Development of a theoretical model for upright postural control in lower limb prosthesis users

David F Rusaw,Sigurd Edholm,Karin L L Hallstedt,Jessika Runesson,Cleveland T Barnett,Rasmus Alinder

doi:10.1038/s41598-021-87657-2

David F Rusaw, Sigurd Edholm + Show 4 more

Open Access

https://doi.org/10.1038/s41598-021-87657-2

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Abstract

Methods used to assess quiet standing in unilateral prosthesis users often assume validity of an inverted pendulum model despite this being shown as invalid in some instances. The aim of the current study was to evaluate the validity of a proposed unilaterally-constrained pin-controller model in explaining postural control in unilateral prosthesis users. Prosthesis users were contrasted against the theoretical model as were able-bodied controls that stood on a platform which unilaterally constrained movement of the CoP. All participants completed bouts of quiet standing with eyes open, eyes closed and with feedback on inter-limb weight bearing asymmetry. Correlation coefficients were used to infer inverted pendulum behavior in both the anteroposterior and mediolateral directions and were derived from both kinematic (body attached markers) and kinetic (centre of pressure) experimental data. Larger, negative correlation coefficients reflected better model adherence, whilst low or no correlation reflected poorer model adherence. Inverted pendulum behavior derived from kinematic data, indicated coefficients of high magnitude in both mediolateral (all cases range 0.71–0.78) and anteroposterior (0.88–0.91) directions, irrespective of groups. Inverted pendulum behavior derived from kinetic data in the anteroposterior direction indicated validity of the model with large negative coefficients associated with the unconstrained/intact limbs (prosthesis users: − 0.45 to − 0.65, control group: − 0.43 to − 0.72), small coefficients in constrained/prosthetic limbs (prosthesis users: − 0.02 to 0.07, control group: 0.13–0.26) and large negative coefficients in combined conditions (prosthesis users: − 0.36 to − 0.56, control group: − 0.71 to − 0.82). For the mediolateral direction, coefficients were negligible for individual limbs (0.03–0.17) and moderate to large negative correlations, irrespective of group (− 0.31 to − 0.73). Data suggested both prosthesis users’ and able-bodied individuals’ postural control conforms well to that predicted by a unilaterally-constrained pin-controller model, which has implications for the fundamental control of posture in transtibial prosthesis users.

Highlights

Methods used to assess quiet standing in unilateral prosthesis users often assume validity of an inverted pendulum model despite this being shown as invalid in some instances
The inverted pendulum model predicts the relationship between the centre of pressure (CoP) and centre of mass (CoM) to be a negative correlation between the difference in CoP-CoM position and the horizontal CoM acceleration in the same plane in both the anteroposterior and mediolateral directions[8,9,10]
Recent efforts to explore if unilateral prosthesis users can utilize unilateral control in inverted pendulum-based postural control have revealed that they have the ability via residual musculature on the amputated side[23,24] and the passive qualities of the prosthetic c omponentry[21,25]

Summary

Introduction

Methods used to assess quiet standing in unilateral prosthesis users often assume validity of an inverted pendulum model despite this being shown as invalid in some instances. Coefficients were negligible for individual limbs (0.03–0.17) and moderate to large negative correlations, irrespective of group (− 0.31 to − 0.73) Data suggested both prosthesis users’ and ablebodied individuals’ postural control conforms well to that predicted by a unilaterally-constrained pin-controller model, which has implications for the fundamental control of posture in transtibial prosthesis users. Previous research has confirmed that for prosthesis users, there is a clear inter-limb asymmetry in mechanical function that may render the direct application of this single inverted pendulum model to this patient group p roblematic[17,18] For this reason, it is necessary to establish new models of postural control that reflect this unilateral difference in function. Inter-limb differences in function arising during or following physical therapy that attempts to improve balance would be better understood

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