Modelling the line of action for the oblique abdominal muscles using an elliptical torso model.

When modelling the line of action of a muscle, anatomical considerations must be included if the model is to realistically mimic the muscle behaviour. The internal and external oblique muscles are examples of muscles that do not follow a straight line between origin and insertion, instead having to wrap around the torso. A model is presented which describes the shape of the torso using a right elliptical cylinder of varying dimensions. The muscle lines of action are then calculated based on this underlying torso shape. The model has been successfully fitted to the data reported by Stokes and Gardner-Morse (Journal of Biomechanics 32(3) (1999) 311). When compared to a linear model, the use of the torso model results in a 15% increase in the axial twist moment, and decreases in the lateral bend and extension moments (5% and 2%, respectively), able to be generated by the internal and external oblique muscles combined in upright stance. These differences become larger (up to 37%) when the torso is flexed, extended or twisted. The structure of the torso model allows it to be used to model any posture without significant increases in the overall model complexity.