Abstract
This study evaluated gene expression changes in gastrocnemius slow-twitch myosin heavy chain I (MHC I) and fast-twitch (MHC IIa) muscle fibers of collegiate cross-country runners (n = 6, 20±1 y, VO2max = 70±1 ml•kg−1•min−1) during two distinct training phases. In a controlled environment, runners performed identical 8 kilometer runs (30∶18±0∶30 min:s, 89±1% HRmax) while in heavy training (∼72 km/wk) and following a 3 wk taper. Training volume during the taper leading into peak competition was reduced ∼50% which resulted in improved race times and greater cross-section and improved function of MHC IIa fibers. Single muscle fibers were isolated from pre and 4 hour post run biopsies in heavily trained and tapered states to examine the dynamic acute exercise response of the growth-related genes Fibroblast growth factor-inducible 14 (FN14), Myostatin (MSTN), Heat shock protein 72 (HSP72), Muscle ring-finger protein-1 (MURF1), Myogenic factor 6 (MRF4), and Insulin-like growth factor 1 (IGF1) via qPCR. FN14 increased 4.3-fold in MHC IIa fibers with exercise in the tapered state (P<0.05). MSTN was suppressed with exercise in both fiber types and training states (P<0.05) while MURF1 and HSP72 responded to running in MHC IIa and I fibers, respectively, regardless of training state (P<0.05). Robust induction of FN14 (previously shown to strongly correlate with hypertrophy) and greater overall transcriptional flexibility with exercise in the tapered state provides an initial molecular basis for fast-twitch muscle fiber performance gains previously observed after taper in competitive endurance athletes.
Highlights
Athletes routinely reduce training volume prior to a major competition to help facilitate peak performance
No changes were observed with exercise for factorinducible 14 (FN14) in the heavily trained state or Heat shock protein 72 (HSP72), MRF4, or Insulin-like growth factor 1 (IGF1) in the heavily trained or tapered state (Table 2)
The main finding was that FN14, a gene strongly associated with fast-twitch hypertrophy [6,9], was robustly induced in MHC IIa fibers with exercise in the tapered state
Summary
Athletes routinely reduce training volume prior to a major competition to help facilitate peak performance. We were guided into these single muscle fiber gene studies by previous work in cross-country runners (for whom muscle biopsy samples were still available) that had an altered transcriptional response in mixed-muscle homogenate samples after identical 8 km running bouts in the heavily trained versus tapered state [2]. This alteration was intriguing since previous research has shown a blunted transcriptional response to exercise in well-conditioned skeletal muscle [6,8]
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