Fig. 1.

Experimental apparatus (A), angles defining arm posture (B), and a free-body diagram of the forces and torques exerted on the upper arm (C).A, A six-degree-of-freedom force–torque sensor was rigidly coupled to an upper extremity orthosis at one end and a stereotaxic frame at the other. Two coordinate frames are illustrated: an x, y, z, frame that was fixed to the arm/sensor, and an x′, y′,z′ frame (inset) that was fixed in space. In this posture, the axes of the two frames were parallel. Thez- and x-axes were aligned with the long axes of the upper arm and forearm segments of the orthosis, respectively; the y-axis was orthogonal to the plane containing these two axes. B, Three angles were used to define upper arm posture: η, a rotation about the verticalz′-axis; θ, a rotation in a vertical plane passing through the upper arm (measured relative to the verticalz′-axis), and ζ, a rotation about the long axis of the humerus. C, A cylinder representing the upper arm is depicted along with the forces (F_x,F_y, F_z) and torques (T_x, T_y,T_z) recorded at the transducer and the corresponding torques at the shoulder (M_x,M_y, M_z). The shoulder, elbow, and length of the upper arm are labeledS, E, and La, respectively.