Ankle intrinsic stiffness is modulated by postural sway

What Influences Proprioceptive Impairments in Patients with Rheumatic Diseases? Analysis of Different Factors

Rheumatic diseases lead to postural problems, which increase the risk of falls and lead to greater disability. The aim of the present work is to evaluate posture disorders in patients with osteoarthritis (OA) and rheumatoid arthritis (RA), as well as to evaluate the influence of other factors. A total of 71 subjects were enrolled in this study. Joint position sense (JPS) and the functional assessment of proprioception on a balance platform for both lower limbs were examined. The Average Trace Error (ATE), test time (t), and Average Platform Force Variation (AFV) were calculated. Additionally, an equilibrium test was carried out in the one-legged standing position (Single Leg Stance-SLS). The results were compared in several ways and revealed the following: (1) A JPS of 10° plantar flexion in RA obtained significantly worse results when repeating the movement than OA; the ATEs were significantly lower in RA; and RA needed more support during SLS assessment. (2) RA patients with higher DAS28 had statistically significantly higher values in JPS, with 5° plantar flexion and 10° dorsal flexion, SLS assessment, and stabilometric rates. A statistically significant correlation between DAS28 and RA was found in a JPS of 10° plantar flexion. The VAS ruler demonstrated a significant moderate correlation with t. (3) Patients who experienced at least one fall demonstrated higher JPS and t. Our study shows that proprioception is the most influenced by the nature of the disease and the level of disease activity. We can see that the stability and balance functions are also greatly influenced by the patient's falling experience and the level of pain. These findings may be useful in designing an optimal proprioception-enhancing movement training plan.

Identification of Central and Stretch Reflex Contributions to Human Postural Control

Human postural control requires continuous modulation of ankle torque to stabilize the upright stance. The torque is generated by two components: active contributions, due to central control and stretch reflex, and passive mechanisms, due to joint intrinsic stiffness. Identifying the contribution of each component is difficult, since their effects appear together, and standing is controlled in closed-loop. This article presents a novel multiple-input, single-output method to identify central and stretch reflex contributions to human postural control. The model uses ankle muscle EMGs as inputs and requires no kinematic data. Application of the method to data from nine subjects during standing while subjected to perturbations of ankle position demonstrated that active torque accounted for 84.0± 5.5% of the ankle torque. The ankle plantar-flexors collectively produced the largest portion of the active torque through central control, with large inter-subject variability in the relative contributions of the individual muscles. In addition, reflex contribution of the plantar-flexors was substantial in half of the subjects, showing its potentially important functional role; finally, intrinsic contributions, estimated as the residual of the model, contributed to 15% of the torque. This study introduces a new method to quantify the contributions of the central and stretch reflex pathways to postural control; the method also provides an estimate of noisy intrinsic torque with significantly increased signal to noise ratio, suitable for identification of intrinsic stiffness in standing. The method can be used in different experimental conditions and requires minimal a-priori assumption regarding the role of different pathways in postural control.

Patterns of muscle activation and modulation of ankle intrinsic stiffness in different postural operating conditions

Intrinsic stiffness describes the dynamic relationship between imposed angular perturbations to a joint and the resulting torque response, due to intrinsic mechanical properties of muscles and joint, and inertia of the limbs. Recently, we showed that ankle intrinsic stiffness changes substantially with sway in normal standing. In the present study, we documented how ankle intrinsic stiffness changes with postural operating conditions. Subjects stood on an apparatus while subjected to ankle position perturbations in five conditions: normal standing, toe-up and toe-down standing, and backward and forward lean. In each condition, ankle intrinsic stiffness was estimated while its modulation with sway was accounted for. The results demonstrated that ankle intrinsic stiffness varies widely, from 0.08 to 0.75 of critical stiffness, across postural operating conditions; however, it is always smaller than the critical stiffness. Therefore, other contributions are necessary to ensure stable standing. The mean intrinsic stiffness was highest in forward lean and lowest in backward lean. Moreover, within each operating condition, the intrinsic stiffness changed with center-of-pressure position in one of three ways, each associated with a distinct muscle activation pattern; these include 1) monotonically increasing stiffness-center of pressure relation, associated with a progressive increase in triceps surae activation, 2) decreasing-increasing stiffness-center of pressure relation, associated with initial activation of tibialis anterior and later activation of triceps surae, and 3) monotonically decreasing stiffness-center of pressure relation, associated with decreasing activation of tibialis anterior. Thus intrinsic stiffness varies greatly within and across postural operating conditions, and a correct understanding of postural control requires accounting for such variations.NEW & NOTEWORTHY Ankle intrinsic stiffness changes with sway in normal standing. We quantified such changes in different postural operating conditions and demonstrated that the intrinsic stiffness changes in a manner associated with different activation patterns of ankle plantarflexors and dorsiflexors, emerging in different operating conditions. Large modulations of the intrinsic stiffness within and across postural operating conditions show that the stiffness importance and contribution change and must be accounted for in the study of postural control.

A Closed-Loop Method to Identify EMG-Torque Dynamics in Human Balance Control

Human balance control requires continuous modulation of ankle torque by central and spinal activation of the ankle muscles combined with the intrinsic mechanical stiffness of the joint. These components appear together and cannot be measured separately. This work presents a novel multiple-input, single-output, closed-loop identification method that decomposes the ankle torque in human balance control into its central, stretch reflex, and intrinsic components. The method models separate transfer functions for each EMG-torque relation for central and stretch reflex mechanisms and estimates the ankle intrinsic torque from the residuals. The method uses only EMG measurements, requires no kinematic data, and has few parameters, resulting in robust performance. Application of the method to perturbed standing data from two healthy subjects demonstrated that the central and stretch reflex torques accounted for 80-93% of the ankle torque variation, while the intrinsic stiffness was responsible for most of the remaining torque.