An examination of agonist and antagonist motor unit firing properties

The interactions between opposing muscle (i.e. agonist and antagonist) groups can be extremely complex, task-dependent, and are still poorly understood. To identify possible origins of the coordination between antagonistic muscle groups, the common or shared sources of neural input need to be understood. The assessment and manipulation of motor unit firing properties, such as synchronization, can provide information regarding the common inputs to opposing muscles. PURPOSE: The purpose of this study was to introduce various interventions to systematically manipulate both agonist and antagonist motor unit firing properties, and obtain a better understanding of the interactions between the two. METHODS: Muscle activity was detected from the biceps brachii ("agonist") and the triceps brachii ("antagonist") during isometric forearm flexions. The signals from these muscles were decomposed into individual motor unit action potential trains. Subsequently, various firing properties such as mean firing rate, recruitment threshold, and synchronization were calculated. On two separate visits, either the agonist or antagonist muscle was fatigued. During another two visits, either the agonist or antagonist muscle underwent 18 minutes of prolonged stretching, which has been shown to significantly desensitize proprioceptors. RESULTS: During co-activation, the antagonist demonstrated significant motor unit synchronization, but to a lesser extent when compared to the agonist. The antagonist also exhibited a substantially smaller recruitment threshold range and higher average firing rates. Fatigue of the agonist did not show any changes to antagonist motor unit firing properties, despite a significant increase in co-activation. Fatigue of the antagonists produced effects on the motor unit behavior of the agonist, such as decreased motor unit synchronization. It was suggested that group III and IV muscle afferents originating from the antagonist were responsible for the change to the agonist. The stretching interventions provided some mixed results, often providing non-uniform changes across motor unit types. For example, agonist low-threshold motor unit pairs demonstrated an increase in short-term synchronization after agonist stretching, but the high-threshold motor unit pairs exhibited a decrease in synchronization. Future studies to help answer follow-up questions were suggested

Studies on the mammalian muscle spindle

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Impaired reflex sensitivity cause and effect

When a voluntarily contracting human muscle is stretched its surface reflex electromyographic response has both short-latency (M1) and long-latency (M2) components. The M1 component occurs at a latency compatible with monosynaptic reflex activation. The long-latency component results from stimulation of skin and other subcutaneous receptors. In the initial experiment, using the first dorsal interosseous muscle (FDI) of the hand, the reflex sensitivity in normal human subjects was studied by comparing the various components of the electromyographic response generated by briefly stretching the voluntarily contracting muscle in subjects of various ages. It was found that age-related changes occur in the reflex response of human subjects. Although it is already known that the reflex response in human muscle slows with age, the result of the experiment showed that the size of that reflex response becomes smaller. The evidence suggests an impaired reflex sensitivity in older people which could be reflected in other motor control systems within the body. A detailed investigation was then undertaken to discover the causes of this change. Although the reflex response (M1) was found to alter with age the M2 component did not. This would seem to rule out neuromuscular block (NMB). NMB as a cause of the change was investigated and was found not to occur in the paradigm employed in this investigation. Possible changes in the mechanical properties of muscles and joints were looked for in a series of experiments using an accelerometer. No changes large enough to account for the reflex impairment could be found. In a further series of experiments the effects of fatigue and the effects of training on the electrical response of the FDI were studied. In a final series of experiments the changes with age, in the effects of coffee upon blood pressure were investigated.<p