Elsevier

Gait & Posture

Volume 53, March 2017, Pages 241-247
Gait & Posture

Full length article
Compliant support surfaces affect sensory reweighting during balance control

https://doi.org/10.1016/j.gaitpost.2017.02.004Get rights and content

Highlights

  • Standing on foam yields a support surface rotation and a compliance effect on sensory reweighting.

  • A support surface rotation effect leads to down weighting of proprioceptive information.

  • A compliance effect leads to less sensory reweighting.

Abstract

To maintain upright posture and prevent falling, balance control involves the complex interaction between nervous, muscular and sensory systems, such as sensory reweighting. When balance is impaired, compliant foam mats are used in training methods to improve balance control. However, the effect of the compliance of these foam mats on sensory reweighting remains unclear.

In this study, eleven healthy subjects maintained standing balance with their eyes open while continuous support surface (SS) rotations disturbed the proprioception of the ankles. Multisine disturbance torques were applied in 9 trials; three levels of SS compliance, combined with three levels of desired SS rotation amplitude. Two trials were repeated with eyes closed. The corrective ankle torques, in response to the SS rotations, were assessed in frequency response functions (FRF). Lower frequency magnitudes (LFM) were calculated by averaging the FRF magnitudes in a lower frequency window, representative for sensory reweighting.

Results showed that increasing the SS rotation amplitude leads to a decrease in LFM. In addition there was an interaction effect; the decrease in LFM by increasing the SS rotation amplitude was less when the SS was more compliant. Trials with eyes closed had a larger LFM compared to trials with eyes open.

We can conclude that when balance control is trained using foam mats, two different effects should be kept in mind. An increase in SS compliance has a known effect causing larger SS rotations and therefore greater down weighting of proprioceptive information. However, SS compliance itself influences the sensitivity of sensory reweighting to changes in SS rotation amplitude with relatively less reweighting occurring on more compliant surfaces as SS amplitude changes.

Introduction

Human balance control during stance is continuously challenged by the gravitational field. To maintain an upright posture and prevent falling, balance control involves the complex interaction of nervous, muscular and sensory systems. The central nervous system (CNS) receives feedback about the body orientation from three main sensory systems: the visual, proprioceptive and vestibular system. For each sensory system, the feedback is compared to its reference. The CNS integrates this information and generates an ‘error’, representing deviations of body orientation from upright stance. The error signal of each sensory system is weighted in relation to its reliability; the CNS prefers reliable over less reliable sensory information within an adaptive weighting process termed sensory reweighting [1], [2], [3]. Subsequently, the neural controller (NC) generates with a time delay, a corrective, stabilizing torque by selective activation of muscles. This stabilizing torque (together with a torque caused by the intrinsic dynamics of the muscle properties) keeps the body in upright position.

In elderly and in people with neurological, sensory or orthopedic disorders, balance control might be impaired, leading to postural instability and falls [4], [5]. People with impaired balance control often undergo functional balance training that is specifically oriented to improve steadiness while standing on compliant surfaces like foam mats [6], [7]. It is assumed that sensory reweighting will be trained using these foam mats, since proprioceptive information is disturbed by the compliant support surface [8]. However, the effect of the compliance of these foam mats on sensory reweighting and balance control remains unclear due to a causality problem. The compliant support surface, i.e. the surface in contact with the feet, might have an effect on sensory reweighting induced by the compliance itself, but on the other hand also might have an effect on sensory reweighting provoked by support surface rotations induced by the compliance. System identification techniques in combination with specifically designed external disturbances provide a way to disentangle cause and effect in balance control. By externally exciting the system with an unique input that is not related to the internal signals of the system, a causal relation between the external disturbances and output signals can be created. This ‘opens’ the closed loop and generates informative data about a dynamic system such as balance control [9].

In this paper we investigated the effect of compliant support surfaces, comparable to foam mats, on sensory reweighting of proprioceptive information in balance control using system identification techniques, independent of the effect caused by the change in support surface rotation amplitude. Previous studies showed that increasing the amplitude of support surface rotations result in a decrease of the proprioceptive weight (i.e. down weighting), since the proprioception becomes less reliable [1], [3], [10]. Therefore, we hypothesize that increasing the amplitude of the support surface rotations lowers the reliability of the proprioceptive information and thus results in down weighting of proprioceptive information [11]. Due to the compliance effect of the support surface, an increase in compliance might affect this sensory weighting. If a compliance effect is present, sensory reweighting due to increasing support surface rotation will change for different levels of compliance.

Section snippets

Subjects

Eleven healthy young volunteers (age:20–30 years, 8 women, weight: 67.4 ± 8.2 kg, height: 1.85 ± 0.09 m), without any history of balance disorders, musculoskeletal injuries or neurological disorders, participated in this study and gave written informed consent prior to participation. The study was performed according to the principles of the Declaration of Helsinki and approved by the Medical Ethics Committee of Medisch Spectrum Twente, Enschede, the Netherlands.

Apparatus

Fig. 1A shows the Bilateral Ankle

Time series

Fig. 2 shows the time series as response to the disturbances of a typical subject in the medium Kss− medium θss condition with eyes open. The standard error of the SS rotation amplitude is relatively low. The responses of body sway angle and corrective ankle torque are slightly more variable. The power spectral densities corresponding to the time series show that the excited frequencies are present in all signals.

Rigid body dynamics

Fig. 3 shows the mean FRF and standard error of the FRF of the rigid body dynamics

Sensory reweighting

As in previous studies, sensory reweighting is most pronounced at low frequencies where system dynamics are dominated by sensory influences and are minimally affected by other factors such as inertia [1], [3], [10]. In this study, we found a comparable effect of the SS rotation amplitude on the LFM; by increasing the SS rotation amplitude, the LFM decreased. This indicates that the stabilizing mechanism was down weighting the proprioceptive information accompanied by up weighting the vestibular

Conclusion

In this study we investigated the effect of compliant support surfaces by changing the level of stiffness on sensory reweighting of proprioceptive information in human balance control during stance using closed loop system identification techniques. Results indicate sensory down weighting of proprioceptive information occurs with increasing SS rotation amplitude, but this sensory down weighting is also affected by the level of SS stiffness resulting in relatively less sensory reweighting on

Declaration of conflicting interest

The authors whose names are listed above certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject

Acknowledgements

This research is supported by the Dutch Technology Foundation STW (NeuroSIPE #10737 BalRoom) which is part of the Netherlands Organisation for Scientific Research (NWO), and which is partly funded by the Ministry of Economic Affairs.

References (14)

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I.M. Schut and D. Engelhart contributed equally to this work.

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