Murach, K., Raue, U., Willkerson, B., Minchev, K., Jemiolo, B., Bagley, J., Luden, N., & Trappe, S. (2013). Molecular insight into fast-twitch muscle fiber remodeling with taper. Medicine & Science in Sports & Exercise, 45(5), Supplement abstract number 755.

This study evaluated fiber type-specific (MHC I and IIa) growth-related gene response following identical pre-/post-taper 8-km runs in trained runners (N = 6). Pre- and four-hour post-8 km run gastrocnemius muscle biopsies were obtained pre- and post-taper. Fn14, Myostatin, Hsp72, MRF4, MuRF-1, and IGF1 mRNA levels were determined via qPCR.

In MHC IIa fibers, exercise increased Fn14 and decreased Myostatin expression post-taper while only Myostatin decreased pre-taper. In contrast, genes of interest in MHC I fibers remained relatively static post-taper with only an increase in Hsp72. Exercise decreased Myostatin and increased MRF4 pre-taper.

Implication. The post-taper run increase in Fn14 (strongly correlated to hypertrophy) coupled with favorable mRNA expression of potent negative muscle mass regulator Myostatin provides an initial molecular basis for taper-induced MHC IIa growth in these runners. Characterizing this unique model of exaggerated fast-fiber hypertrophy provides new insight into fiber type-specific size regulation with training and taper. This taper supports the principle that tapers mostly restore fast-twitch fibers and that heavy training suppresses much fast-twitch fiber function.

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