Eccentric contraction-induced injury to type I, IIa, and IIa/IIx muscle fibers of elderly adults

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.

Figures

Fig. 1

Experimental protocol. a Shows five superimposed length steps and below those, the corresponding superimposed force responses obtained during a slack test. The time for force redevelopment was plotted against the slack length step and fit by linear regression to yield Vo (per fiber length). b The same fiber was activated and lengthened 0.25 fiber length at a velocity of 0.50 Vo, held at the final length for 100 ms, and then slacked to zero the transducer. The work done on the fiber was calculated as the force–time integral during the eccentric contraction (hatched area). c Shows one posttreatment force evaluation for the same fiber. As in b, a slack step used to establish a consistent force baseline. Pre-treatment and posttreatment force (Po) were evaluated as shown. Control fibers were treated in an identical manner except that the fiber was not lengthened during treatment in b. Vertical and horizontal calibration bars represent 200 μm for length, 0.5 mN for force, and 100 ms for time. Note that force and length calibrations are the same across all panels but that the time scale is compressed in b

Fig. 2

Silver-stained 6% polyacrylamide gel illustrating identification of type I, IIa, and IIx MHC isoforms in skinned segments of human vastus lateralis fibers. Lane 6 contains a human MHC isoform standard. All other lanes were loaded with a single skinned muscle fiber segment

Fig. 3

Mechanical characteristics of the eccentric treatments. a Pre-treatment force. b Maximal force attained during the eccentric contraction. c Ratio of maximal eccentric force and pre-treatment force. d Work done on the fiber during the eccentric contraction. Values are mean ± SE. Gray, black, and gray with hatching represent type I (n = 48), IIa (n = 51), and IIa/IIx (n = 13) fibers, respectively

Fig. 4

Images of a fiber under relaxed and Ca2+-activated conditions, before and after a single eccentric contraction. Note that the two pre-treatment images were not obtained at the same location along the length of the fiber as the two posttreatment images. Also, the images obtained during activation were not necessarily captured when the fiber was at its peak force. Note the reduction in skinned fiber cross-sectional area during Ca2+-activation (Kawai et al. 1993). Not all fibers displayed visible signs of sarcomere disruption as shown here. Scale represents 50 μm

Fig. 5

Modeling of post-eccentric force. Over 91% of variability in post-eccentric force could be explained by a regression model that used pre-eccentric force and fiber MHC isoform content as independent variables. Shown are the raw data points (type I, open circles; type IIa, filled circles; type IIa/IIx, filled triangles) and the regression lines for each fiber type when pre-eccentric force is held constant. The y-intercept of the regression line describing the response of the type IIa fibers was significantly different from that of the type I fibers (p < 0.001) and the type IIa/IIx fibers (p < 0.05). See text for regression coefficients. The dashed line is the line of identity

Fig. 6

Comparing responses of fibers from young and old subject populations to a single eccentric contraction. Multiple linear regression revealed significant interactions between fiber type and age when post-eccentric force was modeled from predictors of pre-eccentric force, fiber MHC isoform content, and age group (r2 = 0.91, p < 0.0001). Pre-eccentric force had a significant effect on post-eccentric force independent of fiber type (slope = 0.88, p < 0.001). With pre-eccentric force held constant, the predicted post-force of type I fibers from young subjects (designated Y-I) was not different (p = 0.689) from that of type I fibers from old subjects (designated O-I), i.e., the y-intercepts of the regression lines are not statistically different. However, at any given pre-eccentric force, force deficits of type IIa fibers from young subjects (designated Y-IIa) following an eccentric contraction would be significantly greater (p < 0.001) than the deficits predicted for type IIa fibers from old subjects (designated O-IIa), i.e., the y-intercepts of these responses are significantly different. Likewise, type IIa/IIx fibers from old subjects (designated O-IIa/IIx) were predicted to have significantly greater (p = 0.041) force deficits than type IIa/IIx fibers of young subjects (designated Y-IIa/IIx). Note that the responses of the young type I, young type IIa, and old type I fibers virtually overlap and cannot be individually distinguished in the figure. The dotted line is the line of identity. Data for young subjects previously reported by Choi and Widrick (2010); data for old subjects from present study