title: Patellofemoral joint kinetics while squatting with and without an external load.
abstract: METHODS
  Single-group repeated measures design.
  
  
  OBJECTIVE
  To quantify patellofemoral joint reaction forces and stress while squatting with and without an external load.
  
  
  BACKGROUND
  Although squatting exercises in the rehabilitation setting are often executed to a relatively shallow depth in order to avoid the higher joint forces associated with increased knee flexion, objective criteria for ranges of motion have not been established.
  
  
  METHODS
  Fifteen healthy adults performed single-repetition squats to 90 degrees of knee flexion without an external load and with an external load (35% of the subject's body weight [BW]). Anthropometric data, three-dimensional kinematics, and ground reaction forces were used to calculate knee extensor moments (inverse dynamics approach), while a biomechanical model of the patellofemoral joint was used to quantify the patellofemoral joint reaction forces and patellofemoral joint stress. Data were analyzed during the eccentric (0-90 degrees) and concentric (90-0 degrees phases of the squat maneuver.
  
  
  RESULTS
  In both conditions, knee extensor moments, patellofemoral joint reaction forces, and patellofemoral joint stress increased significantly with greater knee flexion angles (P < 0.05). Peak patellofemoral joint force and stress was observed at 90 degrees of knee flexion. Patellofemoral joint stress at 45 degrees, 60 degrees, 75 degrees, and 90 degrees of knee flexion during the eccentric phase, and at 75 degrees and 90 degrees during the concentric phase, was significantly greater in the loaded trials versus the unloaded trials.
  
  
  CONCLUSIONS
  The data indicate that during squatting, patellofemoral joint stress increases as the knee flexion angle increases, and that the addition of external resistance further increases patellofemoral joint stress. These findings suggest that in order to limit patellofemoral joint stress during squatting activities, clinicians should consider limiting terminal joint flexion angles and resistance loads.
authors.affiliation: Department of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles 90089-9006, USA.
authors.forename: David A
authors.initials: DA
authors.lastname: Wallace
authors.forename: George J
authors.initials: GJ
authors.lastname: Salem
authors.forename: Ruben
authors.initials: R
authors.lastname: Salinas
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Powers
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D011446:Prospective Studies; D016059:Range of Motion, Articular / Q000502:physiology*; D012016:Reference Values; D013314:Stress, Mechanical; D016474:Weight-Bearing