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COMMENTARY
HIGHLIGHTED TOPICS
Neural Control of Movement
Motor units are the basic functional elements of neuromuscular control. In an article entitled "Low threshold motor unit membrane properties vary with contraction intensity during sustained activation with surface EMG visual feedback," Dr. D. Farina and colleagues (1) illustrate the utility of visual feedback of surface multichannel EMG signals to control the activity of single motor units. After training subjects to vary the force of the abductor pollicis and abductor digiti minimi, these investigators used surface EMG recordings of these muscles to observe a single dominant motor unit and modulate its firing rate around two target values for a brief period. Using this noninvasive feedback technique, Farina and colleagues analyzed properties of single motor units during and after periods of sustained activity. It was found that motor unit conduction velocity changed significantly with motor unit firing rate and as a consequence of sustained activity. The slowing of conduction velocity over time was greater for higher sustained motor unit firing rates. These results indicate that changes in single motor unit conduction velocity is one of the earliest signs of peripheral modifications in the neuromuscular system induced by sustained contractions. This investigation also revealed a correlation between single motor unit conduction velocity and instantaneous firing rate, indicating an effect of interpulse interval on muscle fiber membrane properties. During sustained activity of these muscles, both recruitment of additional motor units and motor unit substitution occurred. Thus this study clearly demonstrates the utility of surface EMG recording and visual feedback in the noninvasive analysis of peripheral properties of the neuromuscular system at the level of motor units.
Although numerous studies have explored muscle fatigue, the causes of task failure during dynamic contractions (the most frequent modality in activities of daily living) have not been carefully investigated. In a study entitled "Limiting mechanisms of force production after repetitive dynamic contractions in human triceps surae," Klass and colleagues (2) assess the mechanisms that limit muscle performance after a series of brief moderate dynamic contractions. These investigators examined the neuromuscular adjustments in the human triceps surae muscle through a combination of voluntary, reflex, and electrically induced contractions. They observed that both the range of motion about the ankle joint and maximal voluntary force were reduced after a fatiguing task and that the force-generating capacity of the muscle was more significantly affected at short lengths than at neutral length. Furthermore, although central fatigue and neuromuscular propagation were not significantly altered after the fatigue task, there was a small modulation in spinal reflex activities. The apparent decline in spinal excitatory afferent inputs to the motoneuron pool did not have a direct deleterious effect on voluntary activation. In contrast, alterations in the time course of twitch and postactivation potentiation indicated that intracellular mechanisms limited force production after the fatiguing task. Contrary to sustained isometric efforts, the force decline after brief dynamic contractions did not involve neural mechanisms. These findings therefore support the "task dependency" concept, that the mechanisms responsible for muscle fatigue depend on the task performed. It also emphasizes the need to investigate the mechanisms of task failure rather than to identify a global cause for muscle fatigue.
Also focusing on muscle fatigue, Dr. G. Sjøgaard and colleagues (3) explore neural control during voluntary shoulder movements and positions in the third featured article in this issue, entitled "Intramuscular pressure and EMG relate during static contractions but dissociate with movement and fatigue." These investigators used measures of electrical muscle activation (EMG) and mechanical muscular response (intramuscular pressure or IMP) to evaluate tissue exertion. Compared with static contractions, both electrical and mechanical peak responses were larger; however, only EMG was velocity dependent, whereas IMP was not. These observations suggest that maintaining shoulder position involves lower exertion than does moving toward the same position (in this study, 90° shoulder abduction). From this point of view, dynamic activity is not considered preferential to static activity, in contrast to what is often stated in occupational settings. While imposing prolonged, sustained shoulder positioning, these investigators evaluated both muscular fatigue and load sharing between shoulder stabilizers (supraspinatus and trapezius muscles) and prime movers (deltoideus muscle). In supraspinatus, EMG amplitude increased, suggesting development of fatigue. Surprisingly, in contrast, IMP did not increase, and for most subjects IMP actually decreased, indicating an attenuation in force contribution. EMG results indicated that fatigue development in the supraspinatus muscle was actually underestimated, since fatigue would have been even larger had the same force been maintained. Interestingly, the large increase in EMG activity in the trapezius and deltoideus muscles may not be due solely to fatigue but may also relate to increases in force development in these muscles. This interpretation is particular to the trapezius muscle and is supported by relatively fast recovery, indicating less fatigue than in the other two muscles. The development of fatigue may modulate load sharing between synergistic muscles and can be revealed by EMG recordings only when used in combination with other measures such as IMP.
REFERENCES
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