Sitting

1. Sitting Rad Zdero, Ph.D. zradovan@uoguelph.ca University of Guelph

2. Outline • Lower Back: Lumbar Disc Force • Lower Back: Muscle Activity • Shoulders and Legs • Design Considerations for Seating

3. Weight Transfer • Main body weight transferred to seat. Some transferred to floor, backrest, and armrests. • Advantages • provides stability to tasks involving visual and motor control • less energy consuming than standing • places less stress on lower extremity joints • lessens pressure on lower extremity circulation

4. Lower Back: Lumbar Disc Force

5. [Hall, 1999] Cervical Vertebrae (C1 to C7) Thoracic Vertebrae (T1 to T12) Lumbar Vertebrae (L1 to L5) Sacrum (S1 to S5)

6. SITTING ERECT (sacrum support) SITTING RELAXED (no sacrum support) STANDING SITTING (leaning forward) SITTING (leaning back) [Chaffin et al., 1999]

7. Chair and Disc Force Disc Force [Andersson et al., 1974a]

8. Office Desk and Disc Force Disc Force [Andersson et al., 1974b]

9. Postures and Disc Force Disc Force [Nachemson, 1975]

10. Backrest and Lumbar Support Lumbar Support Location Inclination Angle

11. Backrest, Lumbar Support, & Disc Force Disc Force [Andersson et al., 1974a]

12. Armrest and Disc Force Disc Force [Andersson et al., 1974a]

13. Changing Posture & Disc Force • studies show increased low back pain (LBP) risk for people required to sit or stand for prolonged periods of time (Grieco, 1986) • best to change work posture and/or occupational task frequently to prevent “postural fixity” (Magora, 1972) • Center for Disease Control (U.S.) 1980 tips: • 10-15 minute breaks for every 2 hours seated work

14. Lower Back: Muscle Activity

15. Erector Spinae Muscle • extends down the back • involved in lateral (sideways) flexion of back & extension activities, e.g. maintaining back posture in any “sitting” position • the greater the EMG (electromyography) muscle activity, the greater the compressive force on discs

16. Muscle Activity & Backrest Inclination [Andersson & Ortengren, 1974] EMG signal in lumbar erector spinae muscles.

17. Muscle Activity & Backrest Inclination Increased backrest inclination lessens the need for spinal support from the erector spinae muscle, thereby lowering its EMG activity (Hosea, 1986). EMG (Arbitrary Units)

18. Muscle Activity & Lumbar Support Change in lumbar support has varying effects on EMG erector spinae muscle activity at different spine segments (Hosea, 1986). EMG (Arbitrary Units)

19. Shoulders Ideal desk and elbow height minimize shoulder EMG muscle activity (i.e. deltoids, trapezii, dorsal) [Engdahl, 1978]

20. Legs Erector Spinae Muscle Activity at L3 [Bendix et al., 1985]

21. Legs Higher seating changes the pelvic angle towards a more vertical posture, decreasing the compressive force on discs. But, stresses on the legs increase, which may cause swelling. [Engdahl, 1978]

22. Design Considerations for Seating

23. Design Considerations GOAL: function, comfort, user friendliness, minimizing spinal disc forces • Lumbar Support: use backrest that has lumbar support allowing for spinal curvature similar to standing • Leg Position: movement of pelvis caused by different positioning of knees and hips, e.g. car seats vs. office chairs • Seat Design: tilted seats, contoured or cushioned support to ideally position pelvis, use of armrest, adjustability, seat width, height, & depth • Desk Design: bottom height, top height, inclination of surface, work surface size, surface friction (Human Factors Society, 1988)

24. Backrest Dimensions • Top height • Bottom height • Center height • Height • Width • Horizontal radius • Vertical radius • Seat angle [Chaffin et al., 1999]

25. Seat Dimensions • Height • Width • Depth (length) • Slope [Chaffin et al., 1999]

28. Chairs for Visual Display Terminal (VDT) Workstations

29. Desks for Visual Display Terminal (VDT) Workstations

30. Keyboard & Terminal for Visual Display Terminal (VDT) Workstations

31. Sources 1 • Andersson et al., “Lumbar disc pressure and myoelectric back muscle activity during sitting: I: studies on an experimental chair”, Scan. J. Rehab. Med., 3:104-114, 1974a. • Andersson et al., “Lumbar disc pressure and myoelectric back muscle activity during sitting: II: studies on an office chair”, Scan. J. Rehab. Med., 3:115-121, 1974b. • Andersson and Ortengren, “Myoelectric back muscle activity during sitting”, Scand.J.Rehab.Med., Supplement 3:73-90, 1974. • Bendix et al., “Comparison of office chairs with fixed forwards and backwards inclining or tiltable seats”, Eur.J.Appl.Physiol., 54:378-385, 1985. • Chaffin et al., Occupational Biomechanics, 1999. • Engdahl, “Specification for office furniture”, In: B.Jonsson, Sitting Work Postures, (in Swedish) 1978.

32. Sources 2 • Grieco, “Sitting posture: an old problem and a new one”, Ergonomics, 29:345-362, 1986. • Hall, Basic Biomechanics, 1999. • Hosea, “Myoelectric analysis of the paraspinal musculature in relation to automobile driving”, Spine, 11:928-936, 1986. • Human Factors Society, ANSI/HFS 100-1988, Santa Monica, California, USA. • Magora, “Investigation of the relation between low back pain and occupation, 3. Physical requirements: sitting, standing, and weight lifting”, Industr. Med. Surg., p.415, 1972. • Nachemson, “Towards a better understanding of back pain”, Rheumatol. Rehabil., 14:129, 1975. • Sanders & McCormick, Human Factors in Engineering and Design, 1993.