|Choi, Seung jun|
Submitted to: Journal of the American Aging Association (AGE)
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/1/2011
Publication Date: 2/1/2012
Citation: Choi, S., Lim, J., Nibaldi, E.G., Phillips, E.M., Frontera, W.R., Fielding, R.A., Widrick, J.J. 2012. Eccentric contraction-induced injury to type I, IIa, and IIa/IIx muscle fibers of elderly adults. Journal of the American Aging Association (AGE). 34(1):215-226. Interpretive Summary: Previous studies in humans and animal models have shown that older muscles are more susceptible to injury than younger muscles. In this study we examined the response of different types of muscle cells (fast twitch vs. slow) from older humans to see if there are additional differences in the response to muscle injury between fast twitch and slow twitch fibers. Using sophisticated techniques we were able to show that in older humans the fast twitch muscle fibers are more susceptible to muscle injury that slow twitch fibers. These findings may help explain why there are more fast twitch fibers lost during the aging process than slow twitch fibers.
Technical Abstract: Muscles of old laboratory rodents experience exaggerated force losses after eccentric contractile activity. We extended this line of inquiry to humans and investigated the influence of fiber myosin heavy chain (MHC) isoform content on the injury process. Skinned muscle fiber segments, prepared from vastus lateralis biopsies of elderly men and women (78+/-2 years, N=8), were subjected to a standardized eccentric contraction (strain, 0.25 fiber length; velocity, 0.50 unloaded shortening velocity). Injury was assessed by evaluating pre- and post-eccentric peak Ca(2+)-activated force per fiber cross-sectional area (F (max)). Over 90% of the variability in post-eccentric F (max) could be explained by a multiple linear regression model consisting of an MHC-independent slope, where injury was directly related to pre-eccentric F (max), and MHC-dependent y-intercepts, where the susceptibility to injury could be described as type IIa/IIx fibers > type IIa fibers > type I fibers. We previously reported that fiber type susceptibility to the same standardized eccentric protocol was type IIa/IIx > type IIa = type I for vastus lateralis fibers of 25-year-old adults (Choi and Widrick, Am J Physiol Cell Physiol 299:C1409-C1417, 2010). Modeling combined data sets revealed significant age by fiber type interactions, with post-eccentric F (max) deficits greater for type IIa and type IIa/IIx fibers from elderly vs. young subjects at constant pre-eccentric F (max). We conclude that the resistance of the myofilament lattice to mechanical strain has deteriorated for type IIa and type IIa/IIx, but not for type I, vastus lateralis fibers of elderly adults.