Influence of posture and voluntary background contraction upon compound muscle action potentials from anterior tibial and soleus muscle following transcranial magnetic stimulation.

Compound muscle action potentials (CMAPs) were recorded from anterior tibial (TA) and soleus (SOL) muscles following transcranial magnetic stimulation of motor cortex in 10 healthy subjects: (1) while standing upright without support, (2) while sitting, and (3) while lying supine. The results of this study demonstrate a significant influence of posture upon the amplitudes of CMAPs. Postural facilitation presented itself, firstly, in terms of a higher incidence of bilateral activation of distal leg muscles during stance and, secondly, as significant enhancement of the amplitude of CMAPs while standing as compared to lying supine. The onset latency, however, did not disclose a significant shortening during stance. To assess the effects of preinnervation subjects voluntarily adjusted the level of TA activity to 5%, 10% and 20% of maximum isometric force respectively before cortex stimulation. Voluntary background contraction resulted in a significant increase of amplitude of CMAPs but, in contrast to postural facilitation, concomitant with a significant decrease in onset latency. These results point to a somewhat different mechanism of facilitation during stance as compared to voluntary preinnervation. But it cannot be decided whether cortical mechanisms, different descending systems, the spinal circuitry or a combination of these factors is responsible for the observed effects.