Effect of recording window and stimulation variables on the statistical technique of motor unit number estimation

A variety of methods are used for the selection of recording window sizes and stimulation current levels for statistical motor unit number estimation (MUNE). This study compares different recording window sizes and stimulation current levels within those windows in the same subjects to determine the effect on MUNE value and reproducibility. Four recording windows of 10% size were compared with four of 5%, with the stimulation current set in the lower quarter, middle half, and upper quarter of the recording window. MUNE for stimulation current set in the lower quarter of the window was 81 (62-103) for 10% recording windows and 120 (108-135) for 5% recording windows, and 91 (61-123) and 133 (120-154) for stimulation current set in the middle half. Increasing the recording window size from 5 to 10% lowers the MUNE value in controls, but tends to improve reproducibility; and setting the stimulation current in the lower quarter of the window, changes the MUNE value minimally, while tending to improve further reproducibility. Excellent reproducibility of MUNE was obtained when applied to a pilot group of 10 amyotrophic lateral sclerosis patients. Based on this study, we conclude that the ideal method for statistical motor unit estimation involves using 10% recording windows and setting the stimulation current in the lower quarter of the recording window.