A study was conducted to analyze the distribution and diameter of muscle fiber types in samples of the medial paravertebral lumbar muscle, i.e., multifidus muscle, obtained from 76 patients who underwent surgery for disc herniation. The samples were compared with 41 control samples of corresponding muscle tissue taken from 41 young healthy subjects who had died a sudden death. Histochemical analysis of fibers associated with myofibrillar adenosine triphosphatase (ATPase) revealed the presence of Type I fibers (slow-twitch fibers) and of Type IIA and IIB fibers (fast-twitch fibers) in both the experimental and control samples. The respective percentage of muscle fibers was calculated and their diameters were measured. Type I fibers predominated in both groups and were significantly larger in diameter than Type IIA and IIB fibers. Both fast-twitch fiber types were distributed in almost equal proportions in the healthy women. In the healthy men, Type IIA fibers prevailed. In the healthy females, the percentage of Type I fibers was found to be slightly higher than in the males, but the diameter of all fiber types was respectively smaller. In the females who had undergone surgery, Type I fibers were significantly larger in diameter than those of the healthy subjects. On the other hand, the diameters of all muscle fiber types were significantly larger, and the percentage of both fast-twitch fibers were Idwer in the samples from men who underwent surgery, as compared to the healthy tissue samples. The morphometric changes in the multifidus muscle at the level of the protruded disc observed by the histochemical method for demonstration of myofibrillar ATPase could not be related to the compressed nerve root in the majority of cases in our study.