Human motor fatigability as evoked by repetitive movements results from a gradual breakdown of surround inhibition

Abstract

Motor fatigability emerges when demanding tasks are executed over an extended period of time. Here, we used repetitive low-force movements that cause a gradual reduction in movement speed (or ‘motor slowing’) to study the central component of fatigability in healthy adults. We show that motor slowing is associated with a gradual increase of net excitability in the motor network and, specifically, in primary motor cortex (M1), which results from overall disinhibition. Importantly, we link performance decrements to a breakdown of surround inhibition in M1, which is associated with high coactivation of antagonistic muscle groups. This is consistent with the model that a loss of inhibitory control might broaden the tuning of population vectors such that movement patterns become more variable, ill-timed and effortful. We propose that the release of inhibition in M1 is an important mechanism underpinning motor fatigability and, potentially, also pathological fatigue as frequently observed in patients with brain disorders.

Anodal transcranial direct current stimulation reduces motor slowing in athletes and non-athletes

“…Regarding the lower extremities, our findings go in line with previous investigations by Bächinger et al [32], revealing MoSlo during a 30-second alternating FTT. Performing additional analysis concerning the influence of recovery, authors suggest that the mechanism which causes MoSlo appears to fully recover during the subsequent break.…”

“…Hence, values were normalized to the first bin of the first test block (initial 0-5 = 100%). In a final step, the presence of MoSlo was defined as a reduction of tapping frequency (deltas) from the first to the last bin of the normalized data of each test block [32].…”

“…The focus was on the question whether MoSlo differs between trained athletes and non-athletes and to what extend the decline can be modulated by means of tDCS. First, we expected the tapping frequency to decrease in both upper and lower extremities [32], regardless of training status and sports. On an exploratory level, we aimed at revealing if athletes would show specific MoSlo patterns as compared to non-athletes [33], and, furthermore, if there are any differential effects between different kinds of sport.…”

Peripheral-central interplay for fatiguing unresisted repetitive movements: a study using muscle ischaemia and M1 neuromodulation

“…During maximal-rate finger tapping (FT), even for periods as short as 30 s, the tapping frequency reduces rapidly, the muscle slows, and the excitability of spinal motor neurons and M1 inhibitory interneurons (operating through GABA b receptors) increases 6 – 8 . Movement deterioration during fatiguing FT is similar to that observed during unresisted RRM executed by other body segments 5 . This observation suggests that fatigue may impact central structures engaged in rhythm formation and stresses the importance of understanding the physiological mechanism of fatigue in many situations, such as activities of daily living, sports, and the care of neurological patients.…”

“…Maximal-rate rhythmic repetitive movements (RRMs) cannot be sustained for very long, even when the movements are unresisted, and it is known that the slowing of muscle relaxation is a major peripheral mechanism of RRM fatigue 1 , 2 . However, there is also growing evidence for a neural origin of fatigue during these types of movements 3 – 5 , and their characteristics clearly differ from those expressed during fatiguing isometric contractions performed with the same body segment and for an equivalent period of time 6 , 7 .…”