BFR Exercise Increases S6K1 Phosphorylation in Type‐I and Type‐II Skeletal Muscle Fibers

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Restriction of blood flow to a contracting muscle during low-intensity resistance exercise (BFR exercise) stimulates mTORC1 signaling and muscle protein synthesis during post-exercise recovery. However, it is unknown if mTORC1 activation is occurring in all muscle fiber types. The aim of our study was to determine whether ribosomal S6 kinase 1 (S6K1), a downstream effector of mTORC1, is activated in primarily fast twitch muscle fibers. Young men performed 4 sets of leg extensions at a 20% 1-RM while a pressure cuff was inflated restricting blood flow to the quadriceps, then released immediately following exercise. Muscle biopsies were collected from the vastus lateralis at baseline, 1 and 3 hr post-exercise. Western blotting confirmed an increase in mTOR, S6K1, and S6 phosphorylation in whole muscle homogenates post-exercise. Epifluorescence microscopy was used to assess mTORC1 activation in individual fiber types of muscle samples. We show that phospho-S6K1 is localized to the perinuclear space prior to exercise and following exercise there is a progressive increase in S6K1 phosphorylation and translocation to the sarcoplasm in type-I and type-II muscle fibers. Our preliminary results suggest that BFR exercise increases mTORC1 activation in both slow and fast-twitch muscle fiber types. Supported by NIH/NIAMS RO1 AR049877, P30AG024832, NIH 1UL1RR029876-01 and T32HD07539.