Aim: 

We previously observed that index tapping is preceded by inhibitory postural adjustments (iAPA) in the ongoing EMG of several upper limb muscles, including Biceps Brachii (BB) and Superior Trapezius (ST). Aim of the present work is to investigate the spinal and supra-spinal contribution to this effect.

Methods: 

Index tapping was triggered by an acoustic signal, while EMG from the prime mover (flexor digitorum superficialis) and from BB or ST was recorded by surface electrodes. Changes in the cortico-spinal excitability were revealed by changes of the cortical motor evoked potentials (cMEPs) size, while spinal excitability was probed by the T-reflex size. Both motor potentials were evoked in BB and ST during the iAPA development and before the go signal.

Results: 

In all 9 subjects, with the hand prone and the postural muscle contracted, a clear inhibition of both BB and ST EMG started 50 23 ms before the prime mover onset. However, when postural muscles were fully relaxed T-reflexes falling in the iAPA window were unmodulated, while cMEPs were effectively reduced in size, suggesting a supra-spinal origin of the inhibition. In each subject, statistical analysis (t-test comparing cMEPs or T-refexes falling in the iAPA window versus those falling prior to the go-signal) gave significance to the effect (p < 0.02). Moreover, a high correlation (p < 0.01) was found between the amplitude of the ongoing EMG and the cMEPs evoked in the relaxed muscle. Lastly, on average, MEP inhibition developed 98 33 ms before the prime mover onset, i.e. about 50 ms prior to the iAPA onset.

Conclusion: 

These results show i) that when postural muscles are not recruited under threshold iAPAs are still associated to the motor command and ii) that during the iAPA spinal motoneurones are more likely dis-facilitated than inhibited. Thus, iAPAs seem to be fully sustained by inhibitory circuits located in the supra-spinal centres.