Effects of inertial load and cervical-spine orientation on a head-tracking task in the alert cat

Simultaneous video-fluoroscopic and neck muscle EMG data were recorded from one cat performing +/-15 degrees sinusoidal (0.25 Hz) head-tracking movements in the sagittal plane in a standing body posture with two initial neck orientations and four inertial loads. Radio-opaque markers were inserted into the anterior/posterior and lateral aspects of the occipital ridge and C(1)-C(7) to measure vertebral displacement. Kinematic data were analyzed, and a computer model was applied to the data to characterize the limits of movement in the cervical spine and to estimate the moment arms of the neck muscles at different orientations of head-neck movement. For each initial neck orientation, the cat utilized a distinct set of vertebral alignments, relative joint movements, and muscle-activation patterns to achieve the same movement outcome. As inertial load increased, vertebral alignments and relative joint movements were constant with a vertically oriented neck but differed when the neck was more horizontally oriented. Different muscle-activation patterns were used to maintain the same kinematic pattern with increased inertial loads. Some muscle EMG response gains (rectus capitis major and splenius capitis) increased with increasing mass, while others (biventer cervicis and occipitoscapularis) demonstrated an initial increase and then a plateau. EMG phases were not affected by changing the mass of the system but were affected by changing neck orientation. The model predicted that muscle moment arms would vary little for the different vertebral alignments, suggesting a robust biomechanical system minimally compensates for small changes in task geometry.