title: Age-related changes affect rat rotator cuff muscle function.
abstract: BACKGROUND
  The influence of age on rotator cuff function and muscle structure remains poorly understood. We hypothesize that normal aging influences rotator cuff function, muscle structure, and regulatory protein expression in an established rat model of aging.
  
  
  METHODS
  Seventeen rats were obtained from the National Institute on Aging. The supraspinatus muscles in 11 middle-aged (12 months old) and 6 old (28 months old) rats were studied for age-related changes in rotator cuff neuromuscular function by in vivo muscle force testing and electromyography (EMG). Changes in muscle structure and molecular changes were assessed with quantitative immunohistochemistry for myogenic determination factor 1 (MyoD) and myogenic factor 5 (Myf5) expression.
  
  
  RESULTS
  Old animals revealed significantly decreased peak tetanic muscle force at 0.5 N and 0.7 N preload tension (P < .05). The age of the animal accounted for 20.9% of variance and significantly influenced muscle force (P = .026). Preload tension significantly influenced muscle force production (P < .001) and accounted for 12.7% of total variance. There was regional heterogeneity in maximal compound motor action potential (CMAP) amplitude in the supraspinatus muscle; the proximal portion had a significantly higher CMAP than the middle and distal portions (P < .05). The expression of muscle regulatory factors MyoD and Myf5 was significantly decreased in old animals compared with middle-aged animals (P < .05).
  
  
  CONCLUSIONS
  The normal aging process in this rat model significantly influenced contractile strength of the supraspinatus muscle and led to decreased expression of muscle regulatory factors. High preload tensions led to a significant decrease in force production in both middle-aged and old animals.
authors.affiliation: Department of Orthopaedic Surgery, Wake Forest School of Medicine, Medical Center Boulevard, Winston-Salem, NC, USA; The Neuroscience Program, Wake Forest University Graduate School of Arts and Sciences, Medical Center Boulevard, Winston-Salem, NC, USA. Electronic address: jplate@wakehealth.edu.
authors.forename: Johannes F
authors.initials: JF
authors.lastname: Plate
authors.affiliation: The Neuroscience Program, Wake Forest University Graduate School of Arts and Sciences, Medical Center Boulevard, Winston-Salem, NC, USA.
authors.forename: Lauren A
authors.initials: LA
authors.lastname: Pace
authors.affiliation: Department of Orthopaedic Surgery, Wake Forest School of Medicine, Medical Center Boulevard, Winston-Salem, NC, USA.
authors.forename: Thorsten M
authors.initials: TM
authors.lastname: Seyler
authors.affiliation: The Neuroscience Program, Wake Forest University Graduate School of Arts and Sciences, Medical Center Boulevard, Winston-Salem, NC, USA.
authors.forename: Ramon J
authors.initials: RJ
authors.lastname: Moreno
authors.affiliation: Department of Orthopaedic Surgery, Wake Forest School of Medicine, Medical Center Boulevard, Winston-Salem, NC, USA.
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Smith
authors.affiliation: Department of Orthopaedic Surgery, Wake Forest School of Medicine, Medical Center Boulevard, Winston-Salem, NC, USA.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Tuohy
authors.affiliation: Department of Orthopaedic Surgery, Wake Forest School of Medicine, Medical Center Boulevard, Winston-Salem, NC, USA.
authors.forename: Sandeep
authors.initials: S
authors.lastname: Mannava
mesh_terms: D000375:Aging / Q000378:metabolism / Q000502:physiology*; D000818:Animals; D004576:Electromyography; D007150:Immunohistochemistry; D008297:Male; D023421:Models, Animal; D009119:Muscle Contraction / Q000502:physiology; D017570:MyoD Protein / Q000096:biosynthesis; D051787:Myogenic Regulatory Factor 5 / Q000096:biosynthesis; D051381:Rats; D017006:Rotator Cuff / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology*
chemical_list: D017570:MyoD Protein; C057363:MyoD1 myogenic differentiation protein; D051787:Myogenic Regulatory Factor 5
keywords: Age; Basic Science; In Vivo Animal Study; electromyography; functional assessment; myogenic regulatory factors; shoulder; supraspinatus