Variation in motor threshold with frequency using kHz frequency alternating current

We investigated the frequency dependence of motor thresholds over the frequency range 1 kHz to 25 kHz. Alternating current (AC), ramped in intensity, was applied transcutaneously, and the induced wrist extensor torque was measured. Plots of log torque versus stimulus voltage were used to accurately determine thresholds. Three kinds of sinusoidal AC stimuli were compared: continuous, 10 ms bursts at 50 Hz, and 50-Hz single-cycle. Differences were attributed to summation of subthreshold depolarizations. The variation in relative thresholds (continuous/single-cycle and burst/single-cycle) indicates that summation occurs more efficiently at higher kHz frequencies. The observed frequency and waveform dependence provides evidence for high-frequency nerve fiber firing rates and fiber dropout when continuous or modulated AC is used, with the effects increasing with AC frequency. The form of the motor response evoked at high frequencies has features that suggest that frequencies above 10-kHz have little or no useful clinical role in rehabilitation procedures.