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

Abstract

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.

Home‐hit improves muscle capillarisation and eNOS/NAD(P)Hoxidase protein ratio in obese individuals with elevated cardiovascular disease risk

“…, ) and HIT studies (Tan et al . ), which show no difference in fibre type‐specific angiogenesis in response to interval training, which is in contrast to previous work in rats showing fibre type differences in response to interval training and MICT (Gute et al . ).…”

“…The findings also extend the previous work of Tan et al . (), which demonstrated that 6 weeks of low‐volume HIT increased capillary contacts in overweight/obese women, while we show that 12 weeks of low‐volume HIT increased capillary density and capillary‐fibre perimeter exchange index. The similar increases in capillarisation with Home‐HIT and supervised Lab‐HIT suggest that Home‐HIT is an effective strategy to increase capillarisation while simultaneously reducing the major barriers to exercise.…”

“…; Tan et al . ). The increase in mitochondrial content probably underpins the improved as mitochondrial biogenesis is a major training adaptation that increases lipid and glucose fuel handling.…”

“…; Tan et al . ), the current intervention started with a low number of intervals and progressed training volume throughout (4 intervals during week 1–2 to 8 intervals during week 11–12).…”

“…; Tan et al . ), the applicability to sedentary obese individuals has been disputed by health experts (Courneya, ; Hardcastle et al . ; Biddle & Batterham, ), who cite the strenuous nature and complex protocols as major barriers in sedentary, exercise‐naïve individuals.…”