A computer graphics based model of the cat head and neck used to examine joint movement, moment generating potential and EMG patterns in voluntary head and neck movements

This thesis begins with a comprehensive biomechanical model of the cat head-neck system designed to calculate force moments produced by each neck muscle and to determine whether they vary sufficiently with neck motion to require the central nervous system to compensate for their changing moment generating capacities. The model is based on quantitative measures of physiological cross sectional area, tendon length, fascicle length, sarcomere length, and muscle attachment points that are used along with quantitative descriptions of cervical vertebrae anatomy and joint kinematics and allows computations to quantify the force and moment generating capacities of individual neck muscles. An important property of the head neck system that can only be derived from a comprehensive quantitative analysis is the influence of changes in moment arms and force generating capacity on moment generating capacity over a range of head and neck postures. Experimental observations of cats executing a 30 degree head and neck tracking task in the saggital plane that form part of this thesis showed that cats use multiple patterns of muscle activation and joint movements to accomplish the task. In some cases, differences in muscle activation patterns were explained by changes in muscle moment arms or force-generating potential. In other cases, differences in muscle activation patterns were observed without changes in muscle moment arms or force-generating potential. Different muscle activation patterns were used when the cat completed the tracking task with increased inertial loads. Joint alignments that were chosen to successfully complete the task resulted in increased muscle moment arms while reducing the lever arm of the largest inertial loads. While changes in moment arms and force-producing capacity do occur with neck motion, the system is capable of generating substantial muscle moments that vary little over a large range of motion. Neck muscle moments do not vary sufficiently with neck motion to require the central nervous system to compensate for their changing moment generating capacities suggesting a robust mechanical system that can execute a wide range of head movements.