A robust ensemble data method for identification of human joint mechanical properties during movement.

This paper describes a perturbation method for the identification of linear time-varying systems with an unknown input (voluntary joint input) using ensemble data. The method separates the unknown input and the perturbation through high-pass filtering and recasts the multi-input single-output system identification into single-input single-output system identification. The method is robust to intertrial variation, and can track changes of system dynamics up to 5 Hz. Analysis and simulation are given for the conditions similar to those for the human arm experiments. Experiments show that mechanical properties of the human elbow joint change with voluntary movement speed and that the mean stiffness with voluntary movement is in the range of the posture and is higher than reported before.

Identification of human joint mechanical properties from single trial data.

A method is presented for estimating the time-varying compliance parameters of the elbow-joint from a single movement. The method separates by frequency the perturbed from the voluntary response, then determines the parameters by exponentially weighted least squares. The tracking performance of the method is established by simulation and by a calibrated mechanical joint. Experimental results are presented on time-varying posture and slow movement.

Comparison of relative accuracy between a mechanical and an optical position tracker for image-guided neurosurgery.

An essential component in the execution of image-guided surgery is a hand-held probe whose spatial position is tracked during the procedure and displayed on a three-dimensional operative workstation. This paper describes an experiment performed in order to compare the accuracy of a mechanically linked pointing device (FARO surgical arm) and an optical position tracker (OPTOTRAK) against a "gold standard."

Time-varying stiffness of human elbow joint during cyclic voluntary movement.

The objective of this study was to determine the extent to which subjects modulate their elbow joint mechanical properties during ongoing arm movement. Small pseudo-random force disturbances were applied to the wrist with an airjet actuator while subjects executed large (1 rad) elbow joint movements. Using a lumped parameter model of the muscle, tendon and proprioceptive feedback dynamics, a time-varying system identification technique was developed to analyze the phasic changes in the elbow joint's mechanical response. The mechanical properties were found to be time-varying, and well approximated by a quasi-linear second-order model. The stiffness of the arm was found to drop during movement. The arm was always underdamped, with the damping ratio changing during movement. Inertia estimates were constant and consistent with previous measurements. Overall, the moving arm was found to be very compliant, with a peak stiffness value less than the lowest value measured during posture, and a natural frequency of less than 3 Hz. Changing the speed of movement, or the load from gravity, changed the stiffness measured, but not in strict proportion to the change in net muscle torque.

An airjet actuator system for identification of the human arm joint mechanical properties.

A system is described for determining the mechanical properties of the human arm during unconstrained posture and movement. An airjet perturbation device is attached to the wrist with a special cuff, and provides high-frequency stochastic perturbations in potentially three orthogonal directions. The airjet operates as a fluidic flip-flop utilizing the Coanda effect, and generates binary force sequences with a steady-state thrust of 4 N, a flat frequency response to 75 Hz, usable thrust to 150 Hz, and a rise time of 1 ms, when the static pressure at the nozzle inlet is 5.5 x 10(5) Pa (80 psi). These operating characteristics are adequate to identify the arm's mechanical properties efficiently and robustly.

Inferring limb coordination strategies from trajectory kinematics.

This paper discusses the method of kinematic modeling and matching to human arm trajectories in order to ascertain the motor control system's coordination strategy. The planning variables of joint angles and endpoint Cartesian coordinates are contrasted, under linear and staggered interpolation strategies. It is shown that distinguishing the two variable planning sets depends critically on the region of the workspace in which movement takes place.

Deducing planning variables from experimental arm trajectories: pitfalls and possibilities.

This paper investigates whether endpoint Cartesian variables or joint variables better describe the planning of human arm movements. For each of the two sets of planning variables, a coordination strategy of linear interpolation is chosen to generate possible trajectories, which are to be compared against experimental trajectories for best match. Joint interpolation generates curved endpoint trajectories called N-leaved roses. Endpoint Cartesian interpolation generates curved joint trajectories, which however can be qualitatively characterized by joint reversal points. Though these two sets of planning variables ordinarily lead to distinct predictions under linear interpolation, three situations are pointed out where the two strategies may be confused. One is a straight line through the shoulder, where the joint trajectories are also straight. Another is any trajectory approaching the outer boundary of reach, where the joint rate ratio always appears to be approaching a constant. A third is a generalization to staggered joint interpolation, where endpoint trajectories virtually identical to straight lines can sometimes be produced. In examining two different sets of experiments, it is proposed that staggered joint interpolation is the underlying planning strategy.

Kinematic features of unrestrained vertical arm movements.

Unrestrained human arm trajectories between point targets have been investigated using a three-dimensional tracking apparatus, the Selspot system. Movements were executed between different points in a vertical plane under varying conditions of speed and hand-held load. In contrast to past results which emphasized the straightness of hand paths, movement regions were discovered in which the hand paths were curved. All movements, whether curved or straight, showed an invariant tangential velocity profile when normalized for speed and distance. The velocity profile invariance with speed and load is interpreted in terms of simplification of the underlying arm dynamics, extending the results of Hollerbach and Flash (Hollerbach, J. M., and T. Flash (1982) Biol. Cybern. 44: 67-77).