KINEMATICS OF THE FREELY MOVING HEAD AND NECK IN THE ALERT CAT

In this study we examined connections between the moment-generating ca pacity of the neck muscles and their patterns of activation during vol untary head-tracking movements. Three cats lying prone were trained to produce sinusoidal (0.25 Hz) tracking movements of the head in the sa gittal plane, and 22.5 degrees and 45 degrees away from the sagittal p lane. Radio-opaque markers were placed in the cervical vertebrae, and intramuscular patch electrodes were implanted in five neck muscles, in cluding biventer cervicis, complexus, splenius capitis, occipitoscapul aris, and rectus capitis posterior major. Videofluoroscopic images of cervical vertebral motion and muscle electromyographic responses were simultaneously recorded. A three-dimensional biomechanical model was d eveloped to estimate how muscle moment arms and force-generating capac ities change during the head-tracking movement. Experimental results d emonstrated that the head and vertebrae moved synchronously, but neith er the muscle activation patterns nor vertebral movements were constan t across trials. Analysis of the biomechanical model revealed that, in some cases, modification of muscle activation patterns was consistent with changes in muscle moment arms or force-generating potential. In other cases, however, changes in muscle activation patterns were obser ved without changes in muscle moment arms or force-generating potentia l. This suggests that the moment-generating potential of muscles is ju st one of the variables that influences which muscles the central nerv ous system will select to participate in a movement.