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 la teralis and a greater peak oxygen uptake (V(over dot)(O2)) were associated with a greater initial rise in top in V(over dot)(O2) {Delta V(over dot)(O2 )/Delta W, where W is work rate} following the onset of heavy constant powe r output exercise (above the lactate threshold, LT). It was unclear if thes e results were true only for heavy exercise, or if the association between fibre type and/or fitness and Delta V(over dot)(O2)/Delta W would also be s een for moderate (< LT) exercise. The purpose of the present study was to c ompare the relationships between fibre type or peak V(over dot)(O2) and Del ta V(over dot)(O2)/Delta W determined for moderate (< LT) and heavy (> LT) exercise intensities during incremental exercise. Nine healthy subjects per formed an incremental ramp test on a cycle ergometer. The V(over dot)(O2)/W slope was calculated for the domain of power outputs up to the LT (S-1), f rom the LT towards peak V(over dot)(O2)(S-2), and over the entire linear po rtion of the Delta V(over dot)(O2)/Delta W response (S-T), and compared to fibre type distribution determined from biopsy of the vastus lateralis, and to peak V(over dot)(O2) (as ml kg(-1) min(-1)). Significant correlations b etween Delta V(over dot)(O2)/Delta W and the proportion of type I fibres we re found for each exercise domain (r is 0.69, 0.71 and 0.84 for S-1, S-2 an d S-T, respectively, P < 0.05). S-1 ranged between about 9 ml min(-1) W-1 f or a low proportion of type I fibres and 11 ml min(-1) W-1 for a high propo rtion of type I fibres. Similar correlations were also found between S-2(r = 0.70) and S-T (r = 0.76) and peak V(over dot)(O2). These results are cons istent 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 V(over dot)(O2), is associated with greater Delta V(over dot)(O2)/Delta W during the initial adjustment to < LT as well as >LT exer cise. These results do not appear to be explained by classical descriptions of the kinetics of adjustment of V(over dot)(O2) following the onset of ra mp or constant power output exercise. They might reflect enhanced motor uni t recruitment in subjects with a greater percentage of type I fibres, and/o r who are more aerobically fit. However, the underlying mechanism for these findings must await further study.