Sensory reweighting of proprioceptive input during balance control as function of age and disease

Background and aim: Sensory (re)weighting is the automated and unconscious process of dynamically combining sensory inputs, e.g. proprioception, graviception and vision, during balance control. Typically, reliable sensory inputs are weighted more than unreliable and noisy sensory inputs, to prevent decline of balance control. Malfunctioning of sensory reweighting in case of sensory deterioration may be an important determinant of impaired balance in elderly with the consequence of physical impairment and falls. In this study, we used closed loop system identification techniques (CLSIT) to assess sensory weighting and reweighting of proprioceptive input of the ankle during upright stance as function of age and disease. Methods: Ten healthy young (age 25.4±2.2 years), ten healthy elderly (age 76.8±1.8 years), ten elderly with cataract (age 76.7±6.8 years) and ten elderly with polyneuropathy (age 73.7±8.0 years) were asked to maintain balance while the proprioceptive input of each ankle was disturbed by rotation of the support surface (SS) around the ankle axes. SS rotations were applied with specific frequency content and the perturbation amplitude increased over trials. Body sway and the total reactive ankle torque were recorded. The sensitivity functions of the ankle torque to the perturbation amplitude was determined using CLSIT. The gain of the sensitivity function (S) describes the ratio of the perturbation amplitude and the ankle torque as function of frequency and represents the proprioceptive weighting. Parameters describing the sensitivity functions were estimated using optimized model fits, of which one was the proprioceptive weight (Wp). Results: Healthy elderly were more sensitive to SS rotations as reflected by a significantly higher gain of S (p<0.001) compared with the young. In comparison with healthy elderly, elderly with a cataract had a significantly higher gain of S (p=0.038), unlike elderly with polyneuropathy (p=0.37). In all groups, the gain of S decreased significantly with increased disturbance amplitude (p<0.001). There was no interaction effect between perturbation amplitude and groups (p=0.68).The estimated Wp was significantly higher in healthy elderly compared with the young (p=0.001). Compared with healthy elderly, elderly with cataract had a significantly higher Wp (p=0.003), unlike elderly with polyneuropathy (p=0.24). In all groups, Wp decreased with increased perturbation amplitude (p<0.001). There was an interaction effect (p=0.001) between perturbation amplitude and groups. Conclusions: Using CLSIT, proprioceptive weighting and reweighting could be established as function of age and cataract; healthy elderly rely more on proprioceptive input compared with the young and elderly with cataract rely even more on proprioceptive input. Assessing the interplay between available sensory inputs is necessary to identify the weakest link in impaired balance as a primary therapeutic target.