Central fatigue after cycling evaluated using peripheral magnetic stimulation

Abstract

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.