Time scale dependence of the center of pressure entropy: What characteristics of the neuromuscular postural control system influence stabilographic entropic half-life?

The center of pressure (COP) movement in studies of postural control reveals a highly regular structure (low entropy) over short time periods and a highly irregular structure over large time scales (high entropy). Entropic half-life (EnHL) is a novel measure that quantifies the time over which short-term temporal correlations in a time series deteriorate to an uncorrelated, random structure. The current study suggested and tested three hypotheses about how characteristics of the neuromuscular postural control system may affect stabilometric EnHL: (H1) control system activity hypothesis: EnHL decreases with increased frequency of control system interventions adjusting COP motion; (H2) abundance of states hypothesis: EnHL decreases with increased number of mechanically equivalent states available to the postural system; and (H3) neurologic process hierarchy hypothesis: EnHL increases if postural control functions shift from the spinal level to the motor cortex. Thirty healthy participants performed quiet stance tests for 90 s in 18 different conditions: stance (bipedal, one-legged, and tandem); footwear (bare foot, regular sports shoe, and rocker sole shoes); and simultaneous cognitive task (two-back working memory task, no challenge). A four-way repeated-measures ANOVA revealed significant changes in EnHL for the different stance positions and for different movement directions (medio-lateral, anterior-posterior). These changes support H1 and H2. Significant differences were also found between rocker sole shoes and normal or barefoot standing, which supports H3. This study contributes to the understanding of how and why EnHL is a useful measure to monitor neuromuscular control of balance.