Site Specific Motor Unit Recruitment During Fatigue in Human Soleus Muscle : a Quantitative EMG Analysis
AuCoin, Rhonda B.G.
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The purpose of this study was to determine whether motor unit recruitment is site specific in the human soleus muscle, particularly during muscular fatigue. Needle electrodes measured the electrical activity from the muscle at sites 1 and 2, located on the lateral border of the soleus, and site 3, located on the medial border. Turns analysis was used to determine if there were differences in recruitment among the three sites during isometric plantarflexion at various intensities. Contractions of 20%, 40%, and 60% MVC were each maintained for 10 seconds. Next, a gradual increase in torque (ramp contraction) from 0-80% MVC was completed in approximately 40 seconds. Finally, a 70% MVC fatiguing contraction was sustained for three minutes. During each of the 10 second contractions, torque, numbers of turns, and amplitude remained constant throughout each contraction. There were significant differences in turn counts among the sites, with the largest differences observed at the lowest intensity (20% MVC). During the ramp contraction, as the torque continued to increase to 80% MVC, the amplitude of the EMG signal showed a corresponding increase. The numbers of turns, however, leveled off at sites l and 2 at 60% and 50% respectively. At site 3, the number of turns had not yet reached a plateau at 70% MVC. There were significant differences observed among the sites, with the largest differences at the lowest intensities. During the 70% MVC fatiguing contraction, torque declined in every subject, with an average decline of 59%. Numbers of turns decreased at all sites by approximately 43% at site 1, 35% at site 2, and 25% at site 3. There was a significant difference among the sites, primarily at the lower torque levels, when torque had decreased as a result of fatigue. Site 3, located at the medial site in the muscle, was significantly different from the others. Fast Fourier transformation (FFf) was used along with turns analysis to analyze the fatigue data. The FFf showed a similar trend to the turns analysis, but with more pronounced differences among the sites. FFf decreased significantly by approximately 20% at site 1, 31 % at site 2, but showed no significant decrease at site 3. Once again, site 3 was quite different from sites l and 2 and the greatest site differences were observed at the lower intensities. These data suggest that recruitment can be site specific in the human soleus muscle during fatigue and isometric contractions of various intensities. These recruitment differences were most evident at the lower intensities, suggesting that more recruitment "options" are available to the neuromuscular system at lower torque levels. During fatigue, the medial site was markedly different from the other sites, which were located laterally. The presence of such differences among muscle sites highlights the importance of consistently sampling from different regions of a muscle.