Influence of muscle fibre type and pedal rate on the V̇O₂-work rate slope during ramp exercise

We hypothesised that the ratio between the increase in oxygen uptake and the increase in work rate (ΔV̇O₂/ΔWR) during ramp cycle exercise would be significantly related to the percentage type II muscle fibres at work rates above the gas exchange threshold (GET) where type II fibres are presumed to be active. We further hypothesised that ramp exercise at higher pedal rates, which would be expected to increase the proportional contribution of type II fibres to the total power delivered, would increase the ΔV̇O₂/ΔWR slope at work rates above the GET. Fourteen healthy subjects [four female; mean (SD): age 25 (3) years, body mass 74.3 (15.1) kg] performed a ramp exercise test to exhaustion (25 W min⁻¹) at a pedal rate of 75 rev min⁻¹, and consented to a muscle biopsy of the vastus lateralis. Eleven of the subjects also performed two further ramp tests at pedal rates of 35 and 115 rev min⁻¹. The ΔV̇O₂/ΔWR slope for exercise <GET (S ₁) was significantly correlated with V̇O₂ peak in ml kg⁻¹ min⁻¹ (r=0.60; P<0.05), whereas the ΔV̇O₂/ΔWR slope for exercise >GET (S ₂) was significantly correlated to percentage type II fibres (r=0.54; P=0.05). The ratio between the ΔV̇O₂/ΔWR slopes for exercise above and below the GET (S ₂/S ₁) was significantly greater at the pedal rate of 115 rev min⁻¹ [1.22 (0.09)] compared to pedal rates of 35 rev min⁻¹ [0.96 (0.02)] and 75 rev min⁻¹ [1.09 (0.05), (P<0.05)]. The greater increase in S ₂ relative to S ₁ in subjects (1) with a high percentage type II fibres, and (2) at a high pedal rate, suggests that a greater recruitment of type II fibres contributes in some manner to the “xs” V̇O₂ observed during ramp exercise.