Skeletal muscle VO2 on-kinetics: set by O-2 delivery or by O-2 utilization? New insights into an old issue

Recent work conducted by our group has expanded knowledge on some basic iss ues related to pulmonary and skeletal muscle O-2 uptake ((V) over dot O-2) on-kinetics. We demonstrated that, in exercising humans juring transitions from unloaded pedaling to loaded pedaling below the ventilatory threshold, alveolar (V) over dot O-2, on-kinetics can be taken as a rather close appro ximation of skeletal muscle (V) over dot O-2 on-kinetics. Experiments condu cted on the isolated in situ joy gastrocnemius preparation have shown that, during transitions from rest to contractions corresponding to similar to 7 0% of the muscle peak (V) over dot O-2, convective O-2 delivery to muscle, intramuscular blood flow ((Q) over dot) versus (V) over dot O-2 maldistribu tion, and peripheral O-2 diffusion are nor limiting factors for skeletal mu scle (V) over dot O-2 on-kinetics. The latter, therefore, appears to be mai nly determined by an intrinsic inertia of skeletal muscle oxidative metabol ism possibly related to acetyl group availability within mitochondria, to r egulatory effects on intracellular respiration related to phosphocreatine s plitting, and/or to other still not precisely identified control mechanism( s). Evidence from the literature suggests that the limiting factors for ske letal muscle (V) over dot O-2 on-kinetics may vary according to the intensi ty of muscular contractions or of exercise.