The unconscious mental inhibiting process of human maximal voluntary contraction

Abstract

Human maximal voluntary contraction (MVC) is believed to be limited by neural inhibition. Motivational goal priming alters background states of the motor system, leading to enhanced MVC. However, the mechanisms that determine the constant inhibition of force exertion in the motor system remain unclear. The primary behavioural goal of MVC is maximal voluntary force exertion. The final expected or desired state of this behavioural goal is explicitly demonstrated with words related to physical exertion, such as 'maximal', irrespective of the possibility of demand-like properties in participants' minds, such as attainability and/or desirability of the goal. For the primed maximal goal state, most trial results fail to meet expectations, demonstrating negative affect that, without awareness, contributes to the mentioned inhibitory mechanism underlying MVC. We therefore speculated that the behavioural goal of MVC contributes to neural inhibitory mechanisms underlying MVC. In our study, we used a previously developed paradigm (Takarada and Nozaki in Scientific Reports 8: 10135, 2018) in which subliminal visual priming stimuli such as the physical exertion-related words "perform" and "exert" were presented to 12 healthy participants and were followed by supraliminal words that were the word "maximal" or neutral.We found that when combined with the term 'maximal' in the consciously visible form, the effect of this subliminal motor-goal priming in inducing pupil dilation and stronger action preparation/execution was abolished without conscious awareness. This is the first objective evidence of motor inhibitory effect-predicting patterns of pupil-linked noradrenergic activity as a signature of a type of mental inhibition underlying the MVC behavioural goal.