COMMENT ON SUPPORT SYSTEM

This system of body support whilst working at a desk or similar, aims to simulate the standing position by maintaining the angle of the pelvis (using an angled seat) so that it is near to the angle when standing, as this is the position of least stress on the spine whilst weight bearing.

The body weight, whilst using the 'Equipoise', is taken by the 'ischial tuberosity' (seat bones). As sitting in this position uses back muscles as in standing, the use of these systems for any length of time results in muscular fatigue; the back support built into the 'Equipoise' to maintain the concavity of the lumbar spine thus avoids the user tiring.

To maintain stability when seated on the angled seat, some thrust is taken by the 'femur' (thigh bones) as their extremities (the knees) rest against an angled knee support. The thrust delivered to the knee support is minimal and contributes little or nothing to the deflection of the supporting structural frame.
 

GENERALLY

The structural frame is made of 25 x 25 mm hollow steel section, all radiused shapes being reduced in section such that the inner and outer flanges are each concave by up to 2 mm, the sides of the radiused sections remaining flat and 25 mm wide.

     The general shape is sketched as in Appendix 1.

The seat was positioned at the innermost (of 7) positions, so that when loaded, the maximum bending moment was produced in the supporting structural frame.
 

RESULTS

Loads were placed on the seat, generally in 25 kg increments, up to a maximum load of 185 kg (almost 30 stone). The deflection of the frame was measured as shown in Appendix 1, and is the maximum deflection within the frame.

The deflection versus loading line was linear, thus indicating that the frame was within the elastic limit whilst loaded. A settlement of 2 mm for the wheeled supports occurred at a load of 130 kg and the wheel system remained in that position thereafter.

Some deflection (up to 5 mm) did occur in the lower horizontal section of the frame as indicated in Appendix 1, the frame almost touching the ground with 185 kg applied.

The maximum deflection measured was 29 mm at the load of 185 kg; this equated to a deflection of approximately 1.5 mm/10 kg. of applied body weight whilst the seat is positioned furthest away from the structural frame and in its 'worst' (structurally) position.

    No sideways deflection of the structural frame was noted.

Other appurtenances on the system were inspected and load tested by hand, these including the adjustable back support, the under seat steel framing and the knee support system; the knee support system can be rotated out of the way to allow of easy movement away from the 'Equipoise'. It is considered that they all are of sufficient strength and rigidity as to support loads of up to 185 kg.

COMMENTS

Overall, the 'Equipoise' system examined is of satisfactory structural design, capable of supporting persons up to 185 kg. With considerable usage, the wheeled supports on the system could fail due to rough surfaces. A failure of one or more of these wheels would not contribute to a fall as the frame itself would rock or distort such that it lowered by approximately 8 mm until the frame rested on the ground, thus giving full support to the body weight applied thereto.

     Tests conducted by Associate Professor R W Upfold.
 

R. W. Upfold.
Associate Professor R W Upfold
 

                                                                              Appendix 1.