Skeletal muscle &OV0312;O2 on-kinetics: set by O2 delivery or by O2 utilization? New insights into an old issue

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Abstract
GRASSI, B. Skeletal muscle V̇O2 on-kinetics: set by O2 delivery or by O2 utilization? New insights into an old issue. Med. Sci. Sports Exerc., Vol. 32, No. 1, pp. 108–116, 2000. Recent work conducted by our group has expanded knowledge on some basic issues related to pulmonary and skeletal muscle O2 uptake (V̇O2) on-kinetics. We demonstrated that, in exercising humans during transitions from unloaded pedaling to loaded pedaling below the ventilatory threshold, alveolar V̇O2 on-kinetics can be taken as a rather close approximation of skeletal muscle V̇O2 on-kinetics. Experiments conducted on the isolated in situ dog gastrocnemius preparation have shown that, during transitions from rest to contractions corresponding to ∼70% of the muscle peak V̇O2, convective O2 delivery to muscle, intramuscular blood flow (𝑄̇) versus V̇O2 maldistribution, and peripheral O2 diffusion are not limiting factors for skeletal muscle V̇O2 on-kinetics. The latter, therefore, appears to be mainly determined by an intrinsic inertia of skeletal muscle oxidative metabolism, possibly related to acetyl group availability within mitochondria, to regulatory effects on intracellular respiration related to phosphocreatine splitting, and/or to other still not precisely identified control mechanism(s). Evidence from the literature suggests that the limiting factors for skeletal muscle V̇O2 on-kinetics may vary according to the intensity of muscular contractions or of exercise.

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