Changes of the force - frequency relationship in the rat medial gastrocnemius muscle after total 1 transection and hemisection of the spinal cord

25 The relationships between the stimulation frequency and the force developed by motor units 26 (MUs) of the medial gastrocnemius muscle were compared between intact rats and animals 27 after total transection or hemisection of the spinal cord at low thoracic level. The experiments 28 on functionally isolated MUs were carried out two weeks, one, three and six months after the 29 spinal cord injury. Axons of investigated MUs were stimulated with trains of pulses at ten 30 progressively increased frequencies (from 1 to 150 Hz) and the force-frequency curves were 31 plotted. The spinal cord hemisection resulted in a considerable leftward shift of force32 frequency curves in all types of MUs. After the total transection a leftward shift of a curve 33 was observed in fast MUs, while a rightward shift in slow MUs. These changes coincided 34 with a decrease of stimulation frequencies necessary to evoke 60% of maximal force. 35 Moreover, the linear correlation between these stimulation frequencies and the twitch 36 contraction time observed in intact rats was disrupted in all groups of animals with the spinal 37 cord injury. The majority of the observed changes reached the maximum one month after 38 injury, while effects evoked by spinal cord hemisection were significantly smaller and nearly 39 constant in the studied period. Results of this study can be important for prediction of changes 40 in force regulation in human muscles after various extends of spinal cord injury and in 41 evaluation of the frequency of functional electrical stimulation used for training of impaired 42 muscles. 43 44 key words: force-frequency relationship, spinal cord injury, motor units, rat 45