Measurements of the Uptake Area of Small-Size Electromyographic Electrodes

Extracellular action potentials from 76 different muscle fibers in the human brachial biceps were recorded, with a 14 lead multielectrode, each leading-off surface being 25 ¿m in diameter. Volume conduction of these action potentials was calculated by representing the low-pass filter characteristics of the muscle tissue by a transfer function with one time constant and an attenuation factor. The radial decline of the action potentials was calculated in steps of 5 ¿m, and the pickup radius of the electrode, defined as the distance at which the peak-to-peak amplitude of the action potentials declines to 200 ¿V, was computed. The pickup radius for the 25 ¿m diameter electrode, assuming an average action potential peak-to-peak amplitude of 6.2 mV was 292 Mm. With this uptake area, a fiber density in the brachial biceps of 1.37 fibers/uptake area (the average number of fibers belonging to one motor unit and included within the electrode uptake area) and a fiber radius of 28 ¿m, the electrode "sees" 34 different motor units.