Morphology and contraction properties of cat lumbar back muscles.

The gross morphology, innervation pattern and contraction properties of lumbar back muscles in the cat were investigated. The medially located multifidi and interspinales are formed by short bundles interconnecting adjacent vertebrae. Laterally located bundles composing longissimus and iliocostalis are attached to the spinous processes by a fascial layer and to the pelvic bone by a well developed intermuscular septum. Different spinal segments innervate the various sections along the muscles in such a way that the myotomes composing a muscle belly seem to be arranged after one another, in a row. The contraction time of a maximal isometric twitch is 34 ms for multifidi and interspinales and 29 ms for longissimus and iliocostalis. No large segmental variations of contraction times were found. The time course of twitch responses and a study of reflex contractions in middle and lower sections of lateral muscles indicate the presence also of a slowly contracting portion located to the longissimus. The initial muscle length is more decisive for the twitch amplitude of medial than of lateral muscles. In medial muscles summation of individual twitches starts at a stimulus frequency of 10--15 Hz and apparent fusion occurs at about 50 Hz. For lateral muscles corresponding values are 15--20 Hz and about 70 Hz. The contractile tension declines markedly in all muscles during a 10 min period of stimulation at 5 Hz.