ACTIVITY OF THORACIC AND LUMBAR EPAXIAL EXTENSORS DURING POSTURAL RESPONSES IN THE CAT

This study examined the role of trunk extensor muscles in the thoracic and lumbar regions during postural adjustments in the freely standing cat. The epaxial extensor muscles participate in the rapid postural r esponses evoked by horizontal translation of the support surface. The muscles segregate into two regional groups separated by a short transi tion zone, according to the spatial pattern of the electromyographic ( EMG) responses. The upper thoracic muscles (T5-9) respond best to post eriorly directed translations, whereas the lumbar muscles (T13 to L7) respond best to anterior translations. The transition group muscles (T 10-12) respond to almost all translations. Muscles group according to vertebral level rather than muscle species. The upper thoracic muscles change little in their response with changes in stance distance (fore -hindpaw separation) and may act to stabilize the intervertebral angle s of the thoracic curvature. Activity in the lumbar muscles increases along with upward rotation of the pelvis (iliac crest) as stance dista nce decreases. Lumbar muscles appear to stabilize the pelvis with resp ect to the lumbar vertebrae (L7-sacral joint), The transition zone mus cles display a change in spatial tuning with stance distance, respondi ng to many directions of translation at short distances and focusing t o respond best to contralateral translations at the long stance distan ce.