title: Induced anterior knee pain immediately reduces involuntary and voluntary quadriceps activation.
abstract: OBJECTIVE
  To examine the immediate effects of experimentally induced anterior knee pain (AKP) on involuntary and voluntary quadriceps strength and activation.
  
  
  METHODS
  Crossover 3 × 3 randomized controlled laboratory study with repeated measures.
  
  
  METHODS
  Human Performance Research Center, Brigham Young University.
  
  
  METHODS
  Thirteen neurologically sound volunteers (age, 21.9 ± 3.2 years).
  
  
  METHODS
  Subjects underwent 3 different conditions (pain, sham, and control). To induce AKP and sham condition, 5% sodium chloride and 0.9% sodium chloride (total volume of 1.0 mL for each condition), respectively, were injected into the infrapatellar fat pad on the dominant leg. No injection was performed for the control condition.
  
  
  METHODS
  The vastus medialis peak Hoffmann reflex normalized by the peak motor response (H:M ratio) was used to measure involuntary quadriceps activation. Quadriceps central activation ratio (CAR) using maximal isometric knee extension torque (N·m) was calculated to assess voluntary quadriceps activation. The visual analog scale was used to measure pain perception.
  
  
  RESULTS
  Our pain model increased perceived pain immediately after the 5% hypertonic saline injection and pain lasted for 12 minutes on average (F40,743 = 16.85, P < 0.001). During the pain condition, subjects showed a 12% decrease in H:M ratio (F2,59 = 8.64, P < 0.001), a 34% decrease in maximal isometric knee extension torque (F2,59 = 5.89, P < 0.01), and a 5% decrease in CAR (F2,59 = 3.83, P = 0.03).
  
  
  CONCLUSIONS
  Our data showed that joint pain may be an independent factor to alter function of the muscles surrounding the painful joint. Both involuntary and voluntary inhibitory pathways may play a role in an immediate reduction of muscle activation.
authors.affiliation: Sports Science Research Centre, Department of Physical Education, Kyungpook National University, Bukgu, Daegu, Korea. jihongpark@knu.ac.kr
authors.forename: Jihong
authors.initials: J
authors.lastname: Park
authors.forename: J Ty
authors.initials: JT
authors.lastname: Hopkins
mesh_terms: D000328:Adult; D018592:Cross-Over Studies; D004576:Electromyography; D006801:Humans; D007719:Knee Joint / Q000503:physiopathology*; D059352:Musculoskeletal Pain / Q000209:etiology / Q000503:physiopathology*; D010147:Pain Measurement; D058748:Pain Perception; D052097:Quadriceps Muscle / Q000503:physiopathology*; D016059:Range of Motion, Articular; D012018:Reflex; D012462:Saline Solution, Hypertonic; D019415:Torque; D055815:Young Adult
chemical_list: D012462:Saline Solution, Hypertonic