Optimal frequencies for electric stimulation using medium-frequency alternating current.

OBJECTIVE

To determine the effect of single cycles of alternating current of various frequencies on sensory and motor thresholds and on relative thresholds (motor threshold/sensory threshold).

DESIGN

Repeated-measures design.

SETTING

Laboratory setting.

PARTICIPANTS

University student and staff volunteers (N=16; mean age, 34y).

INTERVENTIONS

Single cycles of sinewave frequencies between 1 and 35kHz were delivered at 50Hz. The frequencies were applied in a random order.

MAIN OUTCOME MEASURES

The motor and sensory thresholds were recorded at each applied frequency.

RESULTS

Both sensory and motor thresholds showed a smooth decrease to a minimum at approximately 3kHz. The relative threshold reached a minimum close to 9kHz. Comparison with previous studies showed that although absolute thresholds reach a minimum at a frequency that depends on electrode size, the frequency at which the relative threshold is a minimum was independent of electrode size and independent of whether the stimulus was applied as single pulses or in burst mode.

CONCLUSIONS

The optimal frequency for transcutaneous stimulation using medium-frequency alternating current depends on the outcome measure used. It would therefore be desirable for clinical stimulators to provide a selection of carrier frequencies.