A unified approach towards understanding the electromyographic signal

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A whole muscle may contain several hundreds of thousands of individual muscle fibers. These fibers are grouped into functional units (10 – 1000 fibers) activated by a single motor neuron. A single motor neuron and all the muscle fibers that it activates is a call a motor unit. The motor unit is the basic building block of force production. Force can be increased by recruiting additional motor units, or by increasing the number of times per second (rate-coding) that each motor unit is activated. Activating motor units more frequently allows the forces generated by the motor units to overlap in time to a greater degree and sum up more efficiently. Similarly, the nervous system can also recruit groups of motor units more synchronously so that there is a near perfect summation of forces. When a motor unit is recruited by the nervous system, it generates electrical activity that can be monitored by implanting an electrode into the muscle. Unfortunately, indwelling electrodes record from a limited volume of tissue, which is not representative of the whole muscle. Indwelling electrodes can also be painful and their use is being limited. Fortunately, the electrical activity from many motor units can sum up sufficiently to be detected from the skin surface, using surface electromyography. This similar to the use of electrocardiography to record the electrical activity of heart muscle from the skin surface. Surface electromyography has the advantage of recording the electrical activity of a large population of motor units, but the electrical activity of individual motor units is “obscured”. The perplexing dilemma is that surface electromyographic electrodes are the easiest way to measure muscle electrical activity. Yet, the ability of the surface electromyographic signal to provide insight into the neural mechanisms underlying the gradation of muscle force is limited. The proposed series of studies combines indwelling and surface methodologies with modeling and simulation of the electromyographic signal in a unified approach towards developing and validating non-invasive techniques to investigate how the nervous system regulates muscle force during non-fatiguing and fatiguing conditions.