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When a muscle contracts, it produces an electrical signal known as an EMG signal. The EMG signal is made up of discrete discharges known as motor-unit action potentials (MUAPs).

o EMG signal from the brachial biceps muscle recorded using a needle electrode during a moderate voluntary contraction.

o High-pass filtering makes the motor-unit discharges more distinct.

The process of sorting out the discharges of the different motor units is known as EMG decomposition

A one-second segment of the high-pass-filtered EMG signal. It contains MUAP trains from eight separate motor units.

Decomposition can be accomplished using pattern-recognition techniques such as template matching.

In theory, a 10 kHz sampling rate captures all the information in the signal. Interpolation may still be need used to align and compare them accurately.

One of the challenging problems is sorting out the superpositions that occur when several motor units discharge at nearly the same time.

o Click this button to generate a random superposition of four MUAPs.

o Drag the sliders to try to reconstruct the superposition. Click this button to see the correct answer.

Knowing the simultaneous discharge patterns of multiple motor units helps us understand the way that the nervous system controls muscular contractions.

The discharge patterns of 15 motor units during a 20-s-long contraction of the medial gastrocnemius muscle. The recruitment profile closely matched the force output of the muscle (light blue trace), while the instantaneous- firing-rate profiles (red traces) did not.

Knowing the MUAP waveforms helps us determine the way in which motor units are organized within the muscle.

MUAP waveforms recorded at the same site in the tibialis anterior muscle. Analysis shows that they come from groups of motor units with different innervation zones and different sites of insertion onto the tendon.

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Veterans Affairs Palo Alto Rehabilitation Research and Development Center
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National Institute of Neurological Disorders and Stroke