The present study of the back muscles of the rabbit, using enzyme histochemical techniques and stereological methods, was undertaken with the view to gaining a better understanding of the fiber-type make-up of the lumbar musculature of this animal. The muscles considered were the multifidus, sacrospinalis and the intertransversarii between levels L5 and L6. Gross examination reveals that the multifidus and the sacrospinalis form the bulk of the lumbar musculature and that these muscles appear white. Between and deep to these two muscles are the red intertransversarii. Histochemically the multifidus and the sacrospinalis are also similar in their fiber-type composition, mostly types IIA and IIB and the percentage of connective tissue that they contain. The intertransversarii, in contrast, are for all practical purposes composed of only type-I fibers. The intertransversarii contain significantly more connective tissue than the other two muscles. It may be concluded that the bulk of the lumbar musculature of the rabbit is phasic while the intertransversarii, containing only type-I fibers and a high percentage of connective tissue, are postural muscles.
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