Supra-spinal circuits shape inhibitory postural adjustments anticipating voluntary index-finger flexion.

We previously observed that index finger flexion, with the prone hand, is preceded by an inhibitory postural adjustment (iAPA) in the Biceps Brachii (BB) electromyogram (EMG). The aim of this work is to locate the iAPA origin by testing the changes in spinal and cortico-spinal excitability during its development. Subjects were asked to flex their index finger, at will after an acoustic signal, while surface EMGs from Flexor Digitorum Superficialis (prime mover) and from BB (postural muscle) were recorded. In each experiment, tendon (T) reflexes in the resting BB and motor-evoked potentials elicited by magnetic cortical stimulation (CMEPs) in the resting, or in the slightly contracted BB, were collected both during the iAPA development and before the go signal. When BB was fully relaxed, T-reflexes falling in the iAPA temporal window were un-modulated, while CMEPs were significantly inhibited; this inhibition occurred, on average, 37 ms before the iAPA onset indicating that iAPA sets up and develops at supraspinal level. Changes in CMEPs' excitability clearly anticipated the iAPA onset (about 50 ms) also when cortico-spinal excitability was probed in the active BB, thus indicating that this effect is likely due to a de-recruitment of spinal motoneurones. Thus, (1) iAPA is associated with voluntary movement even when postural muscles are at rest, (2) during iAPA spinal motoneurones are more likely disfacilitated than inhibited, and (3) the iAPA timing is regulated similarly to prime mover activation. All together these results suggest that iAPAs is fully sustained by inhibitory circuits located in the supra-spinal centres.