[Areas of low-threshold motor units in the m. extensor digitorum communis--study using concentric needle electrodes]

By a slight voluntary contraction, recording sites were chosen where the electrode picked up activity of only one motor unit. Then the needle was moved in minute steps (as controlled by a slide-gauge) through the area of the motor unit under investigation. Hereby the recorded motor unit potential underwent characteristic changes (Fig. 1). The amplitude was maximal within the centre of the motor unit and decreased sharply within 1 to 2 mm (Fig. 2, 3). Single spike components, when discernible within the motor unit potential complex, could be traced for about 1 mm. With regards to the amplitude profile curves of the whole motor unit potential, it is discussed whether a population of fairly small low-threshold motor units exists in the EDC containing as few as 20-50 muscle fibres scattered over an area with a diameter of 1-2 mm.