Capillary differences with age and muscle fiber type are attenuated by accounting for fiber shape

Age-related declines in skeletal muscle capillarization are well-documented, yet commonly used indices, such as the capillary-to-fiber perimeter exchange index (CFPE), assume uniform myofiber shape. Because fiber size and shape changes with age and fiber type, this assumption may lead to overestimation of true capillary rarefaction. This study aimed to refine capillarization metrics by accounting for fiber shape. Muscle biopsies from 12 young (23 ± 3 years) and 11 older (73 ± 2 years) women were analyzed for capillary-to-fiber ratio (C:F), capillary density (CD), and individual capillary-to-fiber ratio (C:Fi). Analyses were fiber type-specific and performed on both rested and previously exercised legs. C:Fi was normalized to fiber perimeter (CFPE), cross-sectional area (CFCE), and adjusted for the shape factor index (SFI: perimeter²/4π·CSA). Older participants exhibited smaller type II fibers (28%) and greater fiber shape irregularity (8% SFI) compared with young. C:Fi and CFPE showed consistent age-related (from 22% to 34%) and fiber type related (from 12% to 29%) reductions in capillarization. Adjusting CFPE for fiber shape (CFPE_SFIadjusted) reduced age-related differences from 22-25% down to 19-20% (~10-25% relative reduction) and fiber type differences from 12-15% down to 8-9% (~25-45% relative reduction). Normalizing C:Fi to fiber CSA (CFCE) further attenuated or eliminated most differences, with only an 18% difference remaining in type I fibers between age groups. These patterns were consistent in the exercised leg, supporting internal validity. Adjusting CFPE using SFI reduced apparent differences in capillarization between young and old muscle, as well as between fiber types. Shape-sensitive indices may provide a more physiologically accurate assessment of capillary supply in skeletal muscle.