Slow Component of (V) over dotO(2) Kinetics: Mechanistic Bases and Practical Applications

JONES, A. M., B. GRASSI, P. M. CHRISTENSEN, P. KRUSTRUP, J. BANGSBO, and D. C. POOLE. Slow Component of (V) over dotO(2) Kinetics: Mechanistic Bases and Practical Applications. Med. Sci. Sports Exerc., Vol. 43, No. 11, pp. 2046-2062, 2011. The (V) over dotO(2) slow component, a slowly developing increase in (V) over dotO(2) during constant-work-rate exercise performed above the lactate threshold, represents a progressive loss of skeletal muscle contractile efficiency and is associated with the fatigue process. This brief review outlines the current state of knowledge concerning the mechanistic bases of the (V) over dotO(2) slow component and describes practical interventions that can attenuate the slow component and thus enhance exercise tolerance. There is strong evidence that, during constant-work-rate exercise, the development of the (V) over dotO(2) slow component is associated with the progressive recruitment of additional (type II) muscle fibers that are presumed to have lower efficiency. Recent studies, however, indicate that muscle efficiency is also lowered (resulting in a "mirror-image" (V) over dotO(2) slow component) during fatiguing, high-intensity exercise in which additional fiber recruitment is unlikely or impossible. Therefore, it seems that muscle fatigue underpins the (V) over dotO(2) slow component, although the greater fatigue sensitivity of recruited type II fibers might still play a crucial role in the loss of muscle efficiency in both situations. Several interventions can reduce the magnitude of the (V) over dotO(2) slow component, and these are typically associated with an enhanced exercise tolerance. These include endurance training, inspiratory muscle training, priming exercise, dietary nitrate supplementation, and the inspiration of hyperoxic gas. All of these interventions reduce muscle fatigue development either by improving muscle oxidative capacity and thus metabolic stability or by enhancing bulk muscle O-2 delivery or local (Q) over dotO(2)-to-(V) over dotO(2) matching. Future honing of these interventions to maximize their impact on the (V) over dotO(2) slow component might improve sports performance in athletes and exercise tolerance in the elderly or in patient populations.