böning et al. ). However, these studies have always considered steady-state conditions. In the letter to the editor, Dotan points out that Fig. 2 (p. 405) is also indicative of a higher efficiency in the trained cyclists compared to the less-trained counterparts in the present study (3). However, it should be argued that this is merely a comparison between two individuals. The baseline V˙O2 for the ramp exercises was similar in both groups, and in combination with the higher ΔV˙O2/ΔW, the cyclists had, on average, a slightly higher V˙O2 for an identical work rate during the ramp exercises. Furthermore, the slope of the V˙O2/W relationship did not differ between the groups during the steady-state conditions of the step exercise. These results indicate that the non-steady-state conditions of the ramp exercise place specific demands on the individuals and that trained subjects respond differently to these demands compared to less-trained counterparts. This specific way to adjust to the ramp exercise is supported by the different profile of the EMG signal.
We have related this higher ΔV˙O2/ΔW in the ramp exercise to the V˙O2 overshoot phenomenon that has been observed in the transition between constant work rates because this is a situation also characterized by non-steady-state conditions with trained subjects responding in a specific way (6,7). An overshoot in HR has indeed been observed in the study of Koppo et al. (7) and Kilding and Jones (6). However, in both studies, it is reported that this HR overshoot occurred only occasionally and was certainly not consistent with the incidence of the V˙O2 overshoot. Furthermore, it is argued by Koppo et al. (7) that the overshoot in HR is more likely to be a consequence of the mechanisms underpinning the V˙O2 overshoot rather than be the cause. This argument is based on several studies in which alterations in cardiac output did not influence the time course of the V˙O2 response (1,4,5), and this is contradictory to the mechanism proposed by the reviewer. In both studies reporting the V˙O2 overshoot, the main mechanism proposed to mediate this specific phenomenon relates to muscular activity during the transition between the work rates. The elevated iEMG as an expression of an increased muscle fiber recruitment during the ramp exercise will result in an increase in the "activation energy," even when the recruited muscle fiber does not contribute appreciably to the force production. The higher muscle fiber recruitment will therefore increase the O2 cost of the exercise (i.e., the V˙O2 overshoot) and might as well result in an HR overshoot.
It should be noted that the terminology in the article reporting "a lower efficiency" in trained cyclists has a rather negative connotation. The higher ΔV˙O2/ΔW in combination with the elevated iEMG should actually be interpreted as a specific way the cyclists adapt to the continuously changing metabolic demand. It is highly likely that this incorporates certain benefits to the trained subjects, but until now, these benefits and its underlying mechanisms are unknown.
Department of Movement and Sports Sciences
1. Barstow TJ, Lamarra N, Whipp BJ. Modulation of muscle and pulmonary O2
uptakes by circulatory dynamics during exercise. J Appl Physiol
2. Böning D, Gonen Y, Maassen N. Relationship between work load, pedal frequency and physical fitness. Int J Sports Med
3. Boone J, Koppo K, Barstow TJ, Bouckaert J. Aerobic fitness, muscle efficiency, and motor unit recruitment during ramp exercise. Med Sci Sports Exerc
4. Corsi A, Granata AL, Midrio M, Turchetto M. Observations upon the rate-limiting role of blood flow in the regulation of respiration of skeletal muscle during intermittent exercise. In: Howald H, Poortmans JR, editors. Metabolic Adaptation to Prolonged Physical Exercise
. Basel (Switzerland): Birkhauser. 1975, p. 65-9.
5. Grassi B, Gladden LB, Samaja M, Stary CM, Hogan MC. Faster adjustment of O2
delivery does not affect V˙O2
on-kinetics in isolated in situ canine muscle. J Appl Physiol
6. Kilding AE, Jones AM. V˙O2
"overshoot" during moderate-intensity exercise in endurance-trained athletes: the influence of exercise modality. Respir Physiol Neurobiol
7. Koppo K, Whipp BJ, Jones AM, Aeyels D, Bouckaert J. Overshoot in V˙O2
following the onset of moderate-intensity cycle exercise in trained cyclists. Eur J Appl Physiol