SURFACE PIN VERSUS SURFACE DISC ELECTRODES FOR ELECTRODIAGNOSIS
Yayoi Katoh (Yokohama City University, Yokohama, Japan)
James W. Little (University of Washington, Seattle, USA)
This study was undertaken to further develop methods for surface electromyographic (EMG) recording. We produced a I mm diameter pin array with 5 mm spacing and examined its sensitivity for identifying single motor-unit action potentials (MUAPs), compared to conventional 1 cm surface disc electrodes. Three pairs of surface pins and 1 pair of surface disc electrodes were used for simultaneous recording. Pin pairs were oriented: a) 5 mm apart and longitudinal to muscle fibers, b) 15 mm apart and longitudinal, and c) 5 mm apart and oriented transverse. We recorded EMG signal from both the abductor digiti minimi (ADM) and the tibialis anterior (TA)muscles during voluntary contraction from 10 normal subjects, and from 5 patients with lower motoneuron (LMN) weakness and from 5 patients with upper motor neuron (UMN) weakness. We identified single motor-units more often with the pin electrodes than with the surface 1 cm discs. We were even able to follow motor-unit firing rates during maximal effort in some weak patients using the surface pin electrodes. Very large amplitudes (>mean+2s.d. for transformed normal data) were commonly recorded in LMN weak but not UMN weak muscles. Long duration MUAPs were not commonly recorded in either LMN or UMN weak muscles.
Surface pin electrodes are often more useful than conventional 1 cm discs for detecting single MUAPs, measuring their amplitude and assessing their firing rates. Surface EMG recordings can provide useful diagnostic information to help distinguish UMN and LMN weakness. Such surface EMG recordings may be of special merit for serial electrodiagnostic investigation, because they are better tolerated than needle EMG electrodes.
Quantitative EMG and motor unit recruitment threshold measurement: with a concentric needle equipped with a quadrifilar electrode
K. Akaboshi, Y. Masakado, N. Chino (Keio University, Tokyo, Japan)
Abstract: According to Henneman's size principle, small motor units are recruited before large ones. Using the EMG signal decomposition technique, we recorded motor unit action potentials (MUAPs) recruited up to 50% of the maximum voluntary contraction in the muscles of 5 healthy young men. We investigated the quantitative relationships that exist between five MUAP parameters: amplitude, duration, area, thickness, and the size index and the recruitment thresholds of the motor units. In each muscle, the amplitude, duration, area, and size index had significant positive high correlations with the motor unit recruitment thresholds. We conclude that the size principle can be detected in concentric needle EMG recordings, evidence that we must give greater importance to the patient's contraction force during EMG examinations.