Longitudinal decline of lower extremity muscle power in healthy and mobility-limited older adults: influence of muscle mass, strength, composition, neuromuscular activation and single fiber contractile properties
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This longitudinal study examined the major physiological mechanisms that determine the age-related loss of lower extremity muscle power in two distinct groups of older humans. We hypothesized that after ~3 years of follow-up, mobility-limited older adults (mean age: 77.2 ± 4, n = 22, 12 females) would have significantly greater reductions in leg extensor muscle power compared to healthy older adults (74.1 ± 4, n = 26, 12 females).
Mid-thigh muscle size and composition were assessed using computed tomography. Neuromuscular activation was quantified using surface electromyography and vastus lateralis single muscle fibers were studied to evaluate intrinsic muscle contractile properties.
At follow-up, the overall magnitude of muscle power loss was similar between groups: mobility-limited: −8.5 % vs. healthy older: −8.8 %, P > 0.8. Mobility-limited elders had significant reductions in muscle size (−3.8 %, P < 0.01) and strength (−5.9 %, P < 0.02), however, these parameters were preserved in healthy older (P ≥ 0.7). Neuromuscular activation declined significantly within healthy older, but not in mobility-limited participants. Within both groups, the cross-sectional areas of type I and IIA muscle fibers were preserved while substantial increases in single fiber peak force (>30 %), peak power (>200 %) and unloaded shortening velocity (>50 %) were elicited at follow-up.
Different physiological mechanisms contribute to the loss of lower extremity muscle power in healthy older and mobility-limited older adults. Neuromuscular changes may be the critical early determinant of muscle power deficits with aging. In response to major whole muscle decrements, major compensatory mechanisms occur within the contractile properties of surviving single muscle fibers in an attempt to restore overall muscle power and function with advancing age.
KeywordsAging Lower extremity muscle power Single muscle fiber Longitudinal
This research was supported by the National Institute on Aging grant number AG18844 and based upon work supported by the US Department of Agriculture, under Agreement No. 58-1950-0-014. Any opinions, findings, conclusion, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the US Department of Agriculture. This research was also supported by the Boston Claude D. Pepper Older Americans Independence Center (1P30AG031679) and the Boston Rehabilitation Outcomes Center, funded by NIH Infrastructure Grant (1R24HD065688-01A1). This manuscript contributes to the requirements of a Ph.D. thesis supervised by Dr. Michael A. Conway, Trinity College Dublin, Ireland.
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