Kevin G. Keenan

Influence of motor unit properties on the size of the simulated evoked surface EMG potential

The purpose of the study was to quantify the influence of selected motor unit properties on the simulated amplitude and area of evoked muscle potentials detected at the skin surface. The study was restricted to a motor unit population simulating a hand muscle whose potentials were recorded on the skin over the muscle. Peak-to-peak amplitude and area of the evoked potential were calculated from the summed motor unit potentials and compared across conditions that simulated variation in different motor unit properties. The simulations involved varying the number of activated motor units, muscle fiber conduction velocities, axonal conduction velocities, neuronal activation times, the shape of the intracellular action potential, and recording configurations commonly used over hand muscles. The results obtained for the default condition simulated in this study indicated that ~7% of the motor unit potentials were responsible for 50% of the size of the evoked potential. Variation in the amplitude and area of the evoked muscle potential was directly related to the number of active motor units only when the stimulus activated motor units randomly, and not when activation was based on a parameter such as motor unit size. Independent adjustments in motor unit properties had variable effects on the size of the evoked muscle potential, including when the stimulus activated only a subpopulation of motor units. These results provide reference information that can be used to assist in the interpretation of experimentally observed changes in the size of evoked muscle potentials.

Fluctuations in motor output during steady contractions are weakly related across contraction types and between hands.

The presence of differences in motor unit activity across contraction types and between hands suggests that the magnitude of fluctuations in motor output is only weakly related when these conditions are compared. Twenty right‐handed young (24.1 ± 5.3 years) and old (72.5 ± 4.9 years) adults performed three levels (≤40% of maximal force) of isometric and anisometric contractions with the first dorsal interosseous muscle of each hand. The fluctuations in motor output were quantified by the coefficient of variation for force during isometric contractions, and as the standard deviations of acceleration and position during anisometric contractions. There was no effect of age on fluctuations in motor output. The magnitude of the fluctuations in motor output was weakly related across contraction types (r2 < 0.325) and between hands (r2 < 0.262). Furthermore, the standard deviations of acceleration and position during shortening and lengthening contractions were largely unrelated to one another. Because the activity in a motor unit population differed across contraction types and hands during steady contractions, the central nervous system likely employed distinct strategies to accomplish these different tasks. Muscle Nerve, 2005

Fluctuations in motor output of a hand muscle can be altered by the mechanical properties of the position sensor

Fluctuations in motor output are typically quantified by the standard deviation (SD) of displacement or acceleration. The aim of the study was to determine the influence of a linear variable-displacement transducer (LVDT) on the SDs and spectral content of displacement and acceleration during steady isometric and anisometric contractions performed with the first dorsal interosseus muscle. Thirteen young adults supported six loads when performing position-holding and position-tracking tasks when the LVDT either was or was not attached to the index finger. The LVDT reduced the magnitude of the SDs in displacement and acceleration and disrupted the load-dependent modulation of the spectral properties of these signals. When the LVDT was not connected to the finger, the displacement SD was greatest during concentric contractions, the acceleration SD was greatest during eccentric contractions, and there were load-dependent changes in the power density spectra. Although the LVDT may be used for assessing relative changes in displacement, its ability to provide absolute measures of fluctuations in motor output is limited. The results provide baseline measures of the fluctuations in motor output during steady contractions with a hand muscle and how the method used to detect displacement alters these measures.