Distinguishing Two Types of Variability in a Sit-to-Stand Task.

Variability is commonly observed in complex behavior, such as the maintenance of upright posture. The current study examines the value added by using nonlinear measures of variability to identify dynamic stability instead of linear measures that reflect average fluctuations about a mean state. The largest Lyapunov exponent (λ1) and SD were calculated on mediolateral movement as participants performed a sit-to-stand task on a stable and unstable platform. Both measures identified changes in movement across postures, but results diverged when participants stood on the unstable platform. Large SD indicated an increase in movement variability, but small λ1 identified those movements as stable and controlled. The results suggest that a combination of linear and nonlinear analyses is useful in identifying the proportion of observed variability that may be attributed to structured, controlled sources. Nonlinear measures of variability, like λ1, can further be used to make predictions about transitions between stable postures and to identify a system's resistance to disruption from external perturbations. Those features make nonlinear analyses highly applicable to both human movement research and clinical practice.

Thinking on your feet: An analysis of movement and cognition in a sit to stand task.

The maintenance of upright posture involves constant adjustment to external and internal perturbations. This balancing act is often assumed to be an automatic process, but studies suggest that cognitive processes, particularly attention, are necessary for the control of posture. The current study examines the role of attention in balance using a dual-task paradigm. Twenty-four healthy young adults performed a sit-to-stand (STS) task on either a stable or unstable platform while performing a secondary cognitive task of counting backwards aloud. Movement of the upper and lower body was analyzed using the largest Lyapunov exponent (λ1) and standard deviation (SD). Results replicated earlier findings (Gibbons, Amazeen, & Likens, 2018) that the transition from sit to stand was marked by increased variability and a temporary destabilization of postural control. Participants exhibited greater movement variability overall on the unstable platform (large SD), but small λ1 indicated that movement was controlled. During second task performance, SD increased for the upper body only. Further research is necessary to understand the interaction between attention and balance in young adults.

Effects of Foot Placement on Postural Sway in the Anteroposterior and Mediolateral Directions.

The common practice of standardizing foot placement in postural research and in clinical practice may serve to increase postural sway. The focus of this study was to investigate foot placement strategies in the tandem (anteroposterior, AP) and side-to-side (mediolateral, ML) stance in healthy adults. Foot placement was either experimenter-controlled or selected by the participant. Greater sway was observed for the AP stance than the ML stance, where sway was minimal. When foot placement was self-selected, participants recruited additional degrees of freedom by rotating both feet outward to expand the base of support; they narrowed their stance width in the AP stance only. Self-selection served to decrease AP sway for the AP stance and increase ML sway for both the AP and ML stances. A dynamical measure, the largest Lyapunov exponent, supported the finding that self-selection of foot placement serves to stabilize posture. The implication is that improvements in postural control were due primarily to self-selection of foot placement and not to adjustments in stance width. Experimental and perhaps clinical procedures should be revised to allow participants to self-select foot placement during postural tasks.

Congruency effects in interpersonal coordination.

Research on interpersonal coordination has demonstrated that incongruent tasks lead to unintended movements in the orthogonal plane. These effects have been interpreted using both an embodied simulation and coordination dynamics approach. To distinguish between these two perspectives, two experiments examined whether this congruency effect is best defined spatially or anatomically. In the first experiment, participants coordinated congruent and incongruent rhythmic arm movements with an actor. To dissociate spatial and anatomical congruency, the actor was rotated 90° in the coronal plane for half of the trials. In the second experiment, participants coordinated movements of different limbs (leg and arm). Spatial and anatomical congruency was dissociated here by rotating the actor in the transverse plane. In both experiments, the unintended movements associated with the congruency effect emerged as a function of spatial congruency; there was no congruency effect associated with anatomical congruency. The data suggests that these unintended movements represent the recruitment of additional df necessary to stabilize an unstable form of coordination.