title: A clinically applicable EMG-force model to quantify active stabilization of the knee after a lesion of the anterior cruciate ligament.
abstract: OBJECTIVE
  To investigate whether a simple electromyography-force (EMG-force) model can be used to measure different levels of co-contraction about the knee for healthy subjects and patients with an anterior cruciate ligament deficiency.
  
  
  METHODS
  To evaluate an EMG-to-force processing model, two groups of subjects, with and without deficiency of the anterior cruciate ligament, participated in experiments in which surface EMG, kinematics and kinetics about the knee were recorded during isokinetic and functional movements.
  
  
  BACKGROUND
  Clinical and biomechanical evidence supports the hypothesis that higher level of co-contraction of quadriceps and hamstrings provide an active stabilization of the knee to compensate for the lost anterior cruciate ligament. To quantify the level of co-contraction, the contribution of both agonist and antagonist muscles to the net joint moment must be known.
  
  
  METHODS
  Surface EMG levels were calibrated to moment by means of a limited number of isokinetic contractions about the knee. With these calibration values, an estimate of the muscle moments during a vertical jump were obtained and compared with the net joint moment, calculated with inverse dynamics. Also co-contraction indices were determined.
  
  
  RESULTS
  The EMG-force model provided a fair estimate of the net joint moment. The co-contraction index in anterior cruciate ligament deficient subjects was significantly higher (mean 0.54 (SD, 0.04)) compared to healthy subjects (mean 0.25 (SD, 0.07)).
  
  
  CONCLUSIONS
  Although the EMG-to-force processing model is not perfectly accurate, it is appropriate within a clinical context.
  
  
  CONCLUSIONS
  Previous research supports the hypothesis that subjects with an anterior cruciate ligament deficiency compensate the loss of passive stability by developing higher co-activation levels of the knee muscles, i.e. active stabilization. Quantifying co-contraction may serve as a valuable parameter to evaluate clinical interventions and rehabilitation processes. The EMG-force model presented in this study appears to be a useful instrument for this purpose.
authors.affiliation: Department of Rehabilitation Medicine, VU University Medical Center, P.O. Box 7057, 1007 MB, Amsterdam, The Netherlands. cam.doorenbosch@vumc.nl
authors.forename: Caroline A M
authors.initials: CA
authors.lastname: Doorenbosch
authors.forename: Jaap
authors.initials: J
authors.lastname: Harlaar
mesh_terms: D000328:Adult; D016118:Anterior Cruciate Ligament / Q000503:physiopathology*; D000070598:Anterior Cruciate Ligament Injuries; D004576:Electromyography / Q000379:methods*; D005080:Exercise Test; D005260:Female; D006801:Humans; D007551:Isotonic Contraction; D007593:Joint Instability / Q000503:physiopathology*; D007719:Knee Joint / Q000503:physiopathology*; D008297:Male; D008954:Models, Biological; D009068:Movement; D018482:Muscle, Skeletal / Q000503:physiopathology*; D005082:Physical Exertion; D004856:Postural Balance; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013314:Stress, Mechanical; D019415:Torque