title: Association of thigh muscle strength with knee symptoms and radiographic disease stage of osteoarthritis: data from the Osteoarthritis Initiative.
abstract: OBJECTIVE
  To determine whether thigh muscle strength differs between symptomatic and asymptomatic knees, and/or different radiographic strata of knee osteoarthritis (KOA).
  
  
  METHODS
  Isometric extensor and flexor strength were analyzed in 3,809 Osteoarthritis Initiative participants (2,201 women and 1,608 men) with central radiographic Kellgren/Lawrence (K/L) grade readings. Isometric strength measurements were stratified by radiographic disease status (K/L grades 0, 1, 2, and 3/4) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain scores. Age-adjusted, separate-slopes analysis of covariance models was used to compare strength between "symptomatic" (WOMAC score range 5-20) and "asymptomatic" (WOMAC score = 0) legs within and across K/L grade strata. Exploratory analyses focused on strength normalized to body weight and symptom frequency.
  
  
  RESULTS
  Isometric strength was significantly lower in symptomatic than in asymptomatic legs: -11 to -13% for extensor strength and -7 to -16% for flexor strength (P < 0.0001 for both) in men, and -9 to -17% (P = 0.029) for extensor strength, and -10 to -21% (P = 0.049) for flexor strength in women. Similar observations were made for pain frequency strata. Extensor and flexor strength were not significantly different across K/L grade strata in asymptomatic legs in either sex (P ≥ 0.12). However, strength normalized to body weight was lower at higher K/L grades in both sexes (P ≤ 0.02) because the body mass index was greater in participants with more advanced radiographic disease.
  
  
  CONCLUSIONS
  Knee symptoms (i.e., pain) appear to be the relevant determinant of isometric knee extensor and flexor strength in KOA, whereas no direct association between strength and radiographic severity was observed. These findings suggest that the reduction in thigh muscle strength in KOA is related to pain but not to the structural (radiographic) disease status.
authors.affiliation: Paracelsus Medical University, Salzburg, Austria.
authors.forename: Anja
authors.initials: A
authors.lastname: Ruhdorfer
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Wirth
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Hitzl
authors.forename: Michael
authors.initials: M
authors.lastname: Nevitt
authors.forename: Felix
authors.initials: F
authors.lastname: Eckstein
mesh_terms: D000368:Aged; D015992:Body Mass Index; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000000981:diagnostic imaging* / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D053580:Muscle Strength / Q000502:physiology*; D020370:Osteoarthritis, Knee / Q000000981:diagnostic imaging* / Q000503:physiopathology; D010147:Pain Measurement; D052097:Quadriceps Muscle / Q000000981:diagnostic imaging / Q000503:physiopathology*; D011859:Radiography; D012720:Severity of Illness Index