title: 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.
abstract: OBJECTIVE
  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).
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.forename: Kieran F
authors.initials: KF
authors.lastname: Reid
authors.forename: Evan
authors.initials: E
authors.lastname: Pasha
authors.forename: Gheorghe
authors.initials: G
authors.lastname: Doros
authors.forename: David J
authors.initials: DJ
authors.lastname: Clark
authors.forename: Carolynn
authors.initials: C
authors.lastname: Patten
authors.forename: Edward M
authors.initials: EM
authors.lastname: Phillips
authors.forename: Walter R
authors.initials: WR
authors.lastname: Frontera
authors.forename: Roger A
authors.initials: RA
authors.lastname: Fielding
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D035002:Lower Extremity / Q000000981:diagnostic imaging / Q000294:innervation / Q000502:physiology*; D008297:Male; D051346:Mobility Limitation; D009119:Muscle Contraction; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D053580:Muscle Strength; D052097:Quadriceps Muscle / Q000000981:diagnostic imaging / Q000294:innervation / Q000502:physiology*; D014057:Tomography, X-Ray Computed