Overview. In the mid-upright sitting mode we have the 2 worst possible combinations.
- Axial loading.The compression on the L3 disc is x2.5 of that when lying supine (Nachemson).
- Flexion forces tend to tilt the pelvis backwards which flattens the lumbar lordosis and reduces the protective wedge angle of the lower intervertebral discs.
- Misplaced lumbar support. (Gorman J.)
- Hips at 90° due to seat being parallel to floor.
This all results in: The main factors in retropulsion of the IV disc contents :-
- Prolonged axial compression of the spinal joints with joint flexion.
- Failure to maintain the wedge angle of the lower two lumbar joints.
- Lack of variation in posture and loading.
- In addition to the IV disc pathology, there is stretching of the posterior elements (including ligaments) which can become permanent and allow instability. (Ligaments & CTD→)
- Relevant is an investigation using Whole-body Positional MRI (pMRI), by FW. Smith, Bashir W (2007) who found that the upright position, at 90°, caused disc contents to move the most, while the relaxed position (135°/45° reclined) caused disc contents to move the least. This confirms that the upright position is the worst for the back, while the relaxed position is the best.In the upright sitting position the nucleus has migrated posteriorly which can culminate in IV disc protrusion. Hips are at 90°.
(Once the main adverse effects are determined then it becomes possible to consider the options available for remediation. )
PARTIAL REMEDIATIONS of the upright sitting mode.
Requirements for optimising the biomechanics of prolonged sitting in the upright seated mode.
2 existing systems are in use.
- Forward Tilted seat (FTS) ☛ The FTS→
- Back support of some sort. ☛ Lumbar v Iliac support→ (‘Ischial off-load‘ combines these two. FTS & back support, by use of a convex • Pelvic support→)
- 4. The ‘Supine reclined mode’ is slowly coming to be recognised. It is to some extent a full solution except lacking the requirements of the 2M concept.
The relaxed position with hip angle at 135°, the NP is in the safe mid-position. This is practical and preferable and is advocated for the 2Tilt principle in the reclined mode.
The importance of keeping staff exercised is increasingly recognised. The negative consequences of constrained sitting has been described by a number of authorities (Adams and Hutton, 1983; Duncan and Ferguson, 1974; Edlund, There are advantages in keeping staff moving around and inter-reacting in certain types of office. Note that a sit/stand system can always incorporated into a 3M work chair station→ . For more detail See ☛Sit/Stand→
Spinal movement aids muscle proprioception and IV Disc nutrition which has a physiological advantages over constrained seating and also may have a comforting effect if under the control of the user. For a fuller account see ☛Movement→ Also ☛ 2M exercise systems→
The 2TILT (2T, 2M) CONCEPT.
An optimal system that effectively fully remediates all the adverse effects that have been enumerated. Any chair can be compared to it for establishing it’s ergonomic efficiency. An essential tool for chair design. See ☛ The 2 TILT (2T) CONCEPT→
Requirements of the 2Tilt concept. See ☛ 2T Requirements→
The concept is derived from consideration of spinal anatomy, pathology, biomechanics and testing as already described. Each of the possible remediations (above) are found to be incorporated.
An office work-station is easily derived from the 2Tilt (now 2M) chair concept and now renamed the ‘TriModal’ (3M) concept as a sit/stand component is incorporated. See☛workstations→
“Sit/stand desks and pelvic support, semi reclined seating in the UK have yet to become part of the normal office landscape. When they do there will be several million office workers that will be grateful to be free from daily debilitating pain.” John Jukes, 2001.