Modeling of equilibrium point trajectory control in human arm movements

MODELING OF EQUILIBRIUM POINT TRAJECTORY CONTROL IN HUMAN ARM MOVEMENTS by Kai Chen The underlying concept of the Equilibrium Point Hyp othesis (EPH) is that the CNS provides a virtual trajectory of joint motion, repr senting spacing and timing, with actual movement dynamics being produced by interactions of limb inertia, muscle viscosity and speed/position feedback from muscle spindles. To co unter criticisms of the EPH, investigators have proposed the use of complex virt ual trajectories, non-linear damping, stiffness and time varying stiffness to the EPH mod el. While these features allow the EPH to adequately produce human joint velocities, t hey conflict with the EPH’s premise of simple pre-planned monotonic control of movement trajectory. As a result, this study proposed an EPH based method, which provides a simp ler echanism in motor control without conflict with the core advantages of the or iginal approach. This work has proposed relative damping as an addi tion to the EPH model to predict the single and two joint arm movements. Thi s addition results in simulated data that not only closely match experimental angle data , but also match the experimental joint torques. In addition, it is suggested that this mod ifie model can be used to predict the multi-joint angular trajectories with fast and norm al velocities, without the need for time varying or non-linear stiffness and damping, but wi th simple monotonic virtual trajectories. In the following study, this relative damping model has been further enhanced with an EMG-based determination of the vir tual trajectory and with physiologically realistic neuromuscular delays. The results of unobstructed voluntary movement studies suggest that the EPH models use re alistic impedance values and produce desired joint trajectories and joint torque s in unperturbed voluntary arm movement. A subsequent study of obstructed voluntary arm mov ement extended the relative damping concept, and incorporated the influential f actors of the mechanical behavior of the neural, muscular and skeletal system in the con trol and coordination of arm posture and movement. A significant problem of the study is how this information should be used to modify control signals to achieve desired perfor mance. This study used an EPH model to examine changes of controlling signals for arm m ovements in the context of adding perturbation/load in the form of forces/torques. Th e mechanical properties and reflex actions of muscles of the elbow joint were examined . Brief unexpected torque/force pulses of identical magnitude and time duration wer e introduced at different stages of the movement in a random order by a pre-programmed 3 de gree of freedom (DOF) robotic arm (MOOG FCS HapticMaster). The results show that e subjects may maintain the same control parameters (virtual trajectory, stiffn ess and damping) regardless of added perturbations that cause substantial changes in EMG activity and kinematics. MODELING OF EQUILIBRIUM POINT TRAJECTORY CONTROL IN HUMAN ARM MOVEMENTS