Assessment of Multi-Joint Coordination and Adaptation in Standing Balance: A Novel Device and System Identification Technique

Abstract

The ankles and hips play an important role in maintaining standing balance and the coordination between joints adapts with task and conditions, like the disturbance magnitude and type, and changes with age. Assessment of multi-joint coordination requires the application of multiple continuous and independent disturbances and closed loop system identification techniques (CLSIT). This paper presents a novel device, the double inverted pendulum perturbator (DIPP), which can apply disturbing forces at the hip level and between the shoulder blades. In addition to the disturbances, the device can provide force fields to study adaptation of multi-joint coordination. The performance of the DIPP and a novel CLSIT was assessed by identifying a system with known mechanical properties and model simulations. A double inverted pendulum was successfully identified, while force fields were able to keep the pendulum upright. The estimated dynamics were similar as the theoretical derived dynamics. The DIPP has a sufficient bandwidth of 7 Hz to identify multi-joint coordination dynamics. An experiment with human subjects where a stabilizing force field was rendered at the hip (1500 N/m), showed that subjects adapt by lowering their control actions around the ankles. The stiffness from upper and lower segment motion to ankle torque dropped with 30% and 48%, respectively. Our methods allow to study (pathological) changes in multi-joint coordination as well as adaptive capacity to maintain standing balance.

Assessing Standing Balance using MIMO Closed Loop System Identification Techniques

“…The results are according to previous studies in which the same kind of perturbations were applied one by one (Engelhart et al 2014b;Peterka 2002). However, this is the first study in which these perturbations were applied simultaneously.…”

“…Compared with Peterka (2002) increasing the perturbation amplitude of the SS rotation results in a decrease of the magnitude of the sensitivity functions of the SS rotation and an increase of the magnitude of the sensitivity functions of the VS rotation. Results of the joint dynamics and their coupling were comparable with Engelhart et al (2014b). These results indicate that unless applying mechanical and sensory perturbations simultaneously, it is still possible to detect the contribution of the sensory systems and the joint dynamics and their coupling.…”

“…By these perturbations the leg and trunk segment will move, which makes it possible to assess the contribution of the ankle and hip joint and their coupling in standing balance (Engelhart et al 2014b). …”

“…The MIMO approach was used to estimate the dynamics of the ankle and hip joint and their coupling according to (2) (Engelhart et al 2014b). …”

“…Previous research already showed that it is possible to describe underlying mechanisms involved in standing balance using perturbations and multi-input-multi-output closed loop system identification techniques (MIMO-CLSIT) (Boonstra et al 2013;Engelhart et al 2014b;Peterka 2002). In those studies, several systems were perturbed one by one during quiet stance, while the subject kept his balance.…”