ABSTRACT
From the general hypothesis that preparatory adjustments set up before a motor task depend upon the response probability, and are patterned according to the biomechanical requirements of the movement to be performed, four experiments were designed in order to analyze, using reflexogenic methods, the changes in the excitability of spinal motor structures during the preparatory period of different RT paradigms. Principally, it was found that the general depressive control exerted from central nervous structures upon the spinal reflex pathway during preparation was strengthened when the muscle was involved in the execution of the response and when the muscle was located on the preferred body side. However, the depth of such a control did not depend, in a choice—RT condition, upon the probability for the muscle to be involved. Moreover it was only slightly modified when the role played by the muscle in the response—movement was changed from agonistic to antagonistic as well as when the expected force of the movement was increased. In relation to the likely neurophysiological mechanisms underlying inhibitory central influences acting at the spinal level, the functional significance of this relatively widespread depression of the reactivity of spinal reflex pathways is discussed. It is mainly concluded that such a phenomenon has to be considered as the negative component of the preparatory process, that means an active suspension of the execution of the response, and/or a temporary protection of spinal motor structures 140against exogenic influences able to prematurely trigger or disturb the execution of the motor program. Finally, the weak specificity of the changes observed at the spinal level during preparation is regarded as sustaining the idea that preparatory adjustments mainly intervene at earlier stages of the response information processing.
