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HIGHLIGHTED TOPICS
Biomechanics and Mechanotransduction in Cells and Tissues

Effects of resistance training on myosin function studied by the in vitro motility assay in young and older men

¹Department of Experimental Medicine, Human Physiology Unit, University of Pavia, Pavia, Italy; Departments of ²Physiology and ³Clinical Neurosciences, Royal Free & University College Medical School, University College London, London, United Kingdom; and ⁴Interuniversity Institute of Myology, Pavia, Italy

Submitted 1 October 2004 ; accepted in final form 23 January 2005

It is generally believed that the maximum shortening velocity (V_o) of a skeletal muscle fiber type does not vary unless a change in myosin heavy chain (MHC) isoform composition occurs. However, recent findings have shown that V_o of a given fiber type can change after training, suggesting the hypothesis that the function of myosin can vary without a change in isoform. The present study addressed the latter hypothesis by studying the function of isolated myosin isoforms by the use of the in vitro motility assay (IVMA) technique. Four young (age 23–29 yr, YO) and four elderly men (age 68–82 yr, EL) underwent a 12-wk progressive resistance training program of the knee extensor muscles and to one pre- and one posttraining biopsy of the vastus lateralis muscle. The significant increase in one-repetition maximum posttraining in both YO and EL indicated that training was effective. After training, MHC isoform composition showed a shift from MHC_2X toward MHC_2A in YO and no shift in EL. The velocity of sliding (V_f) of actin filaments on pure myosin isoforms extracted from single fibers was studied in IVMA. One hundred sixty IVMA samples were prepared from 480 single fibers, and at least 50 filaments were analyzed in each experiment. Whereas no training-induced change was observed in V_f of myosin isoform 1 either in YO or in EL, a significant increase in V_f of myosin isoform 2A after training was observed in both YO (18%) and EL (19%). The results indicate that resistance training can change the velocity of the myosin molecule.

myosin isoforms; velocity of shortening

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