Human skeletal muscle fiber type-specific responses to sprint interval and moderate-intensity continuous exercise: acute and training-induced changes

Abstract

There are limited and equivocal data regarding potential fiber type-specific differences in the human skeletal muscle response to sprint interval training (SIT), including how this compares to moderate-intensity continuous training (MICT). We examined mixed muscle and fiber type-specific responses to a single session (study 1) and 12 wk (study 2) of MICT and SIT using Western blotting. MICT consisted of 45 min of cycling at ~70% of maximal heart rate and SIT involved 3 x 20-sec 'all-out' sprints interspersed with 2 min of recovery. Changes in signaling proteins involved in mitochondrial biogenesis in mixed muscle and pooled fiber samples were similar after acute MICT and SIT. This included increases in the ratios of phosphorylated to total acetyl CoA carboxylase and p38 mitogen activated protein kinase protein content (main effects, p<0.05). Following training, mitochondrial content markers including the protein content of cytochrome c oxidase subunit IV and NADH:ubiquinone oxidoreductase subunit A9 were increased similarly in mixed muscle and type IIa fibers (main effects, p<0.05). In contrast, only MICT increased these markers of mitochondrial content in type I fibers (interactions, p<0.05). MICT and SIT also similarly increased the content of mitochondrial fusion proteins optic atrophy 1 (OPA1) and mitofusin 2 in mixed muscle, and OPA1 in pooled fibre samples (main effects, p<0.05). In summary, acute MICT and SIT elicited similar fiber type-specific responses of signaling proteins involved in mitochondrial biogenesis, whereas 12 wk of training revealed differential responses of mitochondrial content markers in type I but not type IIa fibers.