DYNASIT - An Assistive Seating System that Controls Sitting Posture and Regulates Body Load Associated with Sitting Related Mobility Problems

Paul van Geffen & Jasper Reenalda

ABSTRACT

Subjects who cannot functionally reposition themselves adopt a passive body posture and often suffer from physical discomfort in long-term sitting. Sitting related mobility problems such as pressure ulcers (PU) and low back pain (LBP) are associated with sustained mechanical tissue loading and dynamic seating interventions are needed. Based on carefully selected functional requirements, this paper describes the design and evaluation of an assistive seating system called Dynasit that controls sitting posture and regulates body load in long-term sitting. Experiments have been performed in which chair configuration, body posture, buttock tissue viability and seat reaction load were monitored when subjected to alternating protocols of chair adjustments. Although the experiments only involved healthy subjects, findings suggest that the Dynasit might be an applicable intervention to control sitting posture and to regulate body load associated with wheelchair seating discomfort. Keywords: dynamic wheelchair (interventions), posture control, body load, pressure ulcers, low back pain

BACKGROUND

Figure 1: Design process of the Dynasit. Based on carefully selected functional and technical requirements, a prototype of the assistive seating system Dynasit was developed. Figure 1: Design process (Click for larger view)

Impairments to the neuromuscular function make many wheelchair dependent subjects suffer from limited trunk stability [1] which results in a passive body posture [2] when subjected to long-term sitting. The inability to change body posture contributes to physical discomfort such as pressure ulcers (PU) [3] and low back pain (LBP) [4]. It is accepted with some certainty that sitting related mobility problems are associated with sustained mechanical tissue loading and that dynamic seating interventions are needed to periodically relieve body structures [5]. For subjects who cannot functionally reposition themselves we developed an assistive seating system called Dynasit that controls sitting posture and regulates body load associated with PU and LBP. This paper describes the design and evaluation of the Dynasit, based on carefully selected functional requirements. In addition it evaluates whether this system might be applicable to regulate body load in subjects suffering from neuromuscular impairments to the trunk and lower extremities. 

APPROACH

To prevent mobility problems in prolonged sitting, we strive to regulate body load from global posture adjustments and from local support surface manipulations.

Assistive Seating System

Figure 2: Mechanical concept for sagittal postural adjustment A: Based on a parallelogram, the mechanical concept for independent body segment actuation is shown. B: adjustable simulator chair with the concept for sagittal postural adjustment. Seat-, parallelogram- and backrest angle are defined by φ, α and β respectively.Figure 2: Mechanical concept (Click for larger view)

We developed a fully adjustable computer-aided (Matlab/Simulink® environment) simulator chair (Fig. 1) that is instrumented with the following features:

  1. A concept that decouples body segments is integrated to control body posture when patients cannot functionally reposition themselves. Based on a parallelogram from which the backrest is actuated (Fig. 2A, nr 1), the mechanical concept for independent body segment actuation in the sagittal plane is shown in figure 2. Actuating the configuration of the parallelogram (Fig. 2A, nr 2) rotates the pelvis in the sagittal plane. The whole mechanism rotates around a pivot located under the tuberosities and is externally actuated (Fig. 2A, nr 3). In the frontal plane, actuating the relative height between the left and right seat part causes the pelvis to rotate sideways. For lateral trunk rotation, the backrest is actuated in the frontal plane, around a pivot point located within the lumbothoracic region.
  2. Built-in measurement for real-time feedback of chair configuration and body posture.
  3. A sledge mounted backrest to minimize friction at the trunk/backrest interface. Frictionless vertical trunk translations are allowed when the pelvis is not properly aligned.
  4. Two adjustable force sensing support elements under the ischial tuberosities. Aligned with the seat pivot point and spaced according to an average inter-tuberal distance of 120 mm, two adjustable force sensing support elements are integrated in the seat plane. Both support elements are independently adjustable and designed to relieve pressure meaning that they only allow tuberal actuation below the support surface (Fig 3).
  5. A force sensing seating plane. For accurate 3D measurements of seat reaction load, 6 uni-axial load cells (FUTEK®, California, USA) are integrated in the seating surface.
Figure 3: Adjustable force sensing seating plane (FSP) with an external pressure mapping device (PMD) placed on top. Two adjustable force sensing tuberal support elements (TSE) are integrated in the FSP. Maximal tuberal adjustment (d) is shown. Figure 3: Adjustable seating plane (Click for larger view)

Furthermore, all mechanical and electrical mechanisms are carefully covered and protected to ensure full safety for the patient and bystanders when applying the Dynasit into practice.  

SYSTEM EVALUATION

The Dynasit was designed to control body posture and body load when subjected to passive and long-term sitting. To evaluate whether the Dynasit satisfied the described functional design criteria, experiments have been performed that were approved by the Local Committee for Medical Ethics of ‘t Roessingh Centre for Rehabilitation (Enschede, the Netherlands).

Experimental Setup

Twenty healthy male subjects were seated in the simulator chair with their ischial tuberosities on top of the tuberal support elements. The chair was individually adjusted and reflective markers were placed on the chair and selected anatomical landmarks. 3D chair configuration and body segment orientations were obtained using an infrared camera motion capturing system (VICON®, Oxford, UK) and an accelerometer (MT®, Xsens, Enschede, the Netherlands) attached on the pelvis. A pressure mapping device (Tekscan®, Boston, USA) was placed over the seat to measure pressure distribution. Skin (<2mm) and subcutaneous (<9mm) subtuberal buttock tissue viability was measured with two non-invasive fibre-optic probes that were attached on the skin under the ischial tuberosities. A diagnostic device (Oxygen-to-see®, LEA, Giessen, Germany) that uses Laser Doppler flowmetry and diffuse reflectance spectroscopy measured subtuberal buttock tissue oxygenation and perfusion. Chair configuration, body segment orientation, buttock tissue viability and seat reaction load were captured when subjected to alternating protocols of posture adjustments and manipulations to the local seating surface.

RESULTS AND INTERPRETATIONS

Strong significant (p<0.05) correlations (r2>0.8) were found between chair configuration, sitting posture, buttock tissue viability, buttock load and low back load [6-9]. These findings suggest that the Dynasit might be effective to control sitting posture and to regulate body load associated with LBP and PU in able-bodied subjects. For clinical use, the concept is still to be tested in subjects without postural stability. The lack of controlled trunk muscle function and problems with spasticity are issues that influence postural response from chair adjustments and it is important to evaluate whether the Dynasit is also an applicable assistive technology in subjects who cannot functionally reposition themselves.

ACKNOWLEGEMENTS

This design study was partly funded by the Dutch Ministry of Economic Affairs, SenterNovem. The authors would like to thank the engineering company Demcon (Oldenzaal, the Netherlands) for developing the experimental simulator chair. The authors also thank Bert Faber (Welzorg Special Products, Oldenzaal, the Netherlands) and PRsella (Oldenzaal, the Netherlands) for sharing their expertise in seating ergonomics. This study was supervised by Peter Veltink (PhD) and Bart Koopman (PhD) from the Institute for Biomedical Technology (BMTi, Enschede, the Netherlands), Michiel Jannink (PhD) and Hans Rietman (PhD) from the Roessingh Research and Development (RRD, Enschede, the Netherlands), and Maarten IJzerman (PhD) from the department of Health Science, University of Twente (Enschede, the Netherlands).

AUTHOR CONTACT INFORMATION

Paul van Geffen, Institute for Biomedical Technology (BMTi), Laboratory of Biomechanical Engineering, Department of Engineering Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, Telephone: +31 53 489 3649, Email: p.vangeffen@ctw.utwente.nl

REFERENCES

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  2. Hastings, J. D., et al. (2003). Wheelchair configuration and postural alignment in persons with spinal cord injury. Archives of Physical Medicine and Rehabilitation, 84, 528-34.
  3. Bouten, C. V., et al. (2003). The etiology of pressure ulcers: skin deep or muscle bound? Archives of Physical Medicine and Rehabilitation, 84, 616-9.
  4. Ferguson, S. A. & Marras, W. S. (1997). A literature review of low back disorder surveillance measures and risk factors. Clinical Biomechanics, 12, 211-226.
  5. Crane, B. A., et al. (2007). A dynamic seating intervention for wheelchair seating discomfort. American Journal of Physical Medicine and Rehabilitation, 86, 988-93.
  6. Reenalda, J., et al. Effects of Actuated Pelvic Rotation on Sitting Forces and Pressure Distribution. in RESNA 2008.
  7. van Geffen, P., et al. A System That Adjusts Chair Configuration for Desired Postural Change. in RESNA 2008.
  8. van Geffen, P., et al. (2008). Decoupled Pelvis Rotation in Sitting: A Passive Motion Technique that Regulates Body Load Associated with Low Back Pain and Pressure Ulcers. Journal of Biomechanical Engineering [under review].
  9. van Geffen, P., et al. (2008). Effects of Sagittal Postural Adjustments on Seat Reaction Load. Journal of Biomechanics [accepted].