Control of free head-neck movements in cats analyzed using a three-dimensional musculoskeletal model

A computer graphics model was used to analyze electromyographic (EMG) and kinematic data from the head and neck of two freely moving cats. The model was used to study two cats performing ±15° sinusoidal (0.25 Hz) head tracking movements in the sagittal plane. Cinefluoroscopic images revealed that vertebral motion described a nearly circular arc in a cat that was standing compared to a diagonal line in a cat that was lying prone. The prone lying animal had more acute angles between the vertebrae. The selection of muscles that were most strongly activated and the timing of muscle activation differed between the two cats. EMG lead vertebral motion in the standing cat, but EMG was in phase with the peak up position of the head in the prone cat. Analysis of the model indicated that there was very little difference in the moment-generating capacity of each muscle between the two postures. Thus, we expect that the differences in the kinematics resulted from altered muscle activation patterns rather than differences in the musculoskeletal system in the two postures.