Time variant system identification of human limb dynamics using wavelets

The dynamic behavior (i.e. admittance) of a human limb results from the interaction between limb inertia, muscles and the central nervous system. System identification techniques assess the dynamic behavior of a limb by analyzing the limb’s response to certain perturbations. Most identification techniques require the system to behave linear and time invariant, i.e. the system’s response to the perturbation must remain unchanged during observation. However it is known that neuromuscular properties change for example with fatigue. Furthermore it has been found that the strength of afferent feedback (e.g. from muscle spindles and Golgi tendon organs) adapts to conditions like task instruction and mechanical load. So far, research mainly focused on the the steady state behavior after the system had been adapted but not on the adaptation process itself. In this study we developed a closed-loop time-variant identification technique based on wavelet cross spectra to continuously identify the admittance, i.e. the dynamic relation between input force (or torque) and the output displacement. This identification technique allowed for measurement of the human joint dynamics as a function of time while the human interacts with a mechanical load. As a second step the afferent feedback strengths were quantified by fitting a neuromuscular control model onto the admittance for each time instant. The model fit produced physiological relevant parameters, like muscle visco-elasticity resulting from (co-)contraction, afferent feedback from muscle spindles and Golgi tendon organs including neural time delays. Simulations demonstrated that the developed method is able to track time-variant behavior. Preliminary results of experimental data showed that human subjects adapt their admittance to an instantaneous change of a viscous load. In particular, the gain of the afferent feedback changed within seconds. The estimated dynamic behavior of the human joint before and after the change of the viscous load resembled the behavior as identified using traditional time-invariant techniques in two separate experiments with constant viscous loads. However, the accuracy of the estimated adaptation time of the system is yet to be determined as the method in its current form is less able to track fast changes in system behavior. Further research into time-variant closed-loop identification is recommended to improve the temporal accuracy

A Gait Rehabilitation Robot for the Training of Subtasks in Walking

Background: In the past decade, different gait rehabilitation robots have been developed. These robots concentrated on relearning gait by repeatedly enforcing a walking pattern for the whole leg or only for the foot. Such training might be less task specific as generally thought, because learning to walk is more complex than learning a position trajectory. We believe active participation of the patient in training and subdivision of the training in several subtasks leads to more comprehensive and functional training. Methods: Using a newly developed actuated exoskeleton (LOPES) in combination with a treadmill, we aimed to selectively and gently support specific subtasks of walking, like knee stabilization, lateral balance, and foot clearance. Foot clearance and knee stabilization subtask had been implemented with dedicated controllers and tested during walking with healthy subjects. Results: Foot clearance could be increased without disrupting the subject’s own initiated walking in the remainder of the gait cycle. During weight acceptation, the knee stabilization ensured by the exoskeleton while the subject relaxed his knee. The gentle control of the robot allowed a comfortable interaction between subject and robot. Conclusion: Selective support of subtasks seems to be a viable method of interaction with the patient to train his or her gait. In the near future, we will implement a complete set of gait subtasks, which makes all kinds of training interventions possible

Identifying intrinsic and reflexive contributions to low-back stabilization

Motor control deficits have been suggested as potential cause and/or effect of a-specific chronic low-back pain and its recurrent behavior. Therefore, the goal of this study is to identify motor control in low-back stabilization by simultaneously quantifying the intrinsic and reflexive contributions. Upper body sway was evoked using continuous force perturbations at the trunk, while subjects performed a resist or relax task. Frequency response functions (FRFs) and coherences of the admittance (kinematics) and reflexes (sEMG) were obtained. In comparison with the relax task, the resist task resulted in a 61% decrease in admittance and a 73% increase in reflex gain below 1.1 Hz. Intrinsic and reflexive contributions were captured by a physiologically-based, neuromuscular model, including proprioceptive feedback from muscle spindles (position and velocity) and Golgi tendon organs (force). This model described on average 90% of the variance in kinematics and 39% of the variance in sEMG, while resulting parameter values were consistent over subjects

Comparison of three local frame definitions for the kinematic analysis of the fingers and the wrist

International audienceBecause the hand is a complex poly-articular limb, numerous methods have been proposed to investigate its kinematics therefore complicating the comparison between studies and the methodological choices. With the objective of overcoming such issues, the present study compared the effect of three local frame definitions on local axis orientations and joint angles of the fingers and the wrist. Three local frames were implemented for each segment. The “Reference” frames were aligned with global axes during a static neutral posture. The “Landmark” frames were computed using palpated bony landmarks. The “Functional” frames included a flexion–extension axis estimated during functional movements. These definitions were compared with regard to the deviations between obtained local segment axes and the evolution of joint (Cardan) angles during two test motions. Each definition resulted in specific local frame orientations with deviations of 15° in average for a given local axis. Interestingly, these deviations produced only slight differences (below 7°) regarding flexion–extension Cardan angles indicating that there is no preferred method when only interested in finger flexion–extension movements. In this case, the Reference method was the easiest to implement, but did not provide physiological results for the thumb. Using the Functional frames reduced the kinematic cross-talk on the secondary and tertiary Cardan angles by up to 20° indicating that the Functional definition is useful when investigating complex three-dimensional movements. Globally, the Landmark definition provides valuable results and, contrary to the other definitions, is applicable for finger deformities or compromised joint rotations