No abstract

The study was conducted to contrast exertion-dependent changes in electromyography (EMG), force fluctuation (FF), and limb acceleration (LA) during isometric contraction to attain a versatile picture of muscle fatigue. Fifteen volunteers performed sustained index abduction at 25 and 75% maximal voluntary contractions (MVC) until failure at the tasks; meanwhile, changes in temporal/spectral features of force, muscle activity of the first dorsal interosseous (FDI), and acceleration of the index and hand were monitored. The results showed a manifest increase in all recorded signals for the 25% MVC paradigm, especially for LA, which demonstrated the largest increment in amplitude. In addition to progressive enhancement of the mechanical coupling of the metacarpophalangeal (MCP) joint, the 25% MVC paradigm added to EMG-FF and EMG-LA coherences (CohEMG-FF and CohEMG-LA) at 8-12 Hz and the shift of the spectral peak of the LA to higher frequencies. In contrast, the 75% MVC paradigm did not modulate significantly the spectral peak of LA. Also, CohEMG-FF, CohEMG-LA at 8-12 Hz, and the mechanical coupling of the MCP joint, were conversely undermined consequent to the high exertion paradigm. The present study suggests that LA was most susceptible to muscle fatigue following sustained contraction at a lower exertion level, and diverging alternations among various physiological signals ascribed to exertion-dependent contributions of central and peripheral origins to muscle fatigue.

Visual feedback and non-visual information play different roles in tracking of an external target. This study explored the respective roles of the visual and non-visual information in eleven healthy volunteers who coupled the manual cursor to a rhythmically moving target of 0.5 Hz under three sensorimotor conditions: eye-alone tracking (EA), eye-hand tracking with visual feedback of manual outputs (EH tracking), and the same tracking without such feedback (EHM tracking). Tracking error, kinematic variables, and movement intermittency (saccade and speed pulse) were contrasted among tracking conditions. The results showed that EHM tracking exhibited larger pursuit gain, less tracking error, and less movement intermittency for the ocular plant than EA tracking. With the vision of manual cursor, EH tracking achieved superior tracking congruency of the ocular and manual effectors with smaller movement intermittency than EHM tracking, except that the rate precision of manual action was similar for both types of tracking. The present study demonstrated that visibility of manual consequences altered mutual relationships between movement intermittency and tracking error. The speed pulse metrics of manual output were linked to ocular tracking error, and saccade events were time-locked to the positional error of manual tracking during EH tracking. In conclusion, peripheral non-visual information is critical to smooth pursuit characteristics and rate control of rhythmic manual tracking. Visual information adds to eye-hand synchrony, underlying improved amplitude control and elaborate error interpretation during oculo-manual tracking.

Force intermittency is one of the major causes of motor variability. Focusing on the dynamics of force intermittency, this study was undertaken to investigate how force trajectory is fine-tuned for static and dynamic force-tracking of a comparable physical load. Twenty-two healthy adults performed two unilateral resistance protocols (static force-tracking at 75% maximal effort and dynamic force-tracking in the range of 50%–100% maximal effort) using the left hand. The electromyographic activity and force profile of the designated hand were monitored. Gripping force was off-line decomposed into a primary movement spectrally identical to the target motion and a force intermittency profile containing numerous force pulses. The results showed that dynamic force-tracking exhibited greater intermittency amplitude and force pulse but a smaller amplitude ratio of primary movement to force intermittency than static force-tracking. Multi-scale entropy analysis revealed that force intermittency during dynamic force-tracking was more complex on a low time scale but more regular on a high time scale than that of static force-tracking. Together with task-dependent force intermittency properties, dynamic force-tracking exhibited a smaller 8–12 Hz muscular oscillation but a more potentiated muscular oscillation at 35–50 Hz than static force-tracking. In conclusion, force intermittency reflects differing trajectory controls for static and dynamic force-tracking. The target goal of dynamic tracking is achieved through trajectory adjustments that are more intricate and more frequent than those of static tracking, pertaining to differing organizations and functioning of muscular oscillations in the alpha and gamma bands.

In light of the interplay among physiological finger tremors, this study was undertaken to investigate the transfer effect of fatigue on coordinative strategies of multiple fingers. Fourteen volunteers performed prolonged position tracking with a loaded middle finger while measures of neuromuscular function, including electromyographic activities of the extensor digitorum (ED)/flexor digitorum superficialis (FDS) and physiological tremors of the index, middle, ring, and little fingers, were monitored. The subjects exhibited inferior tracking congruence and an increase in ED activity at the end of the tracking. Fatigue spread was manifested in a remarkable increase in tremor across fingers, in association with enhanced involuntary tremor coupling among fingers that was topologically organized in relation to the distance of the digits from the middle finger. Principal component analysis suggested that an enhanced 8- to 12-Hz central rhythm contributed primarily to the tremor restructure following fatigue spread. The observed tremor reorganization validated the hypothesis that the effect of fatigue was not limited to the instructed finger and that fatigue functionally decreased independence of the digits. The spreading of fatigue weakens neural inputs that diverge to motor units acting on various digits because of fatigue-related enhancement of common drive at the supraspinal level.