Modelling the mechanical effect of muscles with large attachment sites: application to the shoulder mechanism.

A general theory is described for deriving the mechanical effect of muscles with large attachment sites. In a cadaver experiment the complete attachment sites and bundle distribution of 16 muscles of the shoulder mechanism were recorded. These data were used to calculate the mechanical effect, i.e. the resulting force and moment vector, for a large number (200) and a reduced number (maximal 6) of muscle lines of action. The resulting error between both representations is small. The number of muscle lines of action in the reduced representation depends on the shape of the attachment site and muscle architecture. An important feature of this method is that the necessary number of muscle lines of action is determined afterwards. In the often used centroid line approach the number of muscle lines of action and partitioning of muscles is determined before recording the geometry, leading to unverifiable results.