High‐intensity interval training (HIIT) enhances skeletal muscle oxygen delivery and utilization but data are limited regarding fiber‐specific adaptations in humans. We examined the effect of 18 sessions of HIIT (10 × 60‐sec cycling intervals at ~90% HRmax, interspersed by 60‐sec of recovery) over 6 weeks on markers of microvascular density and oxidative capacity in type I and II fibers in healthy but sedentary young women (Age: 26 ± 7 years; BMI: 30 ± 4 kg·m−2; VO2peak: 2.16 ± 0.45 L·m−1). Immunohistochemical analyses of muscle cross sections revealed a training‐induced increase in capillary contacts per fiber in type I fibers (PRE: 4.38 ± 0.37 vs. POST: 5.17 ± 0.80; main effect, P < 0.05) and type II fibers (PRE: 4.24 ± 0.55 vs. POST: 4.92 ± 0.54; main effect, P < 0.05). The capillary‐to‐fiber ratio also increased after training in type I fibers (PRE: 1.53 ± 1.44 vs. POST: 1.88 ± 0.38; main effect, P < 0.05) and type II fibers (PRE: 1.45 ± 0.19 vs. POST: 1.76 ± 0.27; main effect, P < 0.05). Muscle oxidative capacity as reflected by the protein content of cytochrome oxidase IV also increased after training in type I fibers (PRE: 3500 ± 858 vs. POST: 4442 ± 1377 arbitrary units; main effect, P < 0.01) and type II fibers (PRE: 2632 ± 629 vs. POST: 3863 ± 1307 arbitrary units; main effect, P < 0.01). We conclude that short‐term HIIT in previously inactive women similarly increases markers of capillary density and mitochondrial content in type I and type II fibers.
Physiological Reports – Wiley
Published: Jan 1, 2018
Keywords: ; ; ; ;
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