title: Electromyographic patterns suggest changes in motor unit physiology associated with early osteoarthritis of the knee.
abstract: OBJECTIVE
  To assess characteristics of active motor units (MUs) during volitional vastus medialis (VM) activation in adults with symptomatic knee osteoarthritis (OA) across the spectrum of radiographic severity and age-comparable healthy control volunteers.
  
  
  METHODS
  We evaluated 39 participants (age 65+/-3 years) in whom weight-bearing knee X-rays were assigned a Kellgren & Lawrence (KL) grade (18 with KL grade=0; four each with KL grades=1, 2 and 4; nine with grade 3). Electromyography (EMG) signals were simultaneously acquired using surface [surface EMG (S-EMG)] and intramuscular needle electrodes, and analyzed by decomposition-enhanced spike-triggered averaging to obtain estimates of size [surface-represented MU action potentials (S-MUAP) area], number [MU recruitment index (MURI)] and firing rates [MU firing rates (mFR)] of active MUs at 10%, 20%, 30% and 50% effort relative to maximum voluntary force [maximal voluntary isometric contraction (MVIC)] during isometric knee extension.
  
  
  RESULTS
  Knee extensor MVIC was lower in OA participants, especially at higher KL grades (P=0.05). Taking the observed force differences into account, OA was also associated with activation of larger MUs (S-MUAP area/MVICx%effort; P<0.0001). In contrast, the estimated number of active units (MURI/MVICx%effort) changed differently as effort increased from 10% to 50% and was higher with advanced OA (KL=3, 4) than controls (P=0.0002).
  
  
  CONCLUSIONS
  VM activation changes at the level of the MU with symptomatic knee OA, and this change is influenced by radiographic severity. Poor muscle quality may explain the pattern observed with higher KL grades, but alternative factors (e.g., nerve or joint injury, physical inactivity or muscle composition changes) should be examined in early OA.
authors.affiliation: Clinical Research Branch, National Institute on Aging Intramural Research Program (NIA-IRP), National Institutes of Health, MD 21225, USA. lingsh@grc.nia.nih.gov
authors.forename: S M
authors.initials: SM
authors.lastname: Ling
authors.forename: R A
authors.initials: RA
authors.lastname: Conwit
authors.forename: L
authors.initials: L
authors.lastname: Talbot
authors.forename: M
authors.initials: M
authors.lastname: Shermack
authors.forename: J E
authors.initials: JE
authors.lastname: Wood
authors.forename: E M
authors.initials: EM
authors.lastname: Dredge
authors.forename: M J
authors.initials: MJ
authors.lastname: Weeks
authors.forename: D R
authors.initials: DR
authors.lastname: Abernethy
authors.forename: E J
authors.initials: EJ
authors.lastname: Metter
mesh_terms: D000367:Age Factors; D000368:Aged; D016022:Case-Control Studies; D004576:Electromyography / Q000379:methods; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D051346:Mobility Limitation; D009046:Motor Neurons / Q000502:physiology*; D020370:Osteoarthritis, Knee / Q000503:physiopathology*; D052097:Quadriceps Muscle / Q000502:physiology*; D012044:Regression Analysis