Preparatory process for anticipatory postural adjustments: modulation of leg muscles reflex pathways during...

We have investigated, in 6 standing subjects, the time course of amplitude changes in the short latency (40-60 ms) and long latency (60-80 ms) reflex response components of gastrocnemius medialis (GM) and soleus (S) muscles during the preparatory period (time between a warning signal and a response signal) as a function of the precued direction (pull or push) of arm movement. Subjects maintained their standing posture by visual feedback during the 1.5 s preparatory period of a reaction time task. A warning signal gave advance information concerning the voluntary response to be performed which consisted of either a pull or push movement of the right arm. The excitability of the reflex pathways was evaluated by triggering a rapid rotation of the right ankle joint (dorsiflexion) applied randomly at 100, 300 or 500 ms before the response signal for arm movement. Statistical analysis of EMG amplitude (ANOVA) showed that preparatory effects were different for the two synergistic muscles with both short (spinal) and longer latency components of GM showing generalized facilitation and of S showing generalized inhibition. In addition, the longer latency, presumably supraspinal, component of the two muscles was differentially modulated according to directional advance information, showing relative facilitation for pull as compared to push trials. These reflex response modulations were emphasized in faster reaction time (RT) performers at the end of the preparatory period. It is concluded that postural preparatory processes are reflected at the spinal level in global effects and at the supraspinal level in directionally specific effects.