RESUMO
BACKGROUND: Devices for applying transcutaneous electrical nerve stimulation (TENS) use numerous frequency modulation patterns for decreasing habituation to currents. However, there is no evidence supporting the use of an optimal pattern instead of the others, or even modulated frequencies instead of a fixed frequency. OBJECTIVE: The objectives of this study were to determine the effects of 3 TENS patterns (fixed frequency, random modulation, and 6-second-6-second [6s-6s] modulation) on habituation, to examine the subjective perception of habituation and comfort, and to determine their effects on the mechanical pain threshold (MPT). DESIGN: This study was a randomized, double-blind, sham-controlled crossover trial. SETTING: The study took place in a university research laboratory under attenuated noise conditions at a regulated temperature of 22°C to 26°C. PARTICIPANTS: Thirty-nine volunteers who were healthy participated in this trial. INTERVENTION: The participants received 4 different TENS interventions (fixed frequency, random modulation, 6s-6s modulation, and sham) in random order, with a 24-hour washout period, on the radial nerve. MEASUREMENTS: The main outcome-habituation-was quantified by the increase in current density and the number of times the intensity had to be increased during the session. Secondary outcome variables were subjective perception of habituation and comfort and MPT. RESULTS: Random modulation reduced the number of times the intensity had to be increased because of habituation compared with no modulation (1.6 times; 95% confidence interval [CI] = 0.7-2.6) and 6s-6s modulation (0.8 times; 95% CI = 0.01-1.6). No differences were observed between interventions in terms of an increase in current density, self-perceived habituation, or self-perceived comfort. MPT increased during the intervention in the random-modulation group (4.4 N; 95% CI = 1.8-7.0) and the no-modulation group (5.9 N; 95% CI = 1.9-10.0); there were no significant changes in the other 2 groups. LIMITATIONS: The success of masking or blinding procedures of the participants was not assessed. CONCLUSIONS: Randomly modulated frequencies caused less habituation than nonmodulated frequencies or 6s-6s modulation. Further research on more prolonged (hours-long) interventions in participants with pain is required.
