Russell S. Richardson

Determinants of oxygen uptake: Implications for exercise testing

SummaryFor exercise modalities such as cycling which recruit a substantial muscle mass, muscle oxygen uptake (V̇O2) is the primary determinant of pulmonary V̇O2. Indeed, the kinetic complexities of pulmonary V̇O2 associated with exercise onset and the non-steady states of heavy (>lactate threshold) and severe [>asymptote of power-time relationship for high intensity exercise (Ẇ)] exercise reproduce with close temporal and quantitative fidelity those occurring across the exercising muscles. For moderate (<lactate threshold) exercise and also rapidly incremental work tests, pulmonary (and muscle) V̇O2 increases as a linear function of work rate (≈9 to 11 ml O2/W/min) in accordance with theoretical determinations of muscle efficiency (≈30%). In contrast, for constant load exercise performed in the heavy and severe domains, a slow component of the V̇O2 response is manifest and pulmonary and muscle V̇O2 increase as a function of time as well as work rate beyond the initial transient associated with exercise onset.In these instances, muscle efficiency is reduced as the V̇O2 cost per unit of work becomes elevated, and in the severe domain, this V̇O2 slow component drives V̇O2 to its maximum and fatigue ensues rapidly. At pulmonary maximum oxygen uptake (V̇O2max) during cycling, the maximal cardiac output places a low limiting ceiling on peak muscle blood flow, O2 delivery and thus muscle V̇O2. However, when the exercise is designed to recruit a smaller muscle mass (e.g. leg extensors, 2 to 3kg), mass-specific muscle blood flow and V̇O2 at maximal exercise are 2 to 3 times higher than during conventional cycling. Consequently, for any exercise which recruits more than ≈5 to 6kg of muscle at pulmonary V̇O2max there exists a mitochondrial or V̇O2 reserve capacity within the exercising muscles which cannot be accessed due to oxygen delivery limitations. The implications of these latter findings relate to the design of exercise tests. Specifically, if the purpose of exercise testing is to evaluate the oxidative capacity of a small muscle mass (<5 to 6kg), the testing procedure should be designed to restrict the exercise to those muscles so that a central (cardiac output, muscle O2 delivery) limitation is not invoked. It must be appreciated that exercise which recruits a greater muscle mass will not stress the maximum mass-specific muscle blood flow and V̇O2 but rather the integration of central (cardiorespiratory) and peripheral (muscle O2 diffusing capacity) limitations.

Red Blood Cell Transit Time in Man: Theoretical Effects of Capillary Density

These data indicate that through the reduction in exercise time and recruitment of trained subjects, the exercising muscle in the human dynamic knee-extension model can reach even higher work rates, VO2, and Q than previously reported (Andersen & Saltin, 1985; Andersen et al., 1985; Rowell et al., 1986). Despite these high muscle Q, the achievement of high O2 extractions is possible (Richardson et al., 1993). Previously, this was attributed almost exclusively to the elevated WRMAX, and it was therefore concluded that O2 extraction is not limited by the high Q to any greater extent than in conventional two legged cycle ergometry (Richardson et al., 1993). It is now apparent from the analysis of the data in this paper that it is possible that a difference in capillary density between the subjects in the original studies and the present research may have played a role in the increase in O2 extraction with increasing muscle Q. Although, it should be recognized that a) the capillary density necessary to reduce red cell transit time suitably to match the range of measured femoral venous hemoglobin saturation is high (Gayeski et al., 1988), however it is within the measured values for man (Brodal et al., 1977), and b) where comparable, during low WR knee-extensor exercise and whole body VO2MAX, subjects across the studies did not differ. This would be expected if capillary density differed greatly (Brodal et al., 1977). It can therefore be concluded that both the rapid protocol during knee-extensor exercise and the potential increased capillarity of the trained subjects in the present study may have combined to produce the amplified physiologic and WR responses.

Partial substitution of red blood cells with free hemoglobin solution does not improve maximal O2 uptake of working in situ dog muscle.

It has been proposed (Gayeski at al., 1987; Honig and Gayeski, 1 993) that the O2 carrier-free region between the erythrocyte and the myocyte sarcolemma, which would include the plasma space, may serve as the major source of resistance to the diffusion of oxygen. Federspiel and Popel (1986) have suggested, using theoretical modeling, that the flux of O2 from a red blood cell through the capillary wall is limited to the near vicinity of the red blood cell, and that O2 diffusion contributes negligibly to overall O2 flux out of the capillary in the plasma spaces between erythrocytes.

In vivo and in vitro evidence that in oldest-old humans intrinsic upper- and lower-limb skeletal muscle function is unaffected by ageing and disuse

To parse out the impact of advanced ageing and disuse on skeletal muscle function, we utilized both in vivo and in vitro techniques to comprehensively assess upper‐ and lower‐limb muscle contractile properties in 8 young (YG; 25 ± 6 years) and 8 oldest‐old mobile (OM; 87 ± 5 years) and 8 immobile (OI; 88 ± 4 years) women.