A feasibility study to assess intralimb coordination in stroke rehabilitation: two indices of mechanical impedance by coactivation of agonist muscles

Stroke rehabilitation requires intralimb coordination to achieve natural movement after recovery. Focusing on mechanical impedance by the coactivation of agonist muscles, we performed two experiments to assess the intralimb coordination of a post-stroke subject using two indices of the endpoint stiffness and muscle synergies. The results of the first experiment showed that the endpoint stiffness of a post-stroke subject during posture maintenance estimated from muscle synergy analysis resembled that estimated from the mechanical perturbation method. Based on the validity of proposed muscle synergy analysis shown in the first experiment, the results of the second experiment revealed that muscle activities of both the post-stroke and healthy subjects are composed of three muscle synergies in the circle-tracing task. These muscle synergies were invariant despite being determined from time-variant muscle activities; muscle synergies of the post-stroke subject before rehabilitation were different from those of the healthy subject. In addition, the muscle synergies of the post-stroke subject after rehabilitation resembled those of the healthy subject. It is assumed that the post-stroke subject regained appropriate muscle synergies (i.e., the balance of mechanical impedance) after rehabilitation. This study tested the feasibility for practical uses in the assessment, diagnosis, and interventions for stroke rehabilitation using two indices of muscle synergies and endpoint stiffness.