Force-Generation Capacity of Single Vastus Lateralis Muscle Fibers and Physical Function Decline With Age in African Green Vervet Monkeys

Abstract

Previous studies on the contractile properties of human myofibrils reported increase, decrease, or no change with aging, perhaps due to the differences in physical activity, diet, and other factors. This study examined physical performance and contractile characteristics of myofibrils of vastus lateralis (VL) muscle in young adult and old African green vervet monkeys. Animals were offered the same diet and lived in the same enclosures during development, so we were able to examine skeletal muscle function in vivo and in vitro with fewer potential confounding factors than are typical in human research studies. Fiber atrophy alone did not account for the age-related differences in specific force and maximal power output. Regression modeling used to identify factors contributing to lower fiber force revealed that age is the strongest predictor. Our results support a detrimental effect of aging on the intrinsic force and power generation of myofilament lattice and physical performance in vervet monkeys.Key Words: Skeletal muscle-Physical function-Monkey-Aging.Received December 9, 2011; accepted april 18, 2012 Decision editor: Rafael de cabo, PhD L OSS of mass and force and slower shortening velocity are hallmarks of aging skeletal muscle (1-4). Loss of fibers and selective fast-fiber atrophy (5-7), excitation-contraction uncoupling (8,9), and perturbations to cross-bridge function (10-12) seem to account for the reduced ability of whole muscle to produce force under isometric or shortening conditions in old age.Even though this loss in muscular function with aging is widely accepted, the intrinsic capacity of single muscle fiber to sustain force throughout the lifetime of higher mammals, including humans, is still uncertain. Several studies have reported a significant reduction in the contractile properties of skinned single muscle fiber with aging, including reduced maximal isometric force (P o ), normalized force to fiber cross-sectional area (CSA; P o /cSa), and unloaded shortening velocity (V o ; 11,13-16). These reports indicate that both the size and quality of individual fibers decrease with age. The proposed mechanisms are either fewer strongly bound cross-bridges during maximal activation or the force-generating ability of each cross-bridge is reduced (11,13).In contrast, a number of aging studies have reported preserved single muscle fiber contractile properties (17)(18)(19)(20) or even a slight increase in type I and IIa fibers' maximal force (21,22) and conclude that the intrinsic properties of cross-bridge mechanics are preserved with aging. These contrasting results could derive from diverse factors influencing muscle fiber structure and/or function, such as variation in the subject nutrition, genetics, and history of physical activity (23,24). Furthermore, the participants' diverse education, occupational history, and socioeconomic status complicate interpretation of various outcomes (25).Recently, we developed and applied a battery of physical performance tests modeled after human interventional s...

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