Upper limb redundancy resolution under gravitational loading conditions: Arm postural stability index based on dynamic manipulability analysis

Resistance training may be considered as one promising approach for improving the motor capabilities of post-stroke patients. A successful introduction of this depends on the proper resolution of human arm redundancy under gravitational loading. The spatially heterogeneous changes of the human arm swivel angle (which represents the upper limb redundancy) are studied under different loading conditions, the effects of which are incorporated into a modified dynamic manipulability ellipsoid model. A new scalar index describing the arm postural stability (APSI) is then proposed. As part of the experimental protocol, ten (10) healthy subjects performed multiple reaching tasks with different weights mounted on the forearm. Kinematic data was collected via a ten-camera motion capture system and the corresponding APSI was calculated for each task. APSI is found to have a strong linear correlation with the swivel angle under loading conditions. Furthermore, the data suggest that the swivel angle may serve as an indicator of arm postural stability and task difficulty. The results of additional experiments conducted with three (3) subjects indicate that the external loads could deteriorate the arm's control performance in tasks like line tracing (root mean square deviation from straight lines). These findings may be applicable to robot-based (exoskeleton) resistance therapy, assist-as-needed gravity compensation, and human-like motion control of humanoid robotic systems.