Muscle Contractile Properties and Fatigue Characteristics after Spinal Cord Injury

Abstract

<p>Paralyzed skeletal muscle undergoes several changes after spinal cord injury (SCI) that lead to alterations in contractile properties and a decrease in fatigue resistance. In this thesis, SCI-induced changes in the muscle length-tension relationship were explored first through the identification of the optimal joint angles for force generation in the ankle dorsiflexor and plantarflexor muscles. The length-tension relationship in the dorsiflexor muscles was not found to differ between SCI and able-bodied (AB) participants, while the plantarflexor muscle group showed a significant shift to shorter muscle lengths (and thus smaller joint angles) for force generation after SCI.<br /><br />In the second part of this thesis, muscle fatigue characteristics in the ankle dorsiflexors were examined to determine if a decline in muscle excitability contributed to increased muscle fatigue after SCI. The protocol consisted of a 2 minute trial of intermittent tetanic bursts at 30Hz delivered to the peroneal nerve. There were no changes in M-wave amplitude or area observed over the course of the fatigue trial, despite significant loss in force generating capacity, suggesting the maintenance of muscle excitability during fatigue. Participants with SCI showed a greater degree of fatigue than the AB controls, with a decline of 57% and 43% in PT and IT, respectively. In comparison, the AB group showed an increase of 13% in PT and a decline of 22% in IT at the end of the fatigue trial ..<br /><br />In both studies, there were no significant differences in rates of muscle contraction or relaxation between SCI and AB groups, suggesting the possible preservation of type I muscle fibres in our participants. There was, however, a greater slowing of relaxation rate after fatigue in the SCI group, suggesting some potential impairment in Ca2+ re-uptake. Furthermore, there was a significant increase in the twitch-tetanus ratio noted in the SCI group in both studies, suggesting increased muscle stiffness and decreased elastic ~ compliance after SCI. These results have contributed to our understanding of the effects of paralysis on muscle properties; future work should focus on the identification of the :~ mechanisms of muscle fatigue after SCI and the possible role of spasticity in the maintenance of muscle contractile dynamics and fibre composition in paralyzed muscle.</p>