Capillarisation and oxidative capacity constrain muscle fibre size

Background It has been suggested that angiogenesis during skeletal muscle fibre hypertrophy allows to escape the ‘size constraint’, which is the inverse relationship between oxidative capacity and muscle fibre cross-sectional area (FCSA). It is, however, not yet known whether there are any limitations to the combinations of FCSA, oxidative capacity and capillary supply to a fibre.
Methods To study a wide range of fibre sizes and fibre oxidative capacities, we assessed the FCSA, oxidative capacity and capillary supply to individual fibres from different muscles from mice, recreationally active men and women, and highly resistance-trained men.
Results Irrespective of species and muscle origin, both the oxidative capacity and number of capillaries around a fibre (CAF) per FCSA (CAF/FCSA) showed an upper limit at each FCSA. The upper limit of fibre oxidative capacity was likely determined by diffusion constraints. The upper limit of CAF/FCSA was determined by physical constraints where i) there is no further reduction in maximal diffusion distance to the core of a fibre beyond a CAF of 2, and ii) the reduction in fibre area supplied by a capillary diminishes exponentially with an increase in CAF. The calculated upper limits of oxidative capacity and CAF/FCSA of a fibre of a given FCSA were linearly related.
Discussion and conclusion Irrespective of species, muscle of origin and training status, diffusion limitations and physical limitations to capillary placement around a fibre place an upper limit on the oxidative capacity and capillary supply to a fibre of a given size, respectively.