XXXX forepaw lumbrical muscles are resistant to age-related declines in force production.

@entity19 @entity33110
A progressive loss of skeletal muscle mass and force generating capacity occurs with aging. @entity19 are commonly used in the study of aging-associated changes in muscle size and strength, with most models of aging demonstrating 15-35% reductions in muscle mass, cross-sectional area ( @entity33110 ), maximum isometric force production (Po) and specific force (sPo), which is Po/ @entity33110 . The lumbrical muscle of the @entity19 forepaw is exceptionally small, with corresponding short diffusion distances that make it ideal for in vitro pharmacological studies and measurements of contractile properties. However, the aging-associated changes in lumbrical function have not previously been reported. To address this, we tested the hypothesis that compared to adult (12month old) @entity19 , the forepaw lumbrical muscles of old (30month old) @entity19 exhibit aging-related declines in size and force production similar to those observed in larger limb muscles. We found that the forepaw lumbricals were composed exclusively of fibers with type II myosin heavy chain isoforms, and that the muscles accumulated connective tissue with aging. There were no differences in the number of fibers per whole-muscle cross-section or in muscle fiber @entity33110 . The whole muscle @entity33110 in old @entity19 was increased by 17%, but the total @entity33110 of all muscle fibers in a whole-muscle cross-section was not different. No difference in Po was observed, and while sPo normalized to total muscle @entity33110 was decreased in old @entity19 by 22%, normalizing Po by the total muscle fiber @entity33110 resulted in no difference in sPo. Combined, these results indicate that forepaw lumbrical muscles from 30month old @entity19 are largely protected from the aging-associated declines in size and force production that are typically observed in larger limb muscles.
