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1. Single motor units were recorded with highly selective electrodes from intact tibialis anterior muscle in the adult man. A detailed parametric analysis was made of the discharge patterns during voluntary isometric contractions of different peak forces carried out at various rates of force development. 2. During the smooth tracking of a ramp force, the different motor units recorded from a given muscle site were recruited in a consistant order, each unit becoming active when the muscle developed a certain level of force. The threshold of some of the units in such slow ramp contractions exceeded 8 kg. By contrast, in brisk ballistic contractions reaching a peak force of 12 kg in less than 0-15 sec, the same motor units discharged in a transient burst which largely preceded the muscle force production. 3. In slow tracking ramp contractions, the instantaneous frequency of single motor units was initially rather low (5-15/sec) and it increased as the ramp force augmented. By contrast, in (strong) ballistic contractions, the same units discharged at an unusually high instantaneous frequency (60-120/sec) early in the burst and the firing frequency decreased thereafter. Such hitherto unknown pattern appears characteristic of ballistic contractions and it was not found in even fast tracking ramp contractions achieving 12 kg in only 0-4 sec. 4. The potentials of the different motor units activated are rather crowded at intervals of a few msec in the early burst of a strong ballistic contraction and observations on the rank activation of the different motor units do not provide reliable data for the analysis of the recruitment order of units in ballistic contractions. 5. A new method is described for estimating ballistic force threshold of single motor units. When a large series of brisk ballistic contractions with peak forces ranging from 0-05 to 12 kg was carried out any given motor unit only became active when the ballistic peak force exceeded a certain reproducible value. A detailed analysis of the recruitment order based on these ballistic force thresholds showed it to be virtually identical to the recruitment order of the same units in slow tracking ramp contractions (correlation=0-95). 6. Ballistic contractions are graded in force both by the recruitment of additional motor units in stronger contractions, and by an increase in their rate of firing. These gradation mechanisms are discussed.
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