The insulin-sensitizing effect of a single exercise bout is similar in type I and type II human muscle fibres

Human skeletal muscle consists of slow-twitch (type I) and fast-twitch (type II) muscle fibres. Muscle insulin action, regulating glucose uptake and metabolism, is improved following a single exercise bout. Rodent studies suggest that this phenomenon is confined to specific muscle fibre types. Whether this phenomenon is also confined to specific fibre types in humans has not been described. To investigate this, nine healthy men underwent a euglycemic hyperinsulinemic clamp (EHC) in the recovery from a single bout of one-legged knee-extensor exercise. Pools of type I and type II fibres were prepared from muscle biopsies taken in the rested and prior exercised leg before and after the
EHC. AMPK γ3 and TBC1D4 – two key proteins regulating muscle insulin action following exercise – were higher expressed in type II compared to type I fibres. However, phosphor-regulation of TBC1D4 was similar between fibre types when related to the total amount of TBC1D4 protein. The activating dephosphorylation of glycogen synthase was also similar in the two fibre types. Thus, insulin-induced regulation of key proteins important for transport and intracellular flux of glucose towards glycogen storage in the recovery from exercise, does not differ between fibre types. In conclusion, the insulinsensitizing effect of a single bout of exercise includes both type I and type II fibres in human skeletal muscle. This may be an important observation for future pharmacological strategies targeting muscle insulin sensitivity in humans.