Recordings of isometric force were obtained for twitches and (sub)maximal tetani of gastrocnemius medialis (MG) and tibialis anterior (TA) muscle units in female Wistar rats. We assessed the relationships between unit properties that have all been associated with "speed": (1) the relative degree of peak force attained during repetitive activation at 40 Hz (P40/Pmax), (2) the relative degree of final twitch fusion during the same test burst (Fus-end), and (3) various measures of the time-course of single twitches, including twitch time-to-peak and a parameter referred to as "initial fusion ratio" (Fus-in; relative decline from peak force at 25 ms from twitch onset). The various measures of twitch time-course were significantly correlated to each other with correlation coefficients varying over a fairly wide range (0.35-0.64 for MG; 0.50-0.80 for TA). Twitch time-course was also significantly correlated with Fus-end during the 40-Hz repetitive activation; the highest correlation coefficient (0.69 for MG, 0.80 for TA) was obtained for Fus-in, which was also numerically similar to Fus-end. Thus, the degree of fusion indeed seemed to be largely dependent upon aspects of twitch time-course. However, the relative degree of force mobilization obtained in the same contractions elicited by stimulation at 40 Hz was not consistently better correlated with Fus-end than with measures of single twitch time-course. Furthermore, in fast-twitch units having the same twitch time-to-peak, the force mobilization elicited by stimulation at 40 Hz (P40/Pmax) was the same for MG and TA, while the degree of fusion was significantly smaller for TA than for MG units. The results demonstrate the complexity of the concept of isometric "speed" and underline the need for using several speed indicators in parallel in studies concerning the differentiation of muscle (unit) properties.