Motor-evoked potential changes during human muscular fatigue – A triple stimulation technique study

Objective: To assess motor evoked potential (MEP) changes during human muscular fatigue.

Methods: Twenty-three healthy subjects participated in the study. The exercise protocol comprised a pre-exercise period, a 2 minutes lasting fatiguing exercise by repetitive maximal voluntary contractions (MVC) of the abductor digiti minimi muscle at 0.5Hz, and a 10 minutes lasting post-exercise period. We applied the triple stimulation technique (TST) as previously described by Magistris et al. (1998). Muscular force was recorded during all experiments. TST measurements were performed at defined intervals during the exercise and recovery. In 2 sets of experiments, the responses were either facilitated by a muscular contraction of 20% MVC (experiment 1), or were performed during muscular relaxation (experiment 2).

Results: Force recordings:

In all experiments, the force declined significantly and consistently to approximately 40% of pre-exercise level at the end of the exercise.

TST measurements:

Experiment 1 (responses facilitated by 20% MVC): The TST amplitudes decreased significantly during the fatiguing exercise (–56%) and reached pre-exercise levels within 4min of the recovery period.

Experiment 2 (responses recorded during relaxation): The TST amplitudes increased during the fatiguing exercise by 25% and rapidly decreased during the first 15s of the recovery period (–60%) and remained depressed until the end of the experiments. These changes were statistically different compared to experiment 1.

Conclusion: MEPs undergo profound changes during exercise and recovery. Here we demonstrate that these changes depend heavily on the facilitation maneuvers applied during recording of the MEPs. Responses facilitated by voluntary muscle contractions decrease during exercise, while responses recorded during muscle relaxation increase during exercise. This difference may be due to a differential involvement of supraspinal vs. spinal neurons in central fatigue.

Supported by SNF grant 3200B0–100701