During voluntary muscle contraction, human motoneurones can exhibit specific discharge patterns: single and repetitive doublets. Delayed depolarization has been accepted as the mechanism underlying single doublets. Repetitive doublet firing has been studied much less and its controlling mechanisms remain obscure. The aim of the present study was to examine properties of repetitive doublets in human motoneurones and to consider their underlying potential mechanisms. It was found that 22 of 41 (53.7%) lower-threshold motor units (MUs) in the trapezius and 15 of 42 (35.7%) MUs in triceps brachii displayed repetitive doublets with the mean interspike intervals (ISIs) of 5.5 +/- 1.1 and 6.4 +/- 2.6 ms, respectively. Each doublet was followed by a prolonged post-doublet ISI. The analysis of properties of repetitive doublets showed that they were typically initiated in quiescent motoneurones rather than in firing ones (appearing just at recruitment in an all-or-none manner) and could only be maintained at a certain level of muscle contraction. Repetitive doublets were interrupted either voluntarily (by the subject), or spontaneously with sudden transition from doublet firing to single discharges-the firing behaviour that may be referred to as a firing-pattern "jump". The properties of doublet firing seem to be consistent with traits of motoneurone firing in the presence of plateau potentials reported in animal studies. It was suggested that the potential mechanisms underlying repetitive doublet firing could include a delayed depolarization as the primary determinant, which likely could become persistent probably due to a plateau potential activated in parallel with a common synaptic input.

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