title: Quadriceps femoris muscle weakness and activation failure in patients with symptomatic knee osteoarthritis.
abstract: OBJECTIVE
  Quadriceps weakness is common in patients with knee osteoarthritis (OA), and has been attributed to failure of voluntary activation. Methodological differences may have contributed to previous reports of extensive failure of voluntary activation in patients with osteoarthritis. The purpose of this study was to determine the extent of quadriceps muscle weakness and activation failure in middle aged patients with symptomatic medial knee osteoarthritis using maximum voluntary isometric contractions (MVIC) and a burst superimposition technique.
  
  
  METHODS
  Measurements of quadriceps MVIC and extent of voluntary activation were made in 12 subjects with knee OA and 12 similarly aged uninjured subjects. Voluntary activation was tested by superimposing a train of electrical stimulation on a maximal effort volitional contraction of the quadriceps muscle.
  
  
  RESULTS
  The group of subjects with knee OA had significantly less quadriceps strength relative to body mass index (BMI) than the group of control subjects (p=0.010). No difference in voluntary activation was observed (p=0.233), however, 50% of the OA group, and only 25% of the control group failed to fully activate the quadriceps.
  
  
  CONCLUSIONS
  The finding of quadriceps weakness is consistent with past literature. Providing adequate instruction, feedback, and several attempts to maximally contract the muscle likely yielded greater volitional activation (thus less activation failure) than had been reported previously. This finding has implications for the rehabilitation of weakened quadriceps in patients with knee osteoarthritis.
authors.affiliation: Department of Physical Therapy and Biomechanics and Movement Science Program, 301 McKinly Laboratory, University of Delaware, Newark, DE 19716, USA.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Lewek
authors.forename: Katherine S
authors.initials: KS
authors.lastname: Rudolph
authors.forename: Lynn
authors.initials: L
authors.lastname: Snyder-Mackler
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D008875:Middle Aged; D018908:Muscle Weakness / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D020370:Osteoarthritis, Knee / Q000503:physiopathology*; D013848:Thigh; D014836:Volition