In response to an elevated metabolic rate (V O 2 ), increased microvascular blood-muscle O 2 flux is the product of both augmented O 2 delivery (Q O 2 ) and fractional O 2 extraction. Whole body and exercising limb measurements demonstrate thatQ O 2 and fractional O 2 extraction increase as linear and hyperbolic functions, respectively, ofV O 2 . Given the presence of disparate vascular control mechanisms among different muscle fibre types, we tested the hypothesis that, in response to muscle contractions,Q O 2 would be lower and fractional O 2 extraction (as assessed via microvascular O 2 pressure, P mvO 2 ) higher in fast-versus slow-twitch muscles. Radiolabelled microsphere and phosphorescence quenching techniques were used to measureQ O 2 and P mvO 2 , respectively at rest and across the transition to 1 Hz twitch contractions at low (Lo, 2.5 V) and high intensities (Hi, 4.5 V) in rat (n = 20) soleus (Sol, slow-twitch, type I), mixed gastrocnemius (MG, fast-twitch, type IIa) and white gastrocnemius (WG, fast-twitch, type IIb) muscle. At rest and for Lo and Hi (steady-state values) transitions, P mvO 2 was lower (all P < 0.05) in MG (mmHg: rest, 22.5 ± 1.0; Lo, 15.3 ± 1.3; Hi, 10.2 ± 1.6) and WG (mmHg: rest, 19.0 ± 1.3; Lo, 12.2 ± 1.1; Hi, 9.9 ± 1.1) than in Sol (rest, 33.1 ± 3.2 mmHg; Lo, 19.0 ± 2.3 mmHg; Hi, 18.7 ± 1.8 mmHg), despite lowerV O 2 andQ O 2 in MG and WG under each set of conditions. These data suggest that during submaximal metabolic rates, the relationship betweenQ O 2 and O 2 extraction is dependent on fibre type (at least in the muscles studied herein), such that muscles comprised of fast-twitch fibres display a greater fractional O 2 extraction (i.e. lower P mvO 2 ) than their slow-twitch counterparts. These results also indicate that the greater sustained P mvO 2 in Sol may be important for ensuring high blood-myocyte O 2 flux and therefore a greater oxidative contribution to energetic requirements. The relationship between blood flow (Q) and metabolic rate (V O 2 ) is strong and has been well characterized for both the whole body and the exercising limbs. In the exercising human, legQ andV O 2 are linearly related by the equationQ = S ×V O 2 + I, where S represents the slope (5.3) and I the intercept (2.8 l min −1 ; data from Knight et al. 1992

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To determine whether fibre type affects the O2 exchange characteristics of skeletal muscle at the microcirculatory level we tested the hypothesis that, following the onset of contractions, muscle comprising predominately type I fibres (soleus, Sol, 86 % type I) would, based on demonstrated blood flow responses, exhibit a blunted microvascular PO2 (PO2,m, which is determined by the O2 delivery (Q̇O2) to O2 uptake (V̇O2) ratio) profile (assessed via phosphorescence quenching) compared to muscle of primarily type II fibres (peroneal, Per, 84 % type II). PO2,m was measured at rest, and following the rest‐contractions (twitch, 1 Hz, 2–4 V for 120 s) transition in Sol (n= 6) and Per (n= 6) muscles of Sprague‐Dawley rats. Both muscles exhibited a delay followed by a mono‐exponential decrease in PO2,m to the steady state. However, compared with Sol, Per demonstrated (1) a larger change in baseline minus steady state contracting PO2,m (ΔPO2,m) (Per, 13.4 ± 1.7 mmHg; Sol, 8.6 ± 0.9 mmHg, P < 0.05); (2) a faster mean response time (i.e. time delay (TD) plus time constant (τ); Per, 23.8 ± 1.5 s; Sol, 39.6 ± 4.3 s, P < 0.05); and therefore (3) a greater rate of PO2,m decline (ΔPO2,m/τ; Per, 0.92 ± 0.08 mmHg s−1; Sol, 0.42 ± 0.05 mmHg s−1, P < 0.05). These data demonstrate an increased microvascular pressure head of O2 at any given point after the initial time delay for Sol versus Per following the onset of contractions that is probably due to faster Q̇O2 dynamics relative to those of V̇O2.

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Objective: This investigation tested the hypothesis that the dynamics of muscle microvascular O pressure (PO m, which from rest to contractions and that the nature of that perturbation is dependent upon the severity of cardiac dysfunction. 