Stumbling reactions in man: influence of corticospinal input

The aim of this study was to evaluate the degree of contribution of supraspinal input to the generation of the compensatory leg muscle activation following stance perturbation. Therefore, evoked motor response (EMR) input-output relations of two different motor tasks were compared at 3 distinct periods: (1) the basic period of muscular activity during standing, i.e. when no additional cortical or spinal activity due to the different tasks is to be expected, (2) the pre-movement period with low background activity, when different spinal and cortical inputs to the motoneuronal pool can be assumed and (3) the period of plateau EMG activity of compensatory and voluntary motor task. Transcranial magnetic stimulation (TMS) just below the motor threshold was applied randomly at 19 different time-intervals before and during the onset of stance perturbation and for comparison during an equivalent voluntary foot-dorsiflexion task. Recordings of electromyographic (EMG) activity from the tibialis anterior (TA) and corresponding ankle-joint movements were made from both legs. Forward-directed displacements were induced by randomly-timed ramp impulses of constant acceleration upon a moveable platform. For comparison, leg muscle EMG was recorded during isometric foot dorsiflexion during stance while leaning back against a support. The stance perturbations were followed by a compensatory response (CR) in the TA with a mean onset time of 81 ms. During the basic period of muscular activity and the period of plateau EMG activity there was no significant difference of the input-output relation between stance perturbation and the voluntary motor task. However, in the voluntary task compared with the CR, there was significantly greater input-output relation (facilitation) of the EMR in the TA following TMS, which may be related to an increased cortical influence. In contrast to this result of the CR following stance perturbation, a facilitation of the EMR was described for hand muscles under corresponding conditions of automatic compensation for muscle stretch, suggesting a transcortical reflex loop. This difference in the results from upper and lower extremity muscles favors the assumption of a predominantly spinal generation of the TA-CR following stance perturbation.