Muscle fiber type distribution and nonlinear .VO(2)-power output relationship in cycling

We examined whether reported deviations from linearity of the oxygen uptake (.VO(2))-to-power output (W) relationship during intense cycling exercise correlated with the percentage Type II fibers in the exercising muscle.

Twelve trained young men with known fiber type distribution in the vastus lateralis muscle performed step-increment exercise (40 W.3 min(-1)) to exhaustion.

.VO(2) increased linearly with W up to about 50% .VO(2max) with a regression equation of .VO(2) (mL.min-1) = 661 + 9.73 W and a correlation coefficient (r) of 1.000. Subsequent .VO(2) values were all greater than corresponding linear estimates (P < 0.001 or 0.0001). Peak exercise excess .VO(2) (measured minus estimated .VO(2) assuming linearity) averaged (SD) 434 (192) mL O(2).min-1 or 10.3 (4.7) % .VO(2max). A comprehensive curvilinearity index defined as the sum of measured minus estimated .VO(2) at the four highest completed exercise trials averaged 973 (460) mL O(2).min-1 or 21.5 (9.4) % .VO(2max). Correlations between percentage Type II fibers and either of the two expressions of curvilinearity were nonsignificant. Delta [H+] (arterialized capillary blood) from basal level to peak exercise correlated with the submaximal curvilinearity index (r = 0.59-0.64; P < 0.05) but not with peak excess .VO(2). There was a trend toward a correlation between delta La and curvilinearity index in % .VO(2max)(r = 0.52; P < 0.10) but not with any of the other curvilinearity expressions. The relative ventilatory activity expressed as .V(E)-to-.VO(2) ratio tended to correlate with peak excess .VO(2) (P < 0.10) but not with curvilinearity index. Signals from motion sensors indicate that coactivation of upper-body musculature coincided with deviation from linearity in the .VO(2)-W relationship.

VO2 during step-increment cycling increases linearly with power output up to about 50% .VO(2max)and then curvilinearly. The degree of curvilinearity is not related to muscle fiber type distribution in the vastus lateralis, and only marginally and insignificantly related (P < 0.10) to the relative degree of hyperventilation or to lactate response. Acidosis, on the other hand, correlated significantly with curvilinearity index. The inclusion of isometrically working, upper-body muscular groups during high-intensity cycling may also contribute to the overshoot in oxygen cost.