The physiological properties of 355 motor units (MUs) recorded in the extensor carpi radialis muscles were studied in 34 healthy human subjects during isometric contractions. MU selective twitches were educed from the whole muscle force using the spike-triggered averaging method. The twitch contraction times and twitch forces were measured. From these data it was attempted to estimate the distribution of fast and slow MUs in the muscles studied. Mu recruitment thresholds were systematically measured during stereotyped slow ramp contractions (force increase = 0.25 N. s-1). Degrees of correlation between contraction times, twitch forces and recruitment thresholds were pair analysed by computing simple regression curves and correlation coefficients. The degrees of correlation were compared between 245 Mus recorded in 34 subjects and 66 MUs recorded in a single subject. Analysis of the instantaneous discharge frequency of 132 MUs showed the existence of a remarkable degree of correlation (correlation coefficient, r = -0.75) between the "frequency rise times" (discharge onset to maximal frequency) and the MU twitch contraction times; i.e., the "frequency rise times" increase when the twitch contraction time decrease. The possibility that muscle contraction may be differentially modulated on the basis of this discharge property of the M Us is discussed. The results are compared to previous data and the limitations of the spike-triggered averaging method applied to long muscles in man are extensively discussed.