title: Association between knee extensor strength and EMG activities during squat movement.
abstract: OBJECTIVE
  The present study aimed to clarify how the force-generating capability of quadriceps femoris (QF) is associated to its surface EMG activity during a body mass-based squat movement.
  
  
  METHODS
  Isometric knee extension torque (KET) during maximal voluntary contraction and EMG activities of the rectus femoris and vastus lateralis muscles during a body mass-based squat movement were determined in 53 men and 48 women age 19-90 yr, including 18 frail elderly persons who used the long-term care insurance system. The rectified EMG signals during the squat movement were averaged and normalized as the relative value (%EMG(max)) to that during maximal voluntary contraction. The %EMG(max) values for rectus femoris and vastus lateralis were averaged and used as an index representing the level of muscular activities of QF during the squat movement (QF %EMG(max)).
  
  
  RESULTS
  QF %EMG(max) was nonlinearly related to KET relative to body mass (KET/BM). Linear piecewise continuous regression analysis showed that there was a breakpoint of 1.9 N·m·kg(-1) in the relationship between the two variables. In individuals with KET/BM less than 1.9 N·m·kg(-1), QF %EMG(max) rapidly increased as KET/BM decreased.
  
  
  CONCLUSIONS
  The current results indicate that the activity level of QF during a body mass-based squat movement is influenced by its force generation capability. For individuals with a KET/BM less than 1.9 N·m·kg(-1), body mass-based squat movement is considered to be a fairly high-intensity exercise. The breakpoint of 1.9 N·m·kg(-1) may be assumed to be a threshold level of knee extensor strength, which should be maintained for performing the activities of daily living without great difficulty.
authors.affiliation: National Institute of Fitness and Sports in Kanoya, Kagoshima, Japan.
authors.forename: Eiji
authors.initials: E
authors.lastname: Fujita
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Kanehisa
authors.forename: Yasuhide
authors.initials: Y
authors.lastname: Yoshitake
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Fukunaga
authors.forename: Hidetsugu
authors.initials: H
authors.lastname: Nishizono
mesh_terms: D000203:Activities of Daily Living; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D004576:Electromyography; D005260:Female; D016330:Frail Elderly; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D007717:Knee / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009068:Movement; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D019415:Torque; D014891:Weight Lifting / Q000502:physiology*; D055815:Young Adult