This research investigates the linearity between hand stiffness and the sum of activity of antagonistic muscles, and the strategy of hand force control from the viewpoint of equilibrium point and stiffness. The ratio of EMGs between antagonistic muscles around a joint is defined as Agonist-Antagonist muscle ratio (A-A ratio), and the sum of the EMGs is defined as Agonist-Antagonist muscle activity (A-A activity). Our previous work showed some relations between A-A ratio and equilibrium point and between A-A activity and stiffness. Based on that work, this paper precisely estimates the hand force direction of the upper limb in a plane. In addition, we clarified the linear relationship between A-A activity and hand stiffness. Accordingly we could estimate the equilibrium points of the hand.
