ASSESSMENT OF HUMAN MOTOR UNIT TWITCHES - A COMPARISON OF SPIKE-TRIGGERED AVERAGING AND INTRAMUSCULAR MICROSTIMULATION

We recorded twitches of single motor units (MUs) in the human first do rsal interosseus muscle using either spike-triggered averaging (STA; 2 36 MUs in 12 normal subjects) or low-rate intramuscular microstimulati on of motor axons (MS; 200 MUs in 20 normal subjects). We analysed twi tch force (TF), maximal rate of rise of force (MRRF), contraction time (CT) and half-relaxation time (HRT). MRRF, CT and HRT were significan tly smaller with STA than with IMS whereas TFs were fairly similar. Hi gher stimulus rates (up to 14 Hz) in IMS resembling the voluntary MU f inding rates in STA were associated with a decrease of all twitch para meters because of partial fusion of the twitches (20 MUs). Concerning MRRF, CT and HRT, the reduced values matched those obtained by STA, su ggesting that the underestimation of these parameters in STA can be ma inly attributed to partial fusion. The reduction of TF with high rate IMS but not with STA reveals that other factors such as MU synchroniza tion and non-linear force summation of MU contractions must counteract the effects of partial fusion in STA. We conclude that both STA and I MS are appropriate for assessing TFs in man while the time-dependent p arameters MRRF, CT and HRT will be underestimated with STA.