function measureAnteroPosteriorImbalance(obj)
  scapulaCS = obj.shoulder.scapula.coordSys;

  forceResultant = Vector(scapulaCS.origin, scapulaCS.origin)...
    + obj.SC.forceVector...
    + obj.SS.forceVector...
    + obj.IS.forceVector...
    + obj.TM.forceVector;

  % Take infero-superior orthogonal component <=> project on scapular axial plane
  inferoSuperior = Vector(scapulaCS.origin, scapulaCS.origin + scapulaCS.IS);
  imbalanceVector = inferoSuperior.orthogonalComplementTo(forceResultant);

  lateroMedial = Vector(scapulaCS.origin, scapulaCS.origin - scapulaCS.ML);
  anteroPosterior = Vector(scapulaCS.origin, scapulaCS.origin - scapulaCS.PA);

  % posterior imbalance is positive 
  obj.anteroPosteriorImbalance = ...
    sign(dot(imbalanceVector, anteroPosterior)) *...
    rad2deg(lateroMedial.angle(imbalanceVector));
end