Decomposition‐based quantitative electromyography: Effect of force on motor unit potentials and motor unit number estimates

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Abstract
Decomposition‐based quantitative electromyography (DQEMG) allows for the collection of motor unit potentials (MUPs) over a broad range of force levels. Given the size principle of motor unit recruitment, it may be necessary to control for force when using DQEMG for the purpose of deriving a motor unit number estimate (MUNE). Therefore, this study was performed to examine the effect of force on the physiological characteristics of concentric needle‐ and surface‐detected MUPs and the subsequent impact on MUNEs obtained from the first dorsal interosseous (FDI) muscle sampled using DQEMG. Maximum M waves were elicited in 10 subjects with supramaximal stimulation of the ulnar nerve at the wrist. Intramuscular and surface‐detected EMG signals were collected simultaneously during 30‐s voluntary isometric contractions performed at specific percentages of maximal voluntary contraction (MVC). Decomposition algorithms were used to identify needle‐detected MUPs and their individual MU firing times. These MU firing times were used as triggers to extract their corresponding surface‐detected MUPs (S‐MUPs) using spike‐triggered averaging. A mean S‐MUP was then calculated, the size of which was divided into the maximum M‐wave size to derive a MUNE. Increased levels of contraction had a significant effect on needle‐ and surface‐detected MUP size, firing rate, and MUNE. These results suggest that force level is an important factor to consider when performing quantitative EMG, including MUNEs with this method. Muscle Nerve, 2005

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