A Biomechanical Analysis to Derive Pelvic Tilt from Seating Forces



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_{International Seating Symposium • March 8–10, 2007}

A Biomechanical Analysis to Derive Pelvic Tilt from Seating Forces

Paul van Geffen, PhD Candidate

Bart H.F.J.M. Koopman, PhD

Peter H. Veltink. PhD

Abstract

Background: Among wheelchair dependent patients, a poor sitting posture is often seen which contributes to all kinds of physical problems during long term sitting. Because pelvic tilt is crucial for the adopted sitting posture the possibility to derive pelvic tilt from seating forces was investigated by means of a

biomechanical analysis. Methodology: Pelvic angle was

estimated based on equivalent ‘two-force member’ loading in which segment orientation equals force orientation. The equivalent contact force under the tuberosities were determined and successively compensated for pelvic mass, hip force and passive lumbar torque. Subsequently, equivalent force directions were calculated and compared with pelvic angle. Findings: Situations of minimal lumbar torque seemed an important condition for the possibility to derive pelvic tilt. Interpretations: Measuring seating forces seems useful to derive pelvic tilt and to individualise and control chair adjustments for wheelchair dependent patients.

Background

Among wheelchair dependent patients, a poor sitting posture is often seen [1] which contributes to all kinds of physical problems during long term sitting [2-4]. The inability to reposition implies that adequate variation in sitting posture can only be realized by changing the configuration of the chair. Important factors defining sitting posture are the orientation of the trunk, pelvis and thighs. Especially pelvic tilt is crucial for the adopted posture [5-8]. Contrary to pelvic tilt, desired thigh and trunk orientations can easily be invoked by proper adjustment of the seat and back support. For proper pelvic tilt however, information about the pelvic angle is needed. Gravitational forces of the upper body are guided through the pelvis to the seat and a relation between the pelvic tilt and seating forces is expected [9, 10]. When this relation is predictable, it might be possible to estimate pelvic tilt from seating forces and use this information to control sitting posture. The objective of the present study is therefore to investigate the possibility deriving pelvic tilt from seating forces by means of a biomechanical analysis.

Methodology

In figure 1A, a schematic representation is shown of an adopted sitting posture. Supporting the trunk just above the lumbar spine makes the pelvis function as the foundation for trunk support guiding gravitational forces of the upper body to the seat. Other forces exerted on the pelvis are the pelvic gravitational force and an extra force component in the hip joint exerted from the thighs. The individual pelvic segment including external forces acting on the pelvis is reflected in figure 1B. A passive joint stiffness was

introduced for a limited range of lumbar motion [11]. The

estimation of pelvic tilt ( ) is based on ‘two-force member’ loading [12] in which segment orientation equals force orientation (figure 1C). A rigid body model was developed to derive Feqfor different ranges of pelvic angle ( ), trunk angle ( ) and thigh angle ( ). Analysis was done for an average male subject (length = 1.80 m, mass = 80 Kg). Static equations of equilibrium for the individual body segments were determined and the equivalent force angle ( eq) was calculated for different ranges of (15o – 55o), (12o, 24o_{and 36}o_{) and (0}o_{, 12}o_{, 24}o_{and 36}o_{). The individual influence} of the hip force, pelvic mass and lumbar torque on the force angle (resp. 1, 2, 3) was also investigated.

Ft ?lum Fh Fu Gp B A C _Feq Feq ? ?eq ? ? ?

Figure 1. A: adopted sitting posture in which the trunk and thigh

angle are defined as and B: individual pelvic segment

including external forces and the contact force angle ( ). C:

equivalent two-force member loading in which pelvic angle ( ) equals the equivalent force angle eq.

Findings

Figures 2A-D show respectively , 1, 2, 3 for different ranges

of and when was set to 24o_{. The oblique dotted lines refer to}

the situation when eqequals . In figure 2A it is shown that is

greatly dependent on both and and that therefore it is not

possible to estimate only from Ft. In figure 2B it is shown that different values of ( 1– 4) influenced Fhip and resulted in an offset difference. Compared to the oblique dotted line, figure 2C showed a small slope difference which was caused by the influence of pelvic mass. Contrary to a relatively small influence of pelvic mass, a significant influence of lumbar torque was shown in figure 2D. However, a range of minimal lumbar torque ( lum≈ 0 Nm) is also shown in which 3equals eq. Minimal pelvic

tilt within the range of minimal lumbar toque was defined as *

and is reflected in figure 2D. So far, analysis was done in the situation that was set to 24o_{. To investigate the influence of ,}

3 was also derived for different values of (figure 2E). It is

shown that changing relative to influenced lumand resulted in

different values of 3and also different ranges of minimal lumbar torque. To assure minimal lumbar torque, a limited lumbar angle ( *_{) must be preserved which could be defined as the difference} between *_{and the specific .}



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_{International Seating Symposium • March 8–10, 2007}

Figure 2. A-D show respectively , 1, 2, 3for different ranges

of and when was set to 24o_{. The oblique dotted lines refer to}

the situation when eq equals . In figure D, range of minimal

lumbar torque (lum≈ 0 Nm) is shown and it is reflected how *was derived.

Interpretations

The estimation of pelvic tilt was based on equivalent ‘two-force member’ loading in which segment orientation equals force orientation. The analyses showed a significant influence of lumbar torque on the possibility to derive pelvic tilt. Since knowledge about pelvic angle is needed for estimating lumbar torque, logically it is only possible to derive pelvic tilt when the presence of lumbar torque is prevented. It was shown that the introduced passive joint stiffness resulted in a range of minimal lumbar torque which is an important condition. To assure this condition, a limited lumbar angle ( *_{) must be preserved at all} times. For clinical application, a concept for independent pelvis control in combination with the possibility to estimate hip force are essential to derive pelvic tilt from seating forces. Translating the seat in ventral-dorsal direction affects the orientation of the pelvis and can be used to control pelvic tilt. Hip force can be estimated by measuring the contact force under the thighs independent from the contact force under the tuberosities. A translating seat with force sensors in the front and back part of the seat satisfies the criteria of pelvic control en independent force measurement. Furthermore, to preserve the limited lumbar angle, assuring minimal lumbar torque, excessive seat translations must be coupled to back support tilt. Although experimental validation is necessary, measuring seating forces seems useful to derive pelvic tilt and to individualise and control chair adjustments for wheelchair dependent patients.

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10 20 30 40 50 60 10 20 30 40 50 60 A (o) G₁ G₂ G₃ G₄ Y₁ (o) B 10 20 30 40 50 60 10 20 30 40 50 60 A (o₎ G = G₁,G₂,G₃,G₄ Y₂ (o) C 10 20 30 40 50 60 10 20 30 40 50 60 A (o₎ G = G₁,G₂,G₃,G₄ Y₃ (o) D T_lum = 0Nm A* 10 20 30 40 50 60 10 20 30 40 50 60 A (o) G = G₁,G₂,G₃,G₄ B₁ B₂ B₃ Y₃ (o) E