The present study was carried out to reveal the intramuscular innervation pattern as well as the spatiotemporal activation pattern of different back muscles in rats. It was also tried to define when and why the back muscles are active during the locomotion on a horizontal treadmill. The microscopic dissection of the intramuscular innervation shows that the knowledge of the anatomy is essential for the interpretation of the physiological data. Dissections and Sihler`s staining were used to uncover the intramuscular nerve branches of the back muscles. With the procedure of Sihler`s staining the whole specimen becomes translucent in KOH solution. The indwelling nerve branches were counterstained with Ehrlich's Haematoxylin. So it was possible to check on the dissected patterns and estimate the distribution of nerve branches in undissected muscle areas. The innervation patterns of the muscles differ between the thoracal and lumbar regions. While the thoracal muscles are innervated unisegmentally the lumbar nerve branches extend about three segments in caudal direction. The innervation areas of the medial and lateral branches overlap. Thus the muscles of this region are polysegmentally supplied. During locomotion the spatiotemporal activation of the multifidi muscles of both body sides were measured with 16-channel-electrodes (2 rows of 8 electrodes each). The electrodes were positioned on the surfaces of the muscles between the fourth and sixth lumbar vertebrae. Subsequently the activation patterns were combined with gait characteristics and root means square profiles were calculated for 2366 stance phases. A biphasic activation pattern was observed in both multifidi muscles. The activation seems to be correlated with the touch down of the hind limbs but the analysis results in a different view. It became clear that the muscles stabilize the back during the acceleration phase against occurring torques and shear forces. It's also inhibiting the bouncing of the back. It is astonishing that the activation patterns of the human multifidi muscles are still very similar in spite of the evolution of bipedal locomotion.

Supported by Kompetenzzentrum für Interdisziplinäre Prävention Univ. Jena – BGN.