Lower Limb Muscle Co-Contraction and Coupling Synergy in Exoskeleton Assistance for Load Carriage Walking

Human-robot interaction performance was evaluated, and better exoskeleton structures and human-robot adaptability control strategies were explored. The muscle activity, paired-muscle contraction index, and synergy coupling of the muscles of nine healthy male subjects carrying loads and walking with and without the exoskeleton prototype were investigated through evaluation of the conducted experiment. The results show that muscle activity significantly increased compared with normal load carrying, especially for the planter flexor muscle peronaeus longus. Some design defects were observed in the foot/ankle area of the current exoskeleton prototype, and the paired-muscle mean co-contraction index decreased. However, the co-contraction index range was greater than in normal load carriage. The module complexities were similar for both load carriage conditions, with total variances of 95% and 96%. Three modules could describe nearly 95% of the variance in electromyography data during normal load carriage. However, the same modules could only describe 83%–91% of the variance in muscle activity while walking with the exoskeleton. These results show that the muscle activities in healthy subjects will not be modulated by the central control strategy in the process of adaption to the assistance of the exoskeleton. The plot shape of the synergy weights and the muscle activation are significantly different from the normal gait. Hence, better human-robot interface designs in the ankle/foot area and human- robot interaction strategies referenced from the kinds of flexible but principle neuromuscular recruitment mechanisms, would improve the usability and the human-robot interaction performance of the exoskeleton.