title: Calcium-activated force of human muscle fibers following a standardized eccentric contraction.
abstract: Peak Ca(2+)-activated specific force (force/fiber cross-sectional area) of human chemically skinned vastus lateralis muscle fiber segments was determined before and after a fixed-end contraction or an eccentric contraction of standardized magnitude (+0.25 optimal fiber length) and velocity (0.50 unloaded shortening velocity). Fiber myosin heavy chain (MHC) isoform content was assayed by SDS-PAGE. Posteccentric force deficit, a marker of damage, was similar for type I and IIa fibers but threefold greater for type IIa/IIx hybrid fibers. A fixed-end contraction had no significant effect on force. Multiple linear regression revealed that posteccentric force was explained by a model consisting of a fiber type-independent and a fiber type-specific component (r(2) = 0.91). Preeccentric specific force was directly associated with a greater posteccentric force deficit. When preeccentric force was held constant, type I and IIa fibers showed identical susceptibility to damage, while type IIa/IIx fibers showed a significantly greater force loss. This heightened sensitivity to damage was directly related to the amount of type IIx MHC in the hybrid fiber. Our model reveals a fiber-type sensitivity of the myofilament lattice or cytoskeleton to mechanical strain that can be described as follows: type IIa/IIx > type IIa = type I. If these properties extend to fibers in vivo, then alterations in the number of type IIa/IIx fibers may modify a muscle's susceptibility to eccentric damage.
authors.affiliation: Krivickas Muscle Cell Physiology Laboratory, Department of Physical Medicine and Rehabilitation, Spaulding Rehabilitation Hospital and Harvard Medical School, Boston, Massachusetts 02114, USA.
authors.forename: Seung Jun
authors.initials: SJ
authors.lastname: Choi
authors.forename: Jeffrey J
authors.initials: JJ
authors.lastname: Widrick
mesh_terms: D000328:Adult; D002118:Calcium / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000737:chemistry / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000737:chemistry / Q000502:physiology*; D018995:Myosin Heavy Chains / Q000032:analysis / Q000502:physiology; D052097:Quadriceps Muscle / Q000737:chemistry / Q000502:physiology; D055815:Young Adult
chemical_list: D018995:Myosin Heavy Chains; D002118:Calcium