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The present study proposes a new approach to identify the parameters of both continuous and discrete time Hammerstein systems in frequency domain. A harmonic probing technique is used to derive the linear and higher-order frequency response functions (called the generalized frequency response functions (GFRF)) of both discrete and continuous-time Hammerstein models. The computation of the n-th order...
The dynamics of sway during quiet stance have often been approximated by the movement of an unstable inverted pendulum. Controlling this unstable load requires the nervous system to balance the reliance on feed-forward volitional activation and feedback mechanisms such as stretch reflexes. It has been demonstrated that reflex excitability is heightened when postural stability is threatened by destabilizing...
Using the upper limb to manipulate objects or tools requires maintenance of stable arm posture. The ability to maintain stable postures is dependent on the mechanical properties of the arm, which can be characterized by estimates of endpoint stiffness. In this study we quantified the endpoint stiffness of the human arm during postural interactions with mechanically imposed unstable loads. The purpose...
Stretch reflexes have been considered one of the simplest circuits in the human nervous system. Yet, their role is controversial given that they assist or resist an imposed perturbation depending on the task instruction. Evidence shows that a loud acoustic stimulus applied prior to an impending movement elicits a movement-direction dependent muscle activity. In our study, we found that a perturbation...
Robot-assisted training is increasingly being investigated in upper limb rehabilitation for individuals with stroke. Many studies have suggested that an appropriate synchronization of voluntary motor commands and limb movement is critical for long-term efficacy. Bimanual training is one method for enhancing this synchronization or motor coordination. The purpose of the study was to evaluate the potential...
The purpose of this study was to develop an animal model to evaluate the efficacy of a brain machine interface (BMI) to control a neuroprosthesis intended to restore hand function via functional neuromuscular stimulation (FNS). We have implemented the system in a single primate, whose limb could be temporarily paralyzed by a reversible peripheral nerve block Recordings from the primary motor cortex...
The purpose of this study was to examine how multijoint stretch reflexes are modulated during interactions with stiff and compliant environments. Reflex responses were elicited using a 3D robotic manipulator to perturb arm posture in the three degrees of freedom relevant to functional behaviors. The robot applied controlled displacements and simulated environments with different stiffnesses. Stiff...
The force and position data issued to construct models of joint dynamics are often obtained from closed-loop experiments, where the joint position is perturbed using an actuator configured as a position servo. If the position servo is orders of magnitude staffer than the joint, as is often the case, it is possible to treat the data as if they were obtained in open loop. It may be more relevant to...
Understanding how stretch sensitive reflexes contribute to the coordination of whole arm posture and movement is best understood in the context of multijoint perturbations. However, it is difficult to assess the mechanisms contributing to reflex activity during such unconstrained tasks due to the uncertainties associated with estimating the relative importance of biomechanical and neural coupling...
Discretization of dynamic stiffness models is shown to produce an a-causal impulse response. Expressions are derived for discrete-time impulse responses of first- and second-order derivatives. Enforcing periodicity gives rise to anti-causal components in these models. These models are used to describe the anti-causal components in identified stiffness models
System identification involves creating mathematical models of systems using measurements of their inputs and outputs. Linear time-varying systems form an important sub-class of models that require the use of specialized system identification techniques. One such approach involves expanding the time-varying parameters onto a set of temporal basis functions and then estimating the resulting expansion...
We have developed an algorithm for selecting an optimal set of inputs for use in linear multiple-input, single-output system identification processes. The algorithm provides a decomposition of the system output such that each component is uniquely attributable to a specific input This reduces the complexity of the estimation problem by optimally selecting inputs according to the uniqueness of their...
Several nonparametric system identification techniques have been used to estimate the dynamic joint stiffness of the human elbow. Most studies involved a very stiff environment, but some studies have also shown that stiffness is modified in response to environmental compliance. However, using the same identification technique used under very stiff conditions to do identification under compliant conditions...
Long-latency components of the stretch reflex may aid in organizing multi-joint movements and posture. The aims of this study were to investigate if the M2 response in biceps brachii is mediated through a trans-cortical pathway. Ipsi-lateral and contra-lateral transcranial magnetic stimulation, combined with a variety of ramp-and-hold perturbations in which the subject was instructed to either intervene...
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