During constant-load submaximal exercise above the ventilatory threshold (VT), the primary exponential response of pulmonary oxygen uptake (ΩO2) is supplemented by a ΩO2 slow component (ΩO2 SC) that causes ΩO2 to rise above the anticipated steady-state value. The physiological mechanism responsible for the development of the ΩO2 SC is presently unknown but the recruitment of lower-efficiency type II fibres has been suggested as a plausible explanation (Whipp, 1994). Only one previous study has examined the effect of muscle fibre type on ΩO2 kinetics. Barstow et al. (1996) showed that subjects with a high proportion of type II muscle fibres had a significantly lower primary component (phase II) gain (▓Dgr│ΩO2/▓Dgr│Work rate, ã.W) and a significantly larger contribution of the ΩO2 SC to the end-exercise ΩO2 during heavy cycle exercise. In the present study, we sought to extend the work of Barstow et al. (1996) by examining the influence of muscle fibre type on ΩO2 kinetics in moderate, heavy and severe exercise.
Fourteen recreationally active subjects (mean ± S.D., age 25 ± 4 years; mass 72.6 ± 3.9 kg; peak ΩO2 3.42 ± 0.91 l min-1) participated in this study, which was approved by the Manchester Metropolitan University ethics committee. Subjects performed transitions of 6 min duration from ‘unloaded’ pedalling to power outputs calculated to require 80 % of the ΩO2 at VT (moderate exercise) and 50 % and 70 % of the difference between the ΩO2 at VT and peak ΩO2 (50 % ▓Dgr│: heavy exercise; 70 % ▓Dgr│: severe exercise). The subjects performed 4-8 transitions to moderate exercise and 2-4 transitions to heavy and severe exercise. For each condition, breath-by-breath ΩO2 data were interpolated, time-aligned and ensemble averaged, and then modelled using non-linear regression techniques (Barstow et al. 1996). Muscle fibre type (types I, IIA and IIX) was determined histochemically from a conchotome biopsy of the vastus lateralis muscle according to the staining intensity for mATPase activity. The strength of relationships between variables was determined using Pearson correlation coefficients. Results are reported as means ± S.E.M.
Percentage muscle fibre type was 55 ± 4, 34 ± 3 and 11 ± 3 % for types I, IIA and IIX, respectively. There was no relationship between muscle fibre type and the parameters of the ΩO2 kinetic response for moderate exercise. The time constant of the primary component was negatively related to the % of type I fibres for heavy exercise (r = -0.68, P < 0.01) and positively related to the % of type IIX fibres for both heavy (r = 0.68, P < 0.01) and severe exercise (r = 0.56, P < 0.05). The phase II gain (▓Dgr│ΩO2 /▓Dgr│ ã.W) was correlated with the % type I fibres (r = 0.57, P < 0.05 for both heavy and severe exercise) and the % type IIA fibres (r = -0.62, P < 0.05 for heavy exercise; r = -0.71, P < 0.01 for severe exercise). The amplitude (as % end-exercise ΩO2) of the ΩO2 SC was positively related to the % of type II fibres (IIA and IIX combined) for both heavy (r = 0.74, P < 0.01) and severe exercise (r = 0.64, P < 0.05).
Our results are consistent with the findings of Barstow et al. (1996), and demonstrate that both the gain of the primary component and the amplitude of the ΩO2 SC during constant-load cycle exercise above the VT are influenced by muscle fibre type. The higher mechanical efficiency (i.e. lower primary component gain) of individuals with higher % type II fibres is intriguing given that these fibres are typically less efficient than type I fibres at a given (low) velocity of contraction (e.g. He et al. 2000). The relationship between the primary time constant and muscle fibre type might be explained by differences in the activity of oxidative enzymes and/or aspects of O2 delivery in type I compared with type II (particularly IIX) muscle fibres.