Central fatigue after cycling evaluated using peripheral magnetic stimulation.

UNLABELLED: Central and peripheral mechanisms contribute to fatigue during exercise. Electrical and transcranial magnetic stimulation have been used to assess these fatigue mechanisms. Peripheral magnetic stimulation (PMS) of the femoral nerve is associated with very little subject discomfort and has been shown to elicit quadriceps contractions of >70% maximal voluntary contraction (MVC). PURPOSE: To examine peripheral versus central mechanisms of fatigue in men during prolonged cycling using a peripheral nerve magnetic stimulation-based technique. METHODS: Eleven men (aged 41 +/- 3 yr) cycled for 2 h at approximately 66% of VO2peak (55 +/- 2 mL x kg(-1) x min(-1)) with five 1-min sprints interspersed, followed by a 3-km time trial. Oxygen consumption was measured every 20 min to verify a constant workload. RPE were measured simultaneously and during each sprint using a Borg scale. Quadriceps isometric strength testing was performed in a seated position before and after cycling: 1) MVC, 2) MVC with superimposed magnetic stimulation to measure central activation ratio (CAR), 3) femoral nerve stimulation alone. One-minute recoveries were allowed between contractions. Changes in metabolic measurements over time were analyzed with repeated-measures ANOVA, and strength changes before to after with Student's paired t-tests. RESULTS: HR (P = 0.03) and RPE (P < 0.001) increased over time during the 2 h, and MVC declined by 22% (P = 0.001) indicating fatigue. Force elicited by PMS alone decreased 17% (P < 0.001). CAR decreased from 83% before exercise to 71% (P = 0.005) after exercise indicating a loss of central drive. PMS-induced force was > or =90% of MVC. CONCLUSIONS: Results clearly demonstrate that trained cyclists experience significant central fatigue during prolonged cycling. PMS may be a better technique for identifying central fatigue than the traditionally used interpolated twitch technique.

Transcutaneous magnetic stimulation of the quadriceps via the femoral nerve.

We investigated the efficacy of different frequencies and intensities of magnetic stimulation for activating the quadriceps muscles; a painless method for stimulating the quadriceps would be useful in the rehabilitation of patients who have difficulty in voluntarily activating their muscles after injury or surgery. Eleven subjects underwent magnetic stimulation of the femoral nerve over a range of frequencies and intensities using a MagStim Rapid magnetic stimulator. Magnetic stimulation at 30 Hz at 80% of the power output of the stimulator used was capable of generating 72% of quadriceps maximal voluntary contraction torque. Subjects reported little or no pain during the procedure. Magnetic stimulation of the femoral nerve is a well-tolerated way to activate the quadriceps muscles.