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

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

Human neck reflex adaptation towards the frequency content of anterior-posterior torso perturbations

Introduction: Reflex modulation has been extensively reported during posture maintenance in response to task instructions, and to perturbation type, bandwidth and amplitude. For the head-neck system the modulation of the vestibulocollic (VCR) and cervicocollic (CCR) reflexes is essential to maintain upright head posture during unexpected disturbances. Previous studies have estimated that VCR and CCR contribute equally during perturbations in the sagittal plane; however, their modulation with respect to the properties of the disturbance remains unclear. This study seeks to establish how neck reflexes are modulated during perturbations with varying properties and how each reflex contributes to stabilization behavior. We hypothesized that VCR and CCR (a) modulate according to the perturbation bandwidth, (b) are unaffected by the perturbation amplitude and (c) increase when performing a visual acuity task. Methods: Twelve subjects were perturbed via the torso while restrained in a seated position on a motion platform. The anterior-posterior perturbations varied in bandwidth from 0.3 Hz to a maximum frequency of 1.2, 2.0, 4.0 and 8.0 Hz, at three different amplitudes, and with eyes open and closed. Results: Head kinematics and neck muscle EMG demonstrated significant (P < 0.05) changes due to bandwidth, which through modeling and closed loop identification were attributed to modulation of VCR and CCR gains. VCR and CCR demonstrated dominant contributions to stabilization during high (8.0 Hz) and low bandwidth (1.2 and 2.0 Hz) perturbations respectively, and equivalent contributions during mid bandwidth perturbations (4.0 Hz). However both were attenuated when perturbations exceeded the systems natural frequency (~2-3 Hz). Amplitude had an effect only for the lowest amplitude relative to other conditions attributed to thresholding properties of the semicircular canals. With eyes closed reflex gains decreased, attributed to the reduced ability to discriminate self-motion without vision. Conclusions: To maintain head-upright posture adaptations of neck reflexes are observed to occur due to perturbation frequency and visual task conditions but not amplitude. Estimation of reflex contributions demonstrates that previous literature has underestimated the contribution of CCR, particularly during low frequency perturbations