Muscle activity in rapid multi-degree-of-freedom elbow movements: solutions from a musculoskeletal model

The activity of certain muscles that cross the elbow joint complex (EJC) ar e affected by forearm position and forearm movement during elbow flexion/ e xtension. To investigate whether these changes are based on the musculoskel etal geometry of the joint, a three-dimensional musculotendinoskeletal comp uter model of the EJC was used to estimate individual muscle activity in mu lti-degree-of-freedom (df) rapid (ballistic) elbow movements. It is hypothe sized that this model could reproduce the major features of elbow muscle ac tivity during multi-df elbow movements using dynamic optimal control theory , given a minimum-time performance criterion. Results from the model are pr esented and verified with experimental kinematic and electromyographic data from movements that involved both one-df elbow flexion/extension and two-d f flexion/extension with forearm pronation/supination. The model demonstrat ed how the activity of particular muscles is affected by both forearm posit ion and movement, as measured in these experiments and as previously report ed by others. These changes were most evident in the flexor muscles and lea st evident in the extensor muscles. The model also indicated that, for spec ific one- and two-df movements, activating a muscle that is antagonistic or noncontributory to the movement could reduce the movement time. The major features of muscle activity in multi-df elbow movements appear to be highly dependent on the joint's musculoskeletal geometry and are not strictly bas ed on neural influences or neuroanatomical substrates.