Abstract
Instability of posture in the medial-lateral (ML) direction is a significant risk factor for falls in older adults. Standing on wedges changes the angular position in the ankle joints and improves postural stability seen in reduced postural sway in the ML direction. However, it is not known if standing on wedges improves body stability in the presence of external perturbations. AIM: To investigate muscle activation patterns during standing on the wedges while being exposed to lateral external perturbations. METHODS: Ten healthy young participants stood on a plane surface, on two medial wedges or two lateral wedges and were exposed to external perturbations applied to the lateral part of their right shoulder. Bilateral electromyographic (EMG) activity of dorsal and ventral trunk, thigh and shank muscles was recorded. The indexes of reciprocal (R) activation and co-contraction (C) of muscles were calculated (as the difference and sum of the integrals of EMG activity of the dorsal and ventral muscles) and analyzed during the anticipatory (APA) and compensatory (CPA) phases of postural control. RESULTS: Reciprocal activation of muscles was seen in the shank segment on the side of the perturbation while co-contraction of muscles was seen on the contralateral side. Different activation patterns were seen in the thigh and trunk segments as cocontraction of muscles on the right side and reciprocal activation on the left side. Standing on wedges significantly affected magnitudes of C and R indexes in both shank segments, however, the activation patterns on the left and right side were similar. CONCLUSION: Standing on two wedges affects patterns of muscle activity when dealing with the lateral perturbation: reciprocal activation seen in the shank segment on the side of perturbation allows some flexibility and co-contraction of muscles on the contralateral side creates better stability needed for maintaining equilibrium.