Short-interval intracortical inhibition is not affected by varying the complexity of an isometric task in biceps brachii muscle

Introduction: There is considerable evidence that the excitability of neural networks involved in motor control is modulated in a task-dependant manner. At the level of the primary motor cortex (M1), corticomotoneurons have been shown to be more active during complex tasks. These changes in neural excitability are an adaptive response to the requirements of the motor task. However, the effect of varying the difficulty or complexity of the same task on the excitability of neural circuits is less understood, despite the importance of task-complexity in the rehabilitation of fine motor skill. The purpose of this study was to determine whether taskdependant differences in motor cortical network excitability occur when varying the complexity, via visuomotor feedback of an elbow flexion task. Methods: Focal transcranial magnetic stimulation (TMS) was used to measure corticospinal excitability and short-intervalintracortical inhibition (SICI) of the contralateral biceps brachii (BB) in 10 healthy subjects, performing two isometric tasks of differing visuomotor demand at 5, 20 & 40% of maximum voluntary contraction force (MVC). To produce two tasks with differing visuomotor demand, the scaling of the on-line torque trace was modified. Results: Corticospinal excitability did not differ between tasks (p > 0.05), did differ between torque levels (p < 0.05) and there was no task x torque level interaction effect (p > 0.05). SICI did not differ between tasks (p > 0.05), did between torque levels (p < 0.05) and there was no task x torque level interaction (p > 0.05). Discussion: This study shows that the excitability of intracortical neural networks namely, intracortical inhibition; is reduced during increasing levels of torque production. The present findings demonstrate the influence of target torque levels on disinhibition of corticospinal neurons (i.e. increase M1 excitability), modulated by neurons responsible for SICI. Conclusion: This study has demonstrated that intracortical inhibition is reduced during graded voluntary muscle action, which suggests that force production is modulated by neurons confined to the M1. Interestingly, the level of inhibition is not altered when varying the difficulty of the same task. This finding, which is in contrast to findings on lower arm musculature, suggests that SICI is more markedly modulated by force gradation than by task visuomotor demands at least in the BB muscle. These findings have important clinical implications in patient populations who have increased levels of intracortical inhibition.