The organization of action stability during whole-body pointing: insights from drift and random-walk analyses.

We explored processes reflecting movement stability during holding a pointing posture involving the trunk and lower limbs. In particular, we focused on characteristics of drifts and random walk (RW) in joint configuration spaces involved in shoulder movement, endpoint-to-shoulder movement, and their sum - the endpoint movement. Young, healthy participants performed a pointing movement from a natural standing posture that required motion of the arm, trunk, and leg joints. They held this posture for 60 s with open and closed eyes and with and without an elastic band applying a force in the backward direction. Motion analysis in a two-dimensional action space was used. Drift and RW were quantified within and orthogonal to the uncontrolled manifold (UCM, solution space) within the respective spaces of elemental kinematic variables. Drifts along and orthogonal to the UCM with characteristic times of 2-8 s were observed, primarily along the UCM, where the drifts were slower. RW was marginally anti-persistent (stabilizing) when quantified in the joint configuration spaces. There were no effects of vision on drift and RW characteristics, in contrast to earlier force production tasks. We observed no major effects of the external load, indicating that stability-related behavior was robust to the tested change in external forces. Overall, the findings suggest differences in the organization of stability between force-production and movement tasks, while the underlying neural mechanisms remain to be established. Single-trial analysis of drifts and RW is a promising tool for studies of disordered movement stability.