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The Journal of Neurophysiology Vol. 82 No. 6 December 1999, pp. 3188-3195
Copyright ©1999 by the American Physiological Society
1The Miami Project to Cure Paralysis, Departments of Neurological Surgery and Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33136; 2Department of Physiology, University of Umeå, SE-90187 Umeå, Sweden; and 3Department of Pediatrics, Yale University Medical School, New Haven, Connecticut 06520
Thomas, C. K.,
R. S. Johansson, and
B. Bigland-Ritchie.
Pattern of Pulses That Maximize Force Output From Single Human
Thenar Motor Units. J. Neurophysiol. 82: 3188-3195, 1999. 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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