Figure 1.
Experimental procedure. A, Action of interaction torques. The arrows indicate the directions of joint torques and resulting joint rotations. When a single-joint muscle (red) spanning the shoulder joint contracts, this linear force is transformed into a rotational torque (τs) that flexes the shoulder joint. This action produces an interaction torque (τei) at the elbow causing it to extend. Simultaneous contraction of the biarticular flexor muscle (orange) would both help with the shoulder flexion and counteract the elbow extension caused by the interaction torque. B, Experimental apparatus. The subject's right arm is supported by a robotic exoskeleton (KINARM) so that it allows for movement of shoulder and elbow joints in a horizontal plane. Visual targets are projected on the plane of movement. TMS coil position over M1 is maintained using Brainsight, which calculates relative distance between the coil and the subject's head using reflective markers (trackers). C, Target locations and movement directions for the assistive and resistive conditions. The curved arrows illustrate joint torque directions during voluntary movements in the two conditions. τe, Elbow torque; τei, interaction torque acting on the elbow from the movement of the shoulder; τs, shoulder torque; τsi, interaction torque acting on the shoulder from the movement of the elbow. The arrowheads of τsi point in both directions because this torque changes direction during the assistive condition. The τsi are smaller than τei relative to the net or muscle torques at the corresponding joint. D, The integration method of calculating amplitudes of the MEPs in individual trials. The shaded area represents the area of integration. E, The peak-to-peak method of calculating MEP amplitudes in individual trials. The vertical gray lines show the bin within which the maximum and minimum of an MEP was determined.