Effect of Burst Frequency and Duration of Kilohertz-Frequency Alternating Currents and of Low-Frequency Pulsed Currents on Strength of Contraction, Muscle Fatigue, and Perceived Discomfort

Background Low-frequency pulsed currents (LPCs) and kilohertz-frequency alternating currents (KACs) are used clinically to augment muscle contractions. Treatment effectiveness may be enhanced by selecting stimulation parameters that evoke the strongest contractions with minimal discomfort and fatigue. Objective The objective of this study was to compare maximally induced strength (force-producing capacity) of contractions, muscle fatigue, and discomfort associated with an LPC and with 3 KACs differing in frequency and duration of burst modulation. Design This was a repeated-measures trial, with randomized order of current presentation. Setting The study was conducted in the physical therapy laboratory at the University of Haifa. Subjects Twenty-six volunteers without impairments, with a mean age of 27.4 years (SD=5.0, range=21–45), participated. Intervention All currents were applied in separate sessions to the wrist extensors of each subject. Currents consisted of an LPC with a 50-Hz pulse frequency and 3 KACs with a 2.5-kHz carrier frequency, including the “Russian current” (RC) burst modulated at 50 Hz with 25 cycles per burst and 2 currents burst modulated at 20 or 50 Hz with 10 cycles per burst. Measurement The maximal electrically induced isometric force, the force integral of 21 electrically induced consecutive contractions, and the degree of discomfort were recorded. Results Force of contraction was not affected by type of current. The LPC was least fatiguing, and the RC was most fatiguing, with the 2 other KACs having an intermediate effect. Degree of discomfort was higher with the KAC modulated at 20 Hz. Conclusions When comfort, strength, and fatigue are considered jointly, the LPC is advantageous. Electrically induced fatigue is affected by the number of cycles per second, rather than the number of bursts per second.