The ratio of single twitch tension to maximal tetanic tension (Tw/Tet ratio) represents one of the physiological properties of single motor units. In our study, in agreement with the results of other studies, the mean Tw/Tet ratio of slow motor units was lower than that of fast motor units. But we found that the Tw/Tet ratios of the slow motor units with twitch tension between 0.75 and 1.5 g were higher than those of fast motor units with twitch tension in the same range. A significant linear relation between Tw/Tet ratio and twitch tension was obtained for the three types of motor units. This relationsip could be due to the increasing number of muscle fibres within a motor unit with the twitch tension.