Fatigability, relaxation properties, and electromyographic responses of the human paralyzed soleus muscle. Shields, Richard K. College of Medicine, Physical Therapy Graduate Program, The University of Iowa, Iowa City IA 52242-1008.
APStracts 2:0063N, 1995.
SUMMARY AND CONCLUSIONS
1. The goal of this study was to characterize the fatigability, contractile relaxation properties, electrophysiological responses, and histochemical properties of the human paralyzed soleus muscle to determine its relative plasticity. . Acute (< 6 weeks, n=3) and chronic (> 1 year, n=10) paralyzed individuals had the tibial nerve activated with a 20 hz square wave delivered for 330 msec every second for 4 minutes. The soleus muscle peak torque, one half relaxation time (1/2RT), normalized maximum rate of relaxation (nMRR) and mass muscle action potential amplitude (M-wave) were computed every 30 seconds. A soleus muscle biopsy was evaluated for myosin adenosine triphosphatase enzyme (ATPase) (ph = 9.4, 4.6, 4.2) and nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR). 3. In the chronically paralyzed group the torque was significantly reduced within 30 seconds of the fatigue protocol. The 1/2 RT and nMRR were also significantly changed within 30 seconds supporting that muscle relaxation was prolonged. No significant changes were present at comparable times during the same 4 minute fatigue protocol applied to the acutely paralyzed soleus muscle. M-wave amplitude was significantly reduced in the chronic group, but only at 3 minutes of the fatigue protocol. Conversely, no significant changes occurred to the M-waves of the acute group. 4. The correlation was high between torque and nMRR (r=.88-.97) and torque and 1/2RT (r=.88-.96) for each chronic subject. A close association was also found between 1/2 RT and nMRR (r=.88-.92) for each chronic subject. Because these variables changed minimally in the acutely paralyzed group a lower correlation was present (r=.45-.52). 5. Torque was weakly correlated to M-wave amplitude (r=.55) for the chronically paralyzed group. The greatest change in torque occurred at a time (0-65 seconds) when the least amount of change occurred in the M-wave amplitude suggesting that the source of fatigue was within the contractile mechanism and not attributable to neuromuscular transmission compromise. 6. Despite a close association between torque and relaxation properties during fatigue of the chronically paralyzed soleus muscle, there was a significant dissociation after 5 minutes of recovery. Torque recovered to 60 percent while the relaxation properties were consistently fully recovered. This suggests that the mechanism causing torque reduction covaried with the mechanism leading to prolonged relaxation during fatigue, but during recovery the two mechanisms no longer covaried. M-wave amplitude was also completely recovered at 5 minutes despite continued torque depression. 7. Qualitative histochemical analysis supported that over 90 percent of the sampled chronically paralyzed soleus muscle was comprised of type II fibers (myosin ATPase-9.4) and had reduced oxidative capacity (NADH-TR). The type II fibers were found to be predominantly type IIB (myosin ATPase-4.6, 4.2). 8. Fatigability, relaxation properties, and histochemical analysis supports that the chronically paralyzed soleus muscle functions and stains qualitatively as a composite of type IIB muscle fibers. Conversely, within 6 weeks of paralysis, the fatigability and relaxation property changes were minimal in the human soleus muscle as would be expected in a predominantly slow muscle. 

Received 18 July 1994; accepted in final form 9 February 1995.
APS Manuscript Number J439-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.