Influence of muscle fibre type and fitness on the oxygen uptake/power output slope during incremental exercise in humans

We recently reported that a higher percentage of type I fibres in vastus lateralis and a greater peak oxygen uptake ([Vdot]O2) were associated with a greater initial rise in [Vdot]O2 (Δ[Vdot]O2 /ΔW, where W is work rate) following the onset of heavy constant power output exercise (above the lactate threshold, LT). It was unclear if these results were true only for heavy exercise, or if the association between fibre type and/or fitness and Δ[Vdot]O2 /ΔW would also be seen for moderate (< LT) exercise. The purpose of the present study was to compare the relationships between fibre type or peak [Vdot]O2 and Δ[Vdot]O2 /ΔW determined for moderate (< LT) and heavy (> LT) exercise intensities during incremental exercise. Nine healthy subjects performed an incremental ramp test on a cycle ergometer. The [Vdot]O2 /Wslope was calculated for the domain of power outputs up to the LT (S1), from the LT towards peak [Vdot]O2 (S2), and over the entire linear portion of the Δ[Vdot]O2 /ΔW response (ST), and compared to fibre type distribution determined from biopsy of the vastus lateralis, and to peak [Vdot]O2 (as ml kg-1 min-1). Significant correlations between Δ[Vdot]O2 /ΔW and the proportion of type I fibres were found for each exercise domain (r is 0.69, 0.71 and 0.84 for S1, S2 and ST, respectively, P < 0.05). S1 ranged between about 9 ml min-1 W-1 for a low proportion of type I fibres and 11 ml min-1 W-1 for a high proportion of type I fibres. Similar correlations were also found between S2 (r = 0.70) and ST (r = 0.76) and peak [Vdot]O2. These results are consistent with our previous findings during > LT constant power output exercise, and suggest that the proportion of type I fibres, and possibly fitness as indicated by peak [Vdot]O2, is associated with greater Δ[Vdot]O2 /ΔW during the initial adjustment to < LT as well as > LT exercise. These results do not appear to be explained by classical descriptions of the kinetics of adjustment of [Vdot]O2 following the onset of ramp or constant power output exercise. They might reflect enhanced motor unit recruitment in subjects with a greater percentage of type I fibres, and/or who are more aerobically fit. However, the underlying mechanism for these findings must await further study.