title: Initial electrical stimulation frequency and cramp threshold frequency and force.
abstract: BACKGROUND
  In the electrically induced cramp model, the tibial nerve is stimulated at an initial frequency of 4 Hz with increases in 2-Hz increments until the flexor hallucis brevis cramps. The frequency at which cramping occurs (ie, threshold frequency [TF]) can vary considerably.  A potential limitation is that multiple subthreshold stimulations before TF might induce fatigue, which is operationally defined as a decrease in maximal voluntary isometric contraction (MVIC) force, thereby biasing TF.
  
  
  OBJECTIVE
  To determine if TF is similar when  initially stimulated at 4 Hz or 14 Hz and if MVIC force is different among stimulation frequencies or over time (precramp, 1 minute postcramp, and 5 minutes postcramp).
  
  
  METHODS
  Crossover study.
  
  
  METHODS
  Laboratory.  Patients or  Other Participants: Twenty participants (13 males: age = 20.6 ± 2.9 years, height = 184.4 ± 5.7 cm, mass = 76.3 ± 7.1 kg; 7 females: age = 20.4 ± 3.5 years, height = 166.6 ± 6.0 cm, mass = 62.4 ± 10.0 kg) who were prone  to cramps.
  
  
  METHODS
  Participants performed 20 practice MVICs. After a 5-minute rest, three 2-second MVICs were recorded and averaged for the precramp measurement. Participants were stimulated at either 4 Hz or 14 Hz, and the frequency was increased in 2-Hz increments  from each initial frequency until cramp. The MVIC force was reevaluated at 1 minute and 5 minutes postcramp.
  
  
  METHODS
  The TF and MVIC force.
  
  
  RESULTS
  Initial stimulation frequency did not affect TF (4 Hz = 16.2  ± 3.8 Hz, 14 Hz = 17.1 ± 5.0 Hz; t(19) = 1.2, P = .24). Two participants had inaccurate TFs when initially stimulated at 14 Hz; they cramped at 10 and 12 Hz in the 4-Hz condition. The MVIC force did not differ between initial frequencies (F(1,19)  = 0.9, P = .36) but did differ over time (F(2,38) = 5.1, P = .01). Force was lower at 1 minute postcramp (25.1 ± 10.1 N) than at precramp (28.7 ± 7.8 N; P, .05) but returned to baseline at 5 minutes postcramp (26.7 ± 8.9 N; P  > .05).
  
  
  CONCLUSIONS
  The preferred initial stimulation frequency might be 4 Hz because it did not alter or overestimate TF. The MVIC force was lower at 1 minute postcramp, suggesting the induced cramp rather than the varying electrical frequencies affected force. A 1-  to 5-minute rest should be provided postcramp induction if multiple cramps are induced.
authors.affiliation: North Dakota State University, PO Box #6050, Department #2620, Fargo, ND 58108, USA. kevin.c.miller@ndsu.edu
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Miller
authors.forename: Kenneth L
authors.initials: KL
authors.lastname: Knight
mesh_terms: D018592:Cross-Over Studies; D004558:Electric Stimulation / Q000009:adverse effects*; D004576:Electromyography; D005221:Fatigue; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D009120:Muscle Cramp / Q000209:etiology*; D018482:Muscle, Skeletal / Q000502:physiology; D013979:Tibial Nerve; D055815:Young Adult