Critical power is positively related to skeletal muscle capillarity and type I muscle fibers in endurance trained individuals

The asymptote (critical power; CP) and curvature constant (W′) of the hyperbolic power duration relationship can predict performance within the severe-intensity exercise domain. However, the extent which these parameters relate to skeletal muscle morphology is less clear, particularly in endurance trained individuals who, relative to their lesser trained counterparts, possess skeletal muscles that can support high levels of oxygen transport and oxidative capacity i.e. elevated type I fiber proportion and cross-sectional area (CSA) and capillarity. Fourteen endurance trained males performed a maximal incremental test to determine peak oxygen uptake (V̇ O2peak; 63.2 ± 4.1 ml.min-1.kg-1), maximal aerobic power (406 ± 63 W), and 3-5 constant load tests to task failure for the determination of CP (303 ± 52 W) and W' (17.0 ± 3.0 kJ). Skeletal muscle biopsies were obtained from the vastus lateralis and analyzed for % fiber type proportion, CSA and indices of capillarity. CP was positively correlated with the % proportion (r = 0.79; P = 0.001) and CSA (r = 0.73; P =0.003) of type I fibers, capillary to fiber ratio (r = 0.88; P < 0.001) and capillary contacts around type I fibers (r = 0.94; P < 0.001) and type II fibers (r = 0.68; P = 0.008). W' was not correlated with any morphological variables. These data between CP and skeletal muscle capillarity. Our findings support the assertion that CP is an important parameter of aerobic function and offer novel insights into the physiological bases of CP.