Muscle synergy analysis of human adaptation to a variable-stiffness exoskeleton: Human walk with a knee exoskeleton with pneumatic artificial muscles

This paper presents a variable-stiffness knee exoskeleton to enhance human walking. The developed exoskeleton is an agonist-antagonist system with pneumatic artificial muscles (PAMs), and its equilibrium-joint angle and joint stiffness are separately controlled using on our concepts of the agonist-antagonist muscle-pair ratio (A-A ratio) and agonist-antagonist muscle-pair activity (A-A activity). We focus on human adaptation to the variable-stiffness assistance of the exoskeleton, and explore the stiffness control strategy of the device to reduce the subject's excess muscle activity. Muscle synergy analysis of A-A activity indicates that our stiffness-control approach based on EMG analysis leads to successful performance of the human musculoskeletal system with exoskeleton assistance.