Influence of Muscle Fibre Type and Fitness on the Oxygen Uptake/Power Output Slope During Incremental Exercise in Humans

Abstract

We recently reported that a higher percentage of type I fibres in vastus lateralis and a greater peak oxygen uptake (V̇O2) were associated with a greater initial rise in V̇O2 (ΔV̇O2/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 ΔV̇O2/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 V̇O2 and ΔV̇O2/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 V̇O2/Wslope was calculated for the domain of power outputs up to the LT (S1), from the LT towards peak V̇O2 (S2), and over the entire linear portion of the ΔV̇O2/ΔW response (ST), and compared to fibre type distribution determined from biopsy of the vastus lateralis, and to peak V̇O2 (as ml kg‐1 min‐1). Significant correlations between ΔV̇O2/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 V̇O2. 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 V̇O2, is associated with greater ΔV̇O2/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 V̇O2 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.

Endurance training decreases the non‐linearity in the oxygen uptake–power output relationship in humans

“…Assuming that S 1 (Δ /ΔPO) as a reciprocal of the delta efficiency (ΔPO/Δ ) represents the efficiency of contracting muscles (see e.g. Barstow et al 2000), our data suggest that the endurance training decreased the muscle efficiency during cycling in the moderate‐intensity domain. In contrast, at the same time this training increased the gross efficiency, as judged by a significant lowering in the /PO ratio below LT (see Results and Fig.…”

“…This result shows that moderate‐intensity endurance training in its early stage exerts various effects on the factors determining both the delta efficiency (ΔPO/Δ ) and gross efficiency (PO/ ). It should be mentioned that Barstow et al (2000) reported that S 1 is positively correlated with the percentage of type I muscle fibres, suggesting that the delta efficiency in type I muscle fibres during moderate‐intensity exercise is less than in type II. Interestingly, it was reported that the delta efficiency in sprinters and in endurance athletes during cycling in the moderate‐intensity domain was not significantly different, whereas the gross efficiency was higher in distance runners (Stuart et al 1981).…”

“…Moreover, for slope comparisons (see Barstow et al 2000), obtained during the maximal incremental cycling, in the range of power output beginning from the power output at the LT up to the maximal power output reached during the incremental exercise ( Obs. PO max ) was described as the following linear function: where PO is the power output, a 2 the intercept and S 2 the slope of the –PO relationship above the LT.…”

Exploratory Study on Oxygen Consumption On-kinetics During Treadmill Walking in Women With Systemic Lupus Erythematosus

“…Proposed regulators57 of the fast component include central circulatory oxygen delivery58–60, capillary blood flow dynamics24–26, and dynamics of muscle fiber recruitment24, 61. Mitochondrial inertia and respiratory chain inhibition have also been suggested as fast component regulators27–29, 27, 28, 62.…”

“…Because the anaerobic energy contribution is indirectly determined by the rate at which aerobic energy output increases, slowing of VO 2 on-kinetics may impair the ability to sustain physical activity. VO 2 on-kinetics appears to be regulated by microcirculatory dynamics24–26, and mitochondrial function27–29. Pathological adaptations occurring in the muscle capillary bed of patients with SLE could impair microvascular reactivity and oxygen diffusion30–34.…”