title: Multiaxis muscle strength in ACL deficient and reconstructed knees: compensatory mechanism.
abstract: OBJECTIVE
  It is unclear how muscle strength in tibial rotation and knee abduction change following anterior cruciate ligament (ACL) injury and reconstruction. Such strength changes are likely, considering the oblique orientation of the ACL and the constraint provided by the ACL at various tibial rotation and adduction positions. The purposes of this study were to evaluate multiaxis muscle strength in ACL deficient and reconstructed knees and to gain insights into potential compensatory mechanisms adopted by the patients.
  
  
  METHODS
  Muscle strength in tibial internal-external rotation, abduction-adduction, and flexion-extension were investigated in 19 chronic ACL deficient, 18 acute ACL deficient, 21 ACL reconstructed, and 23 normal subjects. The strength ratios of flexion/extension, abduction/adduction, and internal/external rotation were determined for each subject and compared across the different populations.
  
  
  RESULTS
  The chronic ACL deficient patients showed significantly lower strength ratio in internal/external rotation than that of the normal controls and acute ACL deficient subjects (P = 0.02), indicating a compensatory mechanism developed by the patients to unload the ACL and/or to avoid unstable knee positions. For ACL reconstructed patients, the internal/external rotation strength ratio became closer to their counterparts in normal controls than that of chronic ACL deficient patients, presumably reflecting the reduced need for compensation after reconstruction. Furthermore, compared with strength reduction in knee extension, reductions in tibial rotation and abduction strength following ACL reconstruction were less severe and more easy to recover.
  
  
  CONCLUSIONS
  A better understanding of changes in multiaxis muscle strength and the associated compensatory mechanism will help us evaluate treatment outcome more accurately and develop more effective treatment modalities with focus on muscles that help protect and unload the ACL.
authors.affiliation: Department of Physical Medicine and Rehabilitation, Northwestern University, Chicago, IL, USA. l-zhang@northwestern.edu
authors.forename: Li-Qun
authors.initials: LQ
authors.lastname: Zhang
authors.forename: Gordon W
authors.initials: GW
authors.lastname: Nuber
authors.forename: Mark K
authors.initials: MK
authors.lastname: Bowen
authors.forename: Jason L
authors.initials: JL
authors.lastname: Koh
authors.forename: Jesse P
authors.initials: JP
authors.lastname: Butler
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D016118:Anterior Cruciate Ligament / Q000601:surgery*; D000070598:Anterior Cruciate Ligament Injuries; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D007718:Knee Injuries / Q000503:physiopathology* / Q000534:rehabilitation; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D019651:Plastic Surgery Procedures / Q000534:rehabilitation; D020127:Recovery of Function; D012399:Rotation; D012421:Rupture; D013977:Tibia / Q000503:physiopathology; D019415:Torque