Proceedings of The Physiological Society
University College London December 2005 (2006) Proc Physiol Soc 1, PC13
Supporting muscles are less active following self-triggered than externally induced unloading during human walking
Baken, Bernke Christianne Maria; Hauser, Carmen; Geurts, Alexander; Dietz, Volker; Duysens, Jacques;
1. Dept. of Rehabilitation Medicine, Radboud University Nijmegen Medical Center, Nijmegen, Netherlands. 2. Spinal Cord Injury Center, University Hospital Balgrist, Zurich, Switzerland. 3. SMK-Research, Sint Maartenskliniek, Nijmegen, Netherlands.
During arm movements it is known that anticipation of unloading can result in a reduction of unwanted reactions (anticipatory postural adjustments, APAs) . Similarly, during gait, self-triggered electrical stimulation of the sural nerve results in smaller reflex responses as compared to computer-triggered stimulation . This reduction is seen as a switch from reactive to proactive motor control. The question remains whether such a switch occurs during mechanical gait perturbations as well. In the lower limb, sudden unloading has already been used as perturbation. Upper limbs are not normally used as support during gait, except for crutch walking. Sudden unloading can then be introduced by experimentally inducing a crutch collapse. EMG activity of 12 subjects was investigated in 8 upper arm muscles following sudden unloading, induced either by the subjects themselves or externally, during the stance phase of bilateral crutch walking on a treadmill. This unloading was caused by 7 cm shortening of the crutch. Following the self-triggered shortening of the crutch, the normalized EMG (electromyogram) activity was substantially lower in the muscles after the collapse as compared to computer-triggered shortening. The reduction was present in all muscles studied and the amount of reduction differed for the various muscles. In almost all muscles the reduction in EMG amplitude was largest in the 70 ms period just following collapse (mean of all muscles: 34%, SD 21.4%) and the difference subsided in later periods (mean 24% and 17%, SD 11% and 14%). Upper arm supporting muscle activity following unloading during crutch-walking can effectively be reduced by anticipation of the self-triggered unloading, consistent with a switch from reactive to proactive control. Apparently, APAs during gait perturbations can be elicited in the upper limb as well, provided the arms are involved in the control of body support.
Where applicable, experiments conform with Society ethical requirements