The perception-action dynamics of action competency are altered by both physical and observational training.

Action competency is defined as the ability of an individual to self-evaluate their own performance capabilities. The current experiment demonstrated that physical and observational training with a motor skill alters action competency ratings in a similar manner. Using a pre-test and post-test protocol, the results revealed that action competency is constrained prior to training by the intrinsic dynamics of relative phase (ϕ), with in-phase (ϕ = 0°) and anti-phase (ϕ = 180°) patterns receiving higher competency ratings than other relative phase patterns. After 2 days of training, action competency ratings for two trained relative phase patterns, +60° and +120°, increased following physical practice or observational practice. A transfer test revealed that both physical performance ability and action competency ability transferred to the symmetry partners (-60° and -120°) of the two trained relative phase patterns following physical or observational training. The findings also revealed that relative motion direction acts as categorical information that helps to organize action production and facilitate action competency. The results are interpreted based on the coordination dynamics theory of perception-action coupling, and extend this theory by showing that visual perception, action production, and action competency are all constrained in a consistent manner by the dynamics of the order parameter relative phase. As a whole, the findings revealed that relative motion, relative phase, and possibly relative amplitude information are all distinct sources of information that contribute to the emergence of a kinematic understanding of action in the nervous system.

Reference path generation for upper-arm exoskeletons considering scapulohumeral rhythms.

This paper proposes a reference path generation method for upper-limb rehabilitation exoskeletons considering the scapulohumeral rhythms of the shoulder. The developed method is based on Central Nervous System's (CNS) governing rules for coordination of arm motions, and to the best of our knowledge is the first computational model to consider the motion of the inner shoulder in path generation. Existing reference generation methods which utilize computational models such as minimum jerk, minimum torque, etc, are based on the assumption that the shoulder joint does not move, and the origin of the reference frame is defined at the center of the glenohumeral (GH) joint. These computational methods are generally developed for simple point-to-point reaching movements with limited range of motion (RoM) which justifies the assumption of fixed shoulder center. However, most upper limb motions such as Activities of Daily Living (ADL) tasks include larger scale inward and outward reaching motions, during which the center of shoulder joint moves significantly. The proposed motion planning method can be used in upper-limb exoskeletons with 3 Degrees of Freedom (DoF) in shoulder and 1 DoF in elbow which are capable of supporting the motion of the shoulder girdle by moving the center of shoulder joint. The outputs of the proposed model are compared with the natural motion of arm during ADL tasks, recorded via a motion capture system. Comparison of the results show that the proposed model is able to reproduce human ADL motions, and can effectively be used for reference generation. The results of this study also confirm that neglecting the fine manipulations with wrist and fingers, ADL tasks can be modeled as large RoM reaching tasks from the perspective of elbow-shoulder coordination.

Experimental observations on the human arm motion planning under an elbow joint constraint.

This paper seeks to define the governing strategies by which the human central nervous system (CNS) finds optimal solutions for an arm reaching motion, when an elbow joint is constrained. The compensated arm reaching motion under the joint kinematic constraint is observed by human experiments. We present an experimental protocol, where subjects perform point-to-point reaching tasks with a lightweight elbow brace to restrict the elbow kinematics with minimal effect on the arm dynamics. The human compensatory strategy is analyzed in terms of hand path kinematics (i.e. spatial and temporal characteristics) and the arm postural configuration. The spatial and temporal characteristics of hand path are approximated by the Euclidean geodesic curves and the well known bell-shaped smooth profile, respectively. Furthermore, the contribution of each joint degree-of-freedom (DOF) motion is discussed and its relation to the arm posture selection is elaborated.