title: Force output during fatigue with progressively increasing stimulation frequency.
abstract: There is currently a controversy over whether stimulation frequencies should increase or decrease to optimize force output over time. This study compared changes in thenar muscle force and M-wave amplitude during progressively increasing (20-40 Hz), decreasing (40-20 Hz) and constant (20 Hz) frequency stimulation of the median nerve continuously for 3 min. Twenty-three individuals participated in three sets of experiments. There was no significant difference in the force-time integrals between the three fatigue tasks. The rate of fatigue was not correlated to the number of stimulation pulses delivered (20 Hz: 3,600, 20-40 and 40-20 Hz: 5,400). All fatigue tasks caused a significant reduction in M-wave amplitude and the reduction was largest for the 20-40 Hz protocol. However, multiple linear regression analysis revealed that the M-wave amplitude could not predict the changes in force over time for the 20 Hz or 20-40 Hz protocols. Thus during sustained evoked contractions with stimulation frequencies within the physiological range, frequencies can vary significantly without changing the overall force-time integral.
authors.affiliation: Department of Kinesiology Health Education, 1 University Station, D3700, Bellmont 222, University of Texas, Austin, TX 78712, USA. l.griffin@mail.utexas.edu
authors.forename: L
authors.initials: L
authors.lastname: Griffin
authors.forename: B G
authors.initials: BG
authors.lastname: Jun
authors.forename: C
authors.initials: C
authors.lastname: Covington
authors.forename: B M
authors.initials: BM
authors.lastname: Doucet
mesh_terms: D000328:Adult; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography / Q000379:methods; D005260:Female; D006225:Hand / Q000502:physiology; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D020127:Recovery of Function / Q000502:physiology; D013997:Time Factors