Biomechancis and neural control of movement edition:2 location:Mt. Sterling date:12-17 June
Experimental studies showed that a continuum of ankle and hip strategies is used to restore posture. Postural responses can be modeled by feedback control, with feedback gains that optimize a specific objective . On the one hand, feedback gains that minimize effort have been used to predict muscle activity during perturbed standing. On the other hand, hip and ankle strategies have been predicted by minimizing postural instability and deviation from upright posture. It remains unclear however whether and how effort minimization influences the selection of a specific postural response. We hypothesize that the relative importance of minimizing mechanical effort versus postural instability influences the strategy used to restore upright posture.
Experiments and predictive simulations of the postural response following a backward support surface translation were used to test this hypothesis. The posture of 10 healthy adults was perturbed. Full body kinematics and ground reaction forces were measured and analyzed in OpenSim. Significant correlations were found between the measured hip range of motion, characterizing a hip strategy, and the experimentally determined mechanical effort, metabolic work and muscle activity (R=0.81, R=0.71, R= 0.7, p<0.001). Predictive simulations were used to establish a cause effect relationship between the relative importance of minimizing mechanical effort and the postural response. Therefore, the response of a double inverted pendulum model to the backward surface translation was optimized to minimize an objective function consisting of a weighted sum of (1) postural instability and (2) mechanical work. Ankle strategies were predicted when the minimizing postural instability whereas hip strategies were predicted when minimizing mechanical work (figure 1). Furthermore, there was a significant positive correlation between the relative weight that predicts the measured postural response best and the measured hip range of motion (R=0.70, p<0.001). This shows that the trade-off between effort and postural instability minimization can explain the selection of a specific postural response in the continuum of potential ankle and hip strategies.