title: Normal aging alters in vivo passive biomechanical response of the rat gastrocnemius-Achilles muscle-tendon unit.
abstract: Predisposition to Achilles tendon (AT) ruptures in middle-aged individuals may be associated with age-related changes to inherent passive biomechanical properties of the gastrocnemius-Achilles (GC-AT) muscle-tendon unit, due to known muscle-tendon structural changes in normal aging. The goal of this study was to determine whether the passive biomechanical response of the GC-AT muscle-tendon unit was altered with age in 6 young (8 months) and 6 middle-aged (24 months) F344xBN hybrid rats from the National Institute on Aging colony. Fung's quasilinear viscoelastic (QLV) model was used to determine in vivo history and time-dependent load-relaxation response of the GC-AT. Effective stiffness and modulus were also estimated using linear regression analysis. Fung's QLV revealed a significantly decreased magnitude of the relaxation response (parameter C, p=0.026) in middle-aged animals compared to young animals (0.108±0.007 vs. 0.144±0.015), with similar time-dependent viscous GC-AT properties (τ(1), τ(2)). The product of elastic parameters (A*B), which represents the initial slope of the elastic response, was significantly increased by 50% in middle-aged rats (p=0.014). Estimated GC-AT stiffness increased 28% at peak tensions in middle-aged rats (2.7±0.2 N/mm) compared to young rats (1.9±0.2 N/mm; p=0.036). While the limitations of this animal model must be considered, the changes we describe could be associated with the observation that GC-AT pathology and injury is more common in middle-aged individuals. Further studies are necessary to characterize the load-to-failure behavior of AT in middle-aged compared to young animals.
authors.affiliation: Department of Orthopaedic Surgery, Wake Forest School of Medicine, Winston-Salem, NC 27157-1070, USA. jplate@wakehealth.edu
authors.forename: Johannes F
authors.initials: JF
authors.lastname: Plate
authors.forename: Walter F
authors.initials: WF
authors.lastname: Wiggins
authors.forename: Patrick
authors.initials: P
authors.lastname: Haubruck
authors.forename: Aaron T
authors.initials: AT
authors.lastname: Scott
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Smith
authors.forename: Katherine R
authors.initials: KR
authors.lastname: Saul
authors.forename: Sandeep
authors.initials: S
authors.lastname: Mannava
mesh_terms: D000125:Achilles Tendon / Q000473:pathology / Q000503:physiopathology*; D000293:Adolescent; D000328:Adult; D000375:Aging; D000818:Animals; D004548:Elasticity; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000473:pathology / Q000503:physiopathology*; D051381:Rats; D016474:Weight-Bearing