Force-frequency relationships were examined in 30 human thenar single motor units. The technique of intraneural stimulation was used to stimulate the motor axon in the median nerve proximal to the elbow with a tungsten microelectrode. 2. The stimulation consisted of either single shocks or trains of pulses (1 or 2 s duration) at constant rates varying between 5 and 100 Hz. To control various mechanical artifacts, the stimuli were delivered after electronically resetting the force baseline, and the stimuli were phased to the pulse pressure wave. Thumb flexion and abduction force components were recorded and the magnitude and direction of the resultant force calculated. Electromyographic responses (EMG) were recorded from both the proximal and distal thenar muscle surfaces. 3. For all units, twitch force began to fuse between 5 and 8 Hz and maximum tetanic force was achieved between 30 and 100 Hz. Half-maximum tetanic force was produced at 12 +/- 4 (SD) Hz, as assessed by interpolation using the nearly linear force-frequency relationship between 8 and 30 Hz (on logarithmic frequency coordinates). 4. For the majority of units (n = 19), the strongest force changes in response to variations in stimulation frequency occurred between 5 and 10 Hz (sensitivity 6 +/- 1 mN/Hz). Fewer units showed highest force-frequency sensitivity between 8 and 15 Hz (n = 7; 4 +/- 3 mN/Hz) or 10 and 20 Hz (n = 4; 5 +/- 2 mN/Hz).(ABSTRACT TRUNCATED AT 250 WORDS)
