Central fatigue induced by short-lasting finger tapping and isometric tasks: A study of silent periods evoked at spinal and supraspinal levels

Abstract

The neural substrates of fatigue induced by muscular activity have been addressed in depth in relation to isometric tasks. For these activities, when fatigue develops, it has been noted that the duration of the silent periods (SPs) increases in response to both transcranial magnetic stimulation (TMS) of primary motor cortex or electric cervicomedullary stimulation (CMS). However, fatigue is known to be task-dependent and the mechanisms giving rise to a decrease in motor performance during brief, fast repetitive tasks have been less studied. We hypothesized that fatigue induced by repetitive fast finger tapping may have physiological mechanisms different from those accounting for fatigue during an isometric contraction, even in cases of matched effort durations. In these tasks, we examined the contribution of spinal and supraspinal motor circuits to the production of fatigue. The tapping rate and maximal voluntary contractions (MVC), and TMS- and CMS-evoked SPs were obtained at the time of fatigue, and while subjects maintained maximal muscle activation after fast finger-tapping (or isometric activity) of different durations (10 or 30s). Results showed different mechanisms of fatigue triggered by isometric contraction and repetitive movements, even of short duration. Short-lasting repetitive movements induce fatigue within intracortical inhibitory circuits. They increased TMS-SPs, but not CMS-SPs. On the other hand, isometric contraction had a clear impact on spinal circuits. The consideration of these differences might help to optimize the study of fatigue in physiological conditions and neurological disorders.

The Impacts of Central Fatigue on the Polyphasic Nature of Tapping Performance

“…KA16/26) and supported by the Baskent University Research Fund. Each participant performed FT task tests before and after applying the three different central fatiguing models previously defined in the literature (11,12,20).…”

“…All the models are known to induce central fatigue that originates primarily from the nervous system and are described below in short; FATG-1 (sMVRM, Short Duration Maximal Voluntary Repeated Movement): The participants were asked to tap their right index finger with a maximal rate for 10-s (12). FATG-2 (iMVC, Intermittent Maximal Voluntary Contraction): The participants were asked to generate and sustain previously defined Maximal Voluntary Contraction (MVC) force with their right index finger for 15-s and then rest for 5-s, intermittently for 4-min (adopted from 11).…”

“…These studies, in general, make use of wide range of technological systems (12)(13)(14) and fatiguing models (1). In principle, relatively simple and lower technology yet with higher specificity and accuracy methods always attract scientists and have always ranked in priority at least in routine use.…”

Temporal dynamics of muscle, spinal and cortical excitability and their association with kinematics during three minutes of maximal-rate finger tapping

“…Traditionally, their central expressions of fatigue have been studied at the point at which an activity has been completed 9,10 , which is a limitation because the CNS recovers very quickly when the activity ends 11,12 . Several works recently tested fatigue at the central level immediately following the end of unresisted RRMs without allowing time for CNS recovery 5,13,14 . The reduction in the maximal movement rate was greater after 30 s of finger tapping (ft) than that after 10 s, and the reduction was accompanied by an increase in the excitability of M1 GABA b interneurons, which was more pronounced after 30 s of performing the task 5,13 .…”

“…Based on previous studies employing much shorter task durations 5,14 , our hypothesis is that the ft rate will decrease very rapidly during the first few seconds of the task; then, with task progression, the decrease in the ft rate should attenuate. The ft movement amplitude will change more shallowly 5,13,14 . We previously demonstrated a rapid decrease in the ft rate at 30 s and showed that this decrease occurred with increased levels of spinal excitability.…”