Reflex adaptations during treadmill walking with increased body load

Load dependent reflex adaptations were studied in healthy subjects walking on a split-belt treadmill. Compensatory reflex responses were elicited in t he right leg extensor muscles during mid-stance by a short acceleration of the right treadmill belt. Electromyographic activity (EMG) was recorded fro m the right medial gastrocnemius (GMR), soleus (SO) and tibialis anterior ( TA) muscles of the right leg as well as from the gastrocnemius of the left unperturbed leg (GML). To study the adaptational reflex behavior, multiple measurements were taken during walking with normal (control) and increased body load and after removing the load. In most experiments the compensatory EMG response in the GMR consisted of a short inhibitory and a subsequent e xcitatory component. Both reflex components were larger when the body was l oaded. During the course of continuous loading, divergent reflex adaptation s of different degrees and directions were observed in the subjects. In one group of subjects the reflex response increased to a higher level of EMG a ctivity. In a second group EMG activity first increased and afterwards decr eased to baseline level. A subsequent removal of body loading resulted in a slow adaptation to the control reflex values in both groups. Neither the E MG activity in the GM nor the reflex responses in the GMR after increasing the load changed differently in the two groups. Our results suggest that lo ad information is not simply used in a fixed input/output relationship of t he actual biomechanical conditions of a subject. Load information is rather used to slowly modify the reflex response, to achieve the desired posture during walking.