Objective: To investigate the effects of medium frequency burst-modulated alternating current on the current amplitude necessary to produce maximally tolerated electrically stimulated quadriceps femoris isometric knee extension torque.
Design: Ten healthy volunteers participated in this study. Neuromuscular electrical stimulation was applied to the quadriceps femoris at two different carrier frequencies: 2500 and 5000 Hz. All subjects received both frequencies in a random order. Maximum voluntary isometric contraction torque of knee extension was measured. The current amplitude (in milliamperes) required to produce maximally tolerated isometric contraction knee extension torque at both carrier frequencies was recorded.
Results: The mean maximally tolerated isometric contraction torque (in %maximum voluntary isometric contraction) was 37.6% (SD, +/-20.9) for 2500 Hz and 37.2% (SD, +/-20.9) for 5000 Hz. These values were not significantly different (P = 0.944). However, the mean current amplitude required to produce maximally tolerated isometric contraction torque with 2500 Hz was 91.9 (SD, +/-23.1) mA and with 5000 Hz it was 167.4 (SD, +/-34.5) mA (P < 0.001).
Conclusions: A carrier frequency of 2500 Hz is nearly twice as efficient for generating quadriceps femoris knee extension torque as one at 5000 Hz. Using lower medium frequency burst-modulated alternating current carrier frequencies will allow lower current amplitudes to be used to generate maximum-tolerated contraction torque, and will limit the possibility of reaching the upper limits of current amplitude on an electrotherapeutic device before reaching an individual's maximum tolerable contraction torque.