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. 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. 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). 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). The frequency required to elicit 50% maximum tetanic force was significantly correlated with various measures of contractile rate, including twitch contraction time, twitch (or tetanic) one-half relaxation time, and normalized maximum contraction and relaxation rates. Twelve units were subjected to additional tetanic stimulation, which included tests to measure force fatigue. After this stimulation, the maximum tetanic force from seven units decreased. These units then needed higher stimulation frequencies to produce any given force. The tetanic force from the remaining units showed little or no decline. When stimulated at any given submaximal frequencies, these units produced more force than before the fatigue test. It is concluded that low excitation rates are required to generate 50% maximum force in human thenar motor units and that this frequency may either increase or decrease with activity, depending on whether the force of the unit is fatiguing or potentiating, respectively.
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