Application of spherical and cylindrical wrapping algorithms in a musculoskeletal model of the upper limb

In the modelling of the upper limb, many muscles cannot be represented as a straight line from origin to insertion due to the complex morphology causi ng them to wrap around passive structures. The majority of bony contours th at form these obstructions can be described adequately as simple geometric shapes such as spheres and cylinders. A novel technique for the parameterisation of muscle paths as they wrap aro und such shapes has been developed for use in an upper limb model. The new method involves the definition of moving co-ordinate systems in which the p ath of a wrapped muscle does not move, allowing simplified specification. I n addition, an analytical calculation of the wrapping path around a cylinde r is presented over previous approximate methods. Muscle moment arms were pre-calculated from vector considerations and withi n SIMM by tendon excursion. Close agreement between the two suggests that t he proposed implementations accurately follow the theoretical relationship and can be used with confidence in musculoskeletal models. (C) 2001 Elsevie r Science Ltd. All rights reserved.