title: The effect of burst-duty-cycle parameters of medium-frequency alternating current on maximum electrically induced torque of the quadriceps femoris, discomfort, and tolerated current amplitude in professional soccer players.
abstract: METHODS
  Repeated-measures, within-subject crossover trial.
  
  
  OBJECTIVE
  The primary objective was to assess the effect of the burst-duty-cycle parameters of medium-frequency alternating current on the maximum electrically induced torque of the quadriceps femoris. The secondary objectives were to evaluate the amount of discomfort tolerated and the maximum current amplitude delivered for each electrical-stimulation condition.
  
  
  BACKGROUND
  Neuromuscular electrical stimulation used for muscle strengthening can improve functional performance. However, the electrical-stimulation parameters to achieve optimal outcomes are still unknown. Previous studies have demonstrated that the characteristics of the burst duty cycle of medium-frequency alternating current influence torque-generation levels and perception of sensory discomfort.
  
  
  METHODS
  The maximum electrically induced torque was assessed with a medium-frequency alternating current, with a carrier frequency of 2500 Hz and a modulated frequency of 50 Hz. The current amplitude was gradually increased to the point of the participant's maximum tolerance level. The testing sequence for the 3 burst duty cycles (20%, 35%, and 50%) was performed in a randomized order.
  
  
  RESULTS
  Electrical stimulation using a 20% burst duty cycle produced an electrically induced torque greater than the 35% (P = .01) and 50% (P<.01) burst duty cycles, with no difference between the 35% and 50% burst duty cycles (P = .46). There was no difference in the amount of sensory discomfort produced by the 3 durations of burst duty cycles (P = .34). There was also no difference between the 3 conditions for the maximum current amplitude tolerated (P = .62).
  
  
  CONCLUSIONS
  The burst duty cycle of 20% of medium-frequency alternating current, compared to burst duty cycles of 35% and 50%, produced higher peak torque of the quadriceps femoris in professional soccer players. There was no difference in discomfort produced and current amplitude tolerated between the different burst-duty-cycle conditions.
authors.affiliation: Master's and Doctoral Programs in Physical Therapy, Universidade Cidade de São Paulo, São Paulo, Brazil.
authors.forename: Richard Eloin
authors.initials: RE
authors.lastname: Liebano
authors.forename: Silas
authors.initials: S
authors.lastname: Waszczuk
authors.forename: Juliana Barbosa
authors.initials: JB
authors.lastname: Corrêa
mesh_terms: D000293:Adolescent; D018592:Cross-Over Studies; D004558:Electric Stimulation; D004599:Electric Stimulation Therapy; D006801:Humans; D008297:Male; D052097:Quadriceps Muscle / Q000502:physiology*; D012916:Soccer / Q000502:physiology; D019415:Torque; D055815:Young Adult