Technical Briefs

Can Pelvis Angle be Monitored From Seat Support Forces in Healthy Subjects?

[+] Author and Article Information
Paul van Geffen

Laboratory of Biomechanical Engineering, Department of Engineering Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlandsp.vangeffen@utwente.nl

Peter H. Veltink

Biomedical Signals and Systems, Department of Electrical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

Bart F. Koopman

Laboratory of Biomechanical Engineering, Department of Engineering Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

J Biomech Eng 131(3), 034502 (Dec 23, 2008) (5 pages) doi:10.1115/1.3005345 History: Received June 11, 2008; Revised September 22, 2008; Published December 23, 2008

Individuals who cannot functionally reposition themselves often need dynamic seating interventions that change body posture from automatic chair adjustments. Pelvis alignment directly affects sitting posture, and systems that adjust and monitor pelvis angle simultaneously might be applicable to control body posture in sitting. The present study explores whether it is feasible to monitor pelvis angle from seat support forces. Pelvis angle estimation was based on equivalent “two-force member” loading for which pelvis orientation equals the orientation of the equivalent contact force. Theoretical evaluation was done to derive important conditions for practical application. An instrumented wheelchair was developed for experimental validation in healthy subjects. Seat support forces were measured, and mechanical analysis was done to derive the equivalent contact force from which we estimated the pelvis angle. Model analysis showed a significant influence of pelvis mass, hip force, and lumbar torque on the relation between the actual pelvis angle and the predicted pelvis angle. Proper force compensation and minimal lumbar torque seemed important for accurate pelvis angle estimations. Experimental evaluation showed no body postures that involved a clear relation between the pelvis angle and the orientation of the equivalent contact force. Findings suggest that pelvis angle could not be estimated in healthy individuals under the described experimental seating conditions. Validation experiments with impaired individuals must be performed under different seating conditions to provide a better understanding whether the principle is of interest for clinical application.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Equivalent two-force member loading to estimate pelvis angle (α). Schematic representation of a seated person in the sagittal plane. Support of the trunk above the lumbosacral spine makes the pelvis function as the foundation for trunk support guiding the forces of the upper body to the seat. Hip joint center (HJC) and the center of pelvis mass (cmp) lie on the line connecting the lumbar joint center (LJC) and tuberosities (T). The equivalent contact forces (Feq,t and Feq,l) substitute all forces (Fl, Fh, Ft, and Gp) on the pelvis. The orientation of Feq,t(ψeq) equals α in the absence of lumbar torque (Tl).

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Figure 2

Rigid body model consisting of four body segments (head, trunk, pelvis, and thighs) and two supporting areas (seat and back support). Positive body segmental angles (α, β, and γ), body dimensions, and all body forces are shown.

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Figure 3

Experimental setup. Sagittal view of the instrumented wheelchair with adjustable seat and a back supports. The seat is divided into a front and back part. Two multi-axis load cells are mounted under each part for measurement of the support forces and center of pressure under the pelvis (Ft and cpp) and thighs (Fth and cpth). Reflective markers were placed on the chair (UB, LB, BS, and FS) and selected anatomical landmarks (PSI, ASI, and EL). Chair configuration and body segment orientations were obtained using an infrared motion capturing analysis system. Local reference frames (Ts, Tp, Tb, and Tth) were constructed to compute chair configuration and body segment orientations.

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Figure 4

The experimental protocol involved stepwise lumbothoracic trunk support adjustments for different tilt-in-space chair angles (respectively, 0deg, 5deg, 10deg, 15deg, and 20deg)

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Figure 5

Pelvis angle estimation (α̂) from orientations of Ft(ψ), F1(ψ1), F2(ψ2), and Feq,t(ψeq) respectively. The identity lines (oblique dashed lines) refer to situations in which α equals α̂. (a) The relation between α and α̂ derived from the orientation of Ft(ψ). (b) Influence of hip force (Fh) on α̂. Thigh angle (γ1–4) affected Fh and influenced α̂ that was derived from the orientation of F1(ψ1). (c) Influence of pelvis mass (Gp) on α̂ derived from the orientation of F2(ψ2). (d) Influence of lumbar torque (Tl) on α̂ derived from the orientation of Feq,t(ψeq). The identity line coincides with ψeq when no lumbar torque is present (Tl=0Nm).

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Figure 6

Relations (third order best-fit) between the predicted pelvis angle (α̂) and the actual pelvis angle (α) derived from orientations of Ft(ψ) and Feq,t(ψeq), respectively. For the identity lines (oblique dashed lines), α equals α̂.



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