title: Measuring voluntary quadriceps activation: Effect of visual feedback and stimulus delivery.
abstract: BACKGROUND
  Quadriceps voluntary activation, assessed via the superimposed burst technique, has been extensively studied in a variety of populations as a measure of quadriceps function. However, a variety of stimulus delivery techniques have been employed, which may influence the level of voluntary activation as calculated via the central activation ratio (CAR). The purpose was to determine the effect of visual feedback, stimulus delivery, and perceived discomfort on maximal voluntary isometric contraction (MVIC) peak torque and the CAR.
  
  
  METHODS
  Quadriceps CAR was assessed in 14 individuals on two days using three stimulus delivery methods; (1) manual without visual feedback, (2) manual with visual feedback, and (3) automated with visual feedback.
  
  
  RESULTS
  MVIC peak torque and the CAR were not different between the automated with visual feedback (MVIC=3.25, SE=0.14Nm/kg; CAR=88.63, SE=1.75%) and manual with visual feedback (MVIC=3.26, SE=0.13Nm/kg, P=0.859; CAR=89.06, SE=1.70%, P=0.39) stimulus delivery methods. MVIC (2.99, SE=0.12Nm/kg) and CAR (85.32, SE=2.10%) were significantly lower using manual without visual feedback compared to manual with visual feedback and automated with visual feedback (CAR P<0.001; MVIC P<0.001). Perceived discomfort was lower in the second session (P<0.05).
  
  
  CONCLUSIONS
  Utilizing visual feedback ensures participant MVIC, and may provide a more accurate assessment of quadriceps voluntary activation.
authors.affiliation: Human Movement Science Curriculum, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States; Department of Exercise and Sports Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
authors.forename: Brittney A
authors.initials: BA
authors.lastname: Luc
authors.affiliation: Human Movement Science Curriculum, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States; Department of Exercise and Sports Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
authors.forename: Matthew H
authors.initials: MH
authors.lastname: Harkey
authors.affiliation: Department of Exercise and Sports Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
authors.forename: Gabrielle D
authors.initials: GD
authors.lastname: Arguelles
authors.affiliation: Department of Exercise and Sports Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
authors.forename: J Troy
authors.initials: JT
authors.lastname: Blackburn
authors.affiliation: Department of Exercise and Sports Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
authors.forename: Eric D
authors.initials: ED
authors.lastname: Ryan
authors.affiliation: Department of Exercise and Sports Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
authors.forename: Brian
authors.initials: B
authors.lastname: Pietrosimone
mesh_terms: D000328:Adult; D056228:Feedback, Sensory / Q000502:physiology*; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D053581:Muscle Strength Dynamometer; D018482:Muscle, Skeletal / Q000502:physiology; D010775:Photic Stimulation / Q000379:methods*; D052097:Quadriceps Muscle / Q000502:physiology*; D019415:Torque; D014796:Visual Perception / Q000502:physiology*; D055815:Young Adult
keywords: Agreement; Central activation ratio; Perceived discomfort; Reliability; Superimposed burst