Goal-equivalent drifts in sagittal joint angle configuration as indicators of optimality in postural alignment

The relevance of posture as a constituent of physical health varies depending on one's explanatory framework of disease. Contrasting perspectives within this discussion refer to optimal biomechanics, but often without consistent meaning. The resulting theoretical confusion presents challenges both for applied research and clinical practice. We studied joint angle drift tendencies and sway-stabilizing synergy as indicators of optimality in relaxed bipedal standing. Sixteen subjects completed a series of 4-min bipedal standing trials under varying foot positions. Kinematic time series were segmented on the basis of multivariate changepoint location to quantify directional tendencies associated with slow, unintentional joint angle drifts. Subsequently, time series were segmented into shorter epochs and synergy indices describing center of mass stabilizing joint coordination were quantified for each epoch. Paired t-tests were used to assess the tendency of observed drifts to approximate common endpoints and "Random Effects Within Between" models were used to assess the relationship between synergy indices and Euclidean distance from individually defined drift targets. Our analyses suggest that 1) joint angles in relaxed bipedal standing tend to drift toward specific endpoints, and 2) greater deviation from these drift endpoints associates with greater COM-stabilizing coordinative synergy. We interpret these findings as supporting the presence of latent cost functions associated with individual sagittal joint angle configuration. We discuss our work in the context of the ongoing theoretical and clinical dialogue concerning optimal movement and the relevance of biomechanics in physical health.