Dynamics model for analyzing reaching movements during active and passive torso rotation.

We have developed an inverse dynamics model of unrestrained natural reaching movements. Such movements are usually not planar and often involve complex deformation of the shoulder girdle as well as rotary and linear torso motion. Our model takes as its input kinematic data about the positions of the finger, wrist, elbow, left and right acromion processes, and the sternum and produces the torques and forces developed at the shoulder, elbow, and wrist joints. The model can also be used to simulate the consequences of introducing passive torso rotation or linear acceleration on arm movements and to simulate the consequences of applying mechanical perturbations to the reaching limb. It separately quantifies the contributions of inertial forces resulting from torso rotation and translation. In experimental paradigms involving arm movements, different dynamic components can be present such as active or passive torso rotation and translation, external forces and Coriolis forces. Our model provides a means of evaluating the different sources of force and the total muscle force needed to control the trajectory of the arm in their presence.