Evaluating an integrated musculoskeletal model of the human arm.

A simplified model of the mechanical properties of muscle and of the musculoskeletal geometry was used to predict torques at the shoulder and elbow during arm movements in the sagittal plane. Subjects made movements to 20 targets spaced on the diameter of a circle centered on the initial location of the hand. Movement kinematics and the electromyographic (EMG) activity of nine shoulder and elbow muscles were recorded. Muscle force was predicted using rectified EMG activity as an input to a Hill-type model of muscle dynamics. The model also made simplifying assumptions about muscle geometry. Muscle force was then converted to torque and the individual muscle torques were weighted to provide the best fit to the joint torque computed from the kinematic data. The overall fit of the model was reasonably good, but the goodness of fit was not uniform over all movement directions. The results suggest that the assumptions about the musculo-skeletal geometry, the model of muscle dynamics, and muscles not included in the analysis all contributed to the error.