Reciprocal changes of excitability between tibialis anterior and soleus during the sit-to-stand movement

The excitability of spinal motoneurons is modified by central preparatory commands before muscle activation. In relatively complex long duration motor tasks such as the sit-to-stand (STS) movement, the central nervous system commands have to take into account the inputs from muscle, skin, and joint afferents during muscle contraction. We have investigated the changes occurring in tibialis anterior (TA) and soleus (SOL) motoneuronal excitability prior to and during the STS movement in normal subjects. Twelve healthy volunteers received the instruction to rise from a chair at the perception of an acoustic 'go' signal. Cortical transcranial magnetic stimuli (TMS) or peripheral nerve electrical stimuli (PNS) were used as test stimuli to elicit, respectively, the motor evoked potential (MEP) and the H reflex, at intervals of 50-1500 ms after the 'go' signal. Both the MEP and the H reflex were enhanced in the TA between 100 and 900 ms after the 'go' signal. At the same time there was inhibition of the H reflex but not of the MEP in the SOL. At the end of the STS movement, during quiet standing, the size of both the H reflex and the MEP of the TA were not different from those obtained in the sitting position. However, in SOL, the H reflex was smaller, and the MEP was larger, than at rest. Our observations suggest the participation of several mechanisms of control of motoneuronal excitability during the STS, ultimately leading to a dominant role of presynaptic inhibitory mechanisms in SOL during standing.