CAN THE SIZE PRINCIPLE BE DETECTED IN CONVENTIONAL EMG RECORDINGS

According to Henneman's size principle, small motor units are recruite d before large ones. It is commonly believed that this can be detected in routine conventional EMG recordings even among the earliest recrui ted motor units. That is, the MUP amplitude, area, and thickness shoul d increase with recruitment order. We studied the first four motor uni t potentials (MUPs) recruited within the pickup area of the electrodes . Data were obtained from 179 different sites in monopolar recordings and in 153 concentric recordings from 5 health subjects, In the pooled material, amplitude, area, and thickness increased slightly between c onsecutively recruited MUPs, However, at individual recording sites th e size of consecutively recruited MUPs varied considerably. At some re cording sites the first recruited MUP had the largest amplitude and th e later MUPs has successively smaller amplitudes. We conclude that, at individual recording sites, the size principle cannot be detected in low threshold motor units with monopolar or concentric EMG electrodes. The reason for this is the small uptake area of these electrodes in r elation to the motor unit territory. (C) 1995 John Wiley and Sons, Inc .