Joint angle variation in intentional sit-to-stand transitions

Human motion is highly variable due to the effects of interaction with the environment and the intentionality of movement. Current assistive device control systems struggle to support this variable human motion as they restrict typically uncontrolled degrees of freedom resulting in unnatural, mechanistic and highly repetitive (i.e., artificially invariable) motions. In this paper we provide an analysis of variability in the human joint configuration while performing intentional sit-to-stand (STS) tasks. We use the Uncontrolled Manifold (UCM) which indicates that, for a sit-to-stand transition, the natural variability in movement can be accounted for through the reciprocal compensation (i.e., variation of movement trajectories) of joints in the lower kinetic chain (ankle, knee and hip) to modulate the body's Center of Mass (CoM) trajectory in the sagittal plane. Our results suggest that the task space variability differs across participants, which is reflected in the joint configuration variability, to complete the relatively less constrained intentional task (e.g. STS-to-reach) as opposed to a relatively constrained intentional task (e.g. STS). These results can be helpful in devising task specific and personalized strategies in order to control assistive devices modelled as redundant manipulators.