Fatigue suppresses ipsilateral intracortical facilitation.
ABSTRACT Experimental data in animals and humans have demonstrated connections between right and left motor cortices. Interactions between these cortical areas can be explored with electrical or magnetic stimulation. In the present study we examined the interhemispheric effect of fatigue on intracortical facilitation (ICF) and inhibition (ICI) using a paired-pulse transcranial magnetic stimulation (TMS) paradigm. Ten healthy subjects performed pinch grips with their left hand with 50% maximum voluntary contraction (MVC) until fatigue occurred. In the control experiment, the same number of pinch grips was performed with 5% MVC without inducing fatigue. Motor evoked potentials (MEP) produced by single and paired pulse TMS over the left motor cortex were recorded from right first dorsal interosseous muscle (FDI) and right abductor digiti minimi muscle (ADM) before and after the tasks. ICF of the right FDI was significantly reduced after fatigue ( P=0.0008). Fifteen minutes after finishing the task ICF had returned to baseline values. There was no change of ICF of right FDI in the control experiment without inducing fatigue. In both experiments the right ADM did not show significant MEP changes. Additional control experiments showed that M-responses and F-waves were unchanged in right FDI after performing the fatigue task with left FDI, and TMS test pulse amplitudes were significantly reduced in left FDI after fatigue. Fatigue caused by pinch grips induces a short-lasting and task-specific suppression of intracortical facilitation in the motor cortex of an homologous contralateral hand muscle. These results indicate interhemispheric interactions between the two motor cortices that are still effective after cessation of movements.
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ABSTRACT: We investigated the effect of fatigue produced by timed maximal voluntary contraction (MVC) of the index finger of one of the hands on performance in MVC and accurate cyclic force production tasks in right-handed subjects. Based on earlier studies, we hypothesized that fatigue would produce an increase in the indices of force-stabilizing synergies in both hands as well as between the hands in two-hand tasks. Synergies were defined as co-varied adjustments of commands to fingers (modes) across cycles that stabilized total force. Fatigue caused a significant reduction in the MVC of the exercised as well as the non-exercised hand. Indices of finger enslaving (lack of individuation) increased with fatigue in both hands, although the increase was significant in the exercised hand only. In contrast to the significant effects of fatigue on MVC forces performed by the non-exercised hand, there were no comparable transfer effects on the root mean square errors during accurate force production. During one-hand tasks, both hands showed high indices of force-stabilizing synergies. These indices were larger in the left hand. Fatigue led to a general increase in synergy indices. Exercise by the left hand had stronger effects on synergy indices seen in both hands. Exercise by the right hand showed ipsilateral effects only. Smaller effects of fatigue were observed on accuracy of performance of the force-down segments of the force cycles compared to the force-up segments. For the bimanual tasks, synergies were analyzed at two hierarchical levels, two-hand (four-finger) and within-a-hand (two-finger). An increase in the synergy index with fatigue was observed at the lower (two-finger) level of the hierarchy only. We interpret the lack of effects of fatigue at the upper (two-hand) level as a consequence of a trade-off between synergies at different levels of the hierarchy. The differences between the hands are discussed within the dynamic dominance hypothesis.Human movement science 11/2012; · 2.15 Impact Factor
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ABSTRACT: Plunger operated syringes are typically used by laboratory staff to dispense precise quantities of liquid. Such workers commonly complain of discomfort in the upper extremities as a result of prolonged repetitive actions over the course of their workday. A pipetting task was designed to quantify thumb loading and muscle activity measured by pinch strength and surface electromyography (EMG). The root mean square (RMS) EMG was recorded for the abductor pollicis brevis, extensor pollicis brevis and flexor pollicis longus. Analytical results indicate that pinch strength decreased significantly by 20.9% after pipetting tasks. Females had higher reduction rates in maximum pinch force (31.5%) than males (14.8%). User of a large syringe had negative significant effects on thumb loading and muscle activities; however, no statistically significant difference existed between hand conditions, and no interactive effects existed between syringe size and hand condition. We conclude that syringe size influences applied thumb force and muscle activity.Journal of the Chinese Institute of Industrial Engineers 01/2009; 26(6):493-498.
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ABSTRACT: This study sought to determine whether afferent feedback associated with peripheral muscle fatigue inhibits central motor drive (CMD) and thereby limits endurance exercise performance. On two separate days, 8 males performed constant-load single-leg knee extensor exercise to exhaustion (85% of peak power) with each leg (Leg1 and Leg2). On another day, the performance test was repeated with one leg (Leg1) and consecutively (within 10-s) with the other/contralateral leg (Leg2-post). Exercise-induced quadriceps fatigue was assessed by reductions in potentiated quadriceps twitch-force from pre- to post-exercise (ΔQtw,pot) in response to supra-maximal magnetic femoral nerve stimulation. The output from spinal motoneurons, estimated from quadriceps electromyography (iEMG), was used to reflect changes in CMD. Rating-of-perceived-exertion (RPE) was recorded during exercise. Time to exhaustion (~9.3min) and exercise-induced ΔQtw,pot (~51%) were similar in Leg1 and Leg2 (P>0.5). In the consecutive leg trial, endurance performance of the first leg was similar to that observed during the initial trial (~9.3min, P=0.8); however, time to exhaustion of the consecutively exercising contralateral leg (Leg2-post) was shorter than the initial Leg2 trial (4.7±0.6min vs 9.2±0.4min, P<0.01). Additionally, ΔQtw,pot following Leg2-post was less than Leg2 (33±3% vs 52±3%, P<0.01). Although the slope of iEMG was similar during Leg2 and Leg2-post, end-exercise iEMG following Leg2-post was 26% lower compared to Leg2 (P<0.05). Despite a similar rate-of-rise, RPE was consistently ~28% higher throughout Leg2-post vs Leg2 (P<0.05). In conclusion, this study provides evidence that peripheral fatigue and associated afferent feedback limits the development of peripheral fatigue and compromises endurance exercise performance by inhibiting CMD.Journal of Applied Physiology 05/2013; · 3.43 Impact Factor