We assessed the sequence of nerve impulses that maximize force output from individual human thenar motor units. When these motor units were stimulated intraneurally by a variable sequence of seven pulses, the pattern of pulses that elicited maximum force always started with a short (5-15 ms) interpulse interval termed a 'doublet.' The twitch force summation caused by this 'doublet' elicited, on average, 48 ± 13% (SD) of the maximum tetanic force. The peak amplitude of 'doublet' forces was 3.5 times that of the initial twitches, and twitch potentiation appeared to have little influence on twitch force summation elicited by the 'doublets.' For some units, the second optimal interpulse interval was also short. Peak forces elicited by the third to sixth interpulse intervals did not change substantially when the last interpulse interval was varied between 5 to 55 ms, so maximum force could not be attributed to any unique interpulse interval. Each successive pulse contributed a smaller force increment. When five to seven pulses were delivered in an optimal sequence, the evoked force was close to that recorded during maximal tetanic stimulation. In contrast, maximal force-time integral was evoked with one short interpulse interval (5-15 ms) then substantially longer interpulse intervals (>100 ms). Maximum force and force-time integrals were therefore elicited by different patterns of stimuli. We conclude that a brief initial interpulse interval (5-15 ms) is required to elicit maximum 'doublet' force from human thenar motor units and that near-maximal tetanic forces can be elicited by only five or six additional post-'doublet' pulses if appropriately spaced in time. However, the rate at which these post- 'doublet' stimuli must be provided is fairly uncritical. In contrast, maximum post-'doublet' force-time integrals were obtained at intervals corresponding to motoneuronal firing rates of ~7 Hz, rates close to that typically used to recruit motor units and to maintain weak voluntary contractions.
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