title: A numerical solution to calculate internal-external rotation at the glenohumeral joint.
abstract: OBJECTIVE
  To validate the approach of using angular velocity vectors to quantify internal-external rotation of the humerus.
  
  
  METHODS
  An experimental approach was used to compare predictions of internal-external rotation of the humerus based on angular velocity vectors, and known measurements of internal-external rotation.
  
  
  BACKGROUND
  A primary concern associated with description of glenohumeral biomechanics is measurement of internal-external rotation of the humerus. Euler angles are often used, yet they do not address problems such as Codman's paradox and 'gimbal lock'. Previous work has presented a technique that uses angular velocity vectors to quantify internal-external rotation instead of Euler angles. This approach is promising with regard to providing an independent measure of internal-external rotation of the humerus, and requires validation for subsequent use on humans.
  
  
  METHODS
  A gimbal with three axes of rotation that simulated the rotational d.o.f. of the humerus relative to the glenoid was developed and used to validate the use of angular velocity vectors to quantify internal-external rotation of the humeral shaft portion of the gimbal.
  
  
  RESULTS
  Correlations between calculated and measured rotation values revealed R(2) values of 0.99, slopes at diagonal (1.0), and y-intercepts near zero (-0.6 degrees ).Conclusions. The expression developed here is a valid and useful method for measuring motion of the humerus relative to the glenoid.
  
  
  CONCLUSIONS
  Angular velocity vectors can be used to accurately determine internal-external rotation of the humerus relative to the glenoid, and this may be useful for the development of arthrometers to characterize the glenohumeral joint.
authors.affiliation: Department of Mechanical Engineering, The University of Delaware, Newark, DE, USA.
authors.forename: J E
authors.initials: JE
authors.lastname: Novotny
authors.forename: B D
authors.initials: BD
authors.lastname: Beynnon
authors.forename: C E
authors.initials: CE
authors.lastname: Nichols
mesh_terms: D001696:Biomechanical Phenomena; D006801:Humans; D006811:Humerus / Q000502:physiology; D009068:Movement / Q000502:physiology*; D016059:Range of Motion, Articular; D012399:Rotation; D012785:Shoulder Joint / Q000502:physiology*