We recorded twitches of single motor units (MUs) in the human first dorsal interosseus muscle using either spike-triggered averaging (STA; 236 MUs in 12 normal subjects) or low-rate intramuscular microstimulation of motor axons (IMS; 200 MUs in 20 normal subjects). We analysed twitch force (TF), maximal rate of rise of force (MRRF), contraction time (CT) and half-relaxation time (HRT). MRRF, CT and HRT were significantly smaller with STA than with IMS whereas TFs were fairly similar. Higher stimulus rates (up to 14 Hz) in IMS resembling the voluntary MU firing rates in STA were associated with a decrease of all twitch parameters because of partial fusion of the twitches (20 MUs). Concerning MRRF, CT and HRT, the reduced values matched those obtained by STA, suggesting that the underestimation of these parameters in STA can be mainly attributed to partial fusion. The reduction of TF with high rate IMS but not with STA reveals that other factors such as MU synchronization and non-linear force summation of MU contractions must counteract the effects of partial fusion in STA. We conclude that both STA and IMS are appropriate for assessing TFs in man while the time-dependent parameters MRRF, CT and HRT will be underestimated with STA.