Skeletal muscle fiber‐type‐specific changes in markers of capillary and mitochondrial content after low‐volume interval training in overweight women

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% HR max , 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 ; VO 2peak : 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.