Brain stimulation techniques for the study of fatige induced by repeated movements

Abstract

Fatigue is a very limiting condition in many activities of the daily living and is present in different neurological pathologies. Although the underlying mechanisms are not well understood, it is considered to have a great impact on the different functional capacities of the people who suffer from it. From a motor system perspective, fatigue is divided as central and peripheral. Central fatigue has been related to the inability to execute or maintain muscle force. Notwithstanding, there are many activities of the daily living, like rhythmic repetitive movements such as walking, typing, mouse-clicking, or movements in assembly lines, which can be executed with very low levels of muscle force and also induce fatigue, likely of central origin. For this reason, it seems paradoxical that central expressions of fatigue induced by repetitive movements have hardly been explored. This PhD Thesis presents results from three different studies. We used noninvasive brain stimulation techniques to characterize central expressions of fatigue induced by movements performed repeatedly. All studies enrolled young healthy participants. In the first study, we evaluated the effect of transcranial direct current stimulation on arm reaching movements, performed repeatedly as fast as possible in a reaction time protocol. In the sham stimulation session, we observed the development of fatigue with task progression, which was absent in the case of real stimulation sessions. Regretfully, the methodology used in this study did not permit us to understand physiological mechanism behind this observation. For the second and third studies, we used transcranial magnetic stimulation, stimulation at the level of the cervicomedullary junction (electric and magnetic) and percutaneous nerve stimulation to explore the central expressions of fatigue induced by repetitive movements. We compared its profile to the central manifestations of fatigue during isometric contractions, which is considered the \gold-standard" to study fatigue. These non-invasive brain stimulation techniques allowed us to identify central cortical circuits as the loci of fatigue produced during un-resisted repetitive movements. Excitability of spinal motoneurons was not impaired. This profile is different from observations during isometric maximal voluntary contractions of same duration, for which an evident impact on the excitability of the spinal motoneurons is present. Our results indicate that central expressions of fatigue induced by movements performed repeatedly are different from those generated during isometric contractions, despite a same task-duration and relative effort. Our results are relevant because they permit dissociating central circuits and structures responsible for different types of motor system fatigue. This is important due to the impact of fatigue on activities of the daily living in physiological and clinical conditions, in sports, or ergonomics. Also, these results allow a better understanding of putative approaches to treat motor system fatigue based on neuromodulation techniques, either in physiological as pathological conditions.