Received December 15, 2005
Revised January 20, 2006
Accepted after revision May 18, 2006

¹ University School of Physical Education, Poznañ, Poland
² Nencki Institute of Experimental Biology, Warsaw, Poland

^* To whom correspondence should be addressed. E-mail: celichowski{at}awf.poznan.pl.

	Abstract

The effects of complete transection of the spinal cord at the level of Th9/10 on contractile properties of the motor units (MUs) in the rat medial gastrocnemius (MG) muscle were investigated. Our results indicated that one-month after injury the contraction time (time-to-peak) and half-relaxation time were prolonged and the maximal tetanic force in majority of MUs in the MG muscle of spinal rats was reduced. The resistance to fatigue also decreased in most of MUs in the MG of spinal animals. Moreover, the post-tetanic potentiation of MUs twitches diminished after the spinal cord transection. Criteria of the division of MUs into three types: slow (S), fast fatigue resistant (FR) and fast fatigable (FF) applied in intact animals couldn't be directly used in spinal animals due to changes in contractile properties of MUs. The "sag" phenomenon observed in unfused tetani of fast units in intact animals essentially disappeared in spinal rats and it was only detected in few units, at low frequencies of stimulation only. Therefore the MUs in spinal rats were classified as fast or slow on the basis of an adjusted borderline of 20 ms instead of 18 ms as in intact animals due to a slightly longer contraction time of those fast motor units with the "sag". We conclude that all basic contractile properties of rat motor units in the medial gastrocnemius muscle are significantly changed one month after complete spinal cord transection with the majority of motor units being more fatigable and slower than those of intact rats.

Key Words: Motor control, Muscle, Spinal cord injury