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IE 552 Mechanics of Musculoskeletal System Dr. Andris Freivalds Class #40

IE 552 Mechanics of Musculoskeletal System Dr. Andris Freivalds Class #40. Work Surface Height. Normal Working Area. Max. Optimum. 3-D Workspace. Seated Posture – Key factor ■ Avoid postural rigidity – sit/stand chair ■ Alternate postures and tasks. Key Factors in Office Ergonomics.

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IE 552 Mechanics of Musculoskeletal System Dr. Andris Freivalds Class #40

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  1. IE 552 Mechanics of Musculoskeletal System Dr. Andris Freivalds Class #40 IE 552

  2. Work Surface Height IE 552

  3. Normal Working Area IE 552

  4. Max Optimum 3-D Workspace IE 552

  5. Seated Posture – Key factor■ Avoid postural rigidity – sit/stand chair■ Alternate postures and tasks IE 552

  6. Key Factors in Office Ergonomics • Seated posture • Visual display terminal • Keyboard • Mouse and other input devices • Notebooks and hand-held PCs • Rest breaks • Psychosocial factors IE 552

  7. Visual Display Terminal • Height of VDT screen • Line of sight of -15° (-9° to -45°) below horizontal • Larger angles ↓ ocular exposure (‘dry eyes’) (Yaginuma, Yamada, 1990) • Preferred screen distance • Average of 59 cm (12 - ∞) • Intermediate resting focus (Leibowitz, Owens, 1975) IE 552

  8. IE 552

  9. Seat Values IE 552

  10. Key Factors in Office Ergonomics • Seated posture • Visual display terminal • Keyboard • Mouse and other input devices • Notebooks and hand-held PCs • Rest breaks • Psychosocial factors IE 552

  11. Keyboard – Basic features • Slope: 0-15° (wrist ext.) (typewriter ~ 30°) • Key size: • min 1.2 x 1.2 cm • ~ 2 cm separation • indented • Key resistance: • min 0.5 N (accidental) • max 1.5 N • tactile feedback (elastomer response) 30° 30 IE 552

  12. max min Keyboard – Keying Forces • 4 – 7 times larger than necessary (Armstrong, Foulke, 1994) • force ratio = max min • 6 – 20% MVC (Martin, Armstrong, 1996) • main problem! IE 552

  13. Keyboard – CTS Impairment • CTS impaired exerted twice (10x min) the force as non impaired (Lowe, Freivalds, 1999) • Due to sensory and motor nerve impairment • Vicious cycle - aggravates the problem IE 552

  14. Maltron Keyboard – Split, angled • 40° ulnar deviation, 13% ↑ CT pressure (Werner, Armstrong, 1997) • 76° forearm pronation, muscular fatigue • 50° wrist extension, CT pressures = 2x injury threshold (Rempel, Keir, 1997) • Negative slope keyboard IE 552

  15. Keyboard - Layout • QWERTY – Sholes (1878) • Like printer’s type case? • To prevent key jamming? • Dvorak (1936) alternative • Most keys on home row • Most frequent keys→ strongest fingers • 5% faster? (US Navy) • Not worth retraining millions of typists IE 552

  16. Keyboard – Chord type • One character requires simultaneous activation of two keys • 50% faster (Seibel, 1964) • One-handed use • Small, portable • Special functions: • mail sorting • court stenotyping IE 552

  17. Keyboard – Numeric pads • Unfortunately two alternatives • Telephone is 5% faster, with 50% less errors (Deininger, 1960; Conrad and Hull, 1968) • BSR/HFES 100 recommends either • Worst situation = causes confusion IE 552

  18. Key Factors in Office Ergonomics • Seated posture • Visual display terminal • Keyboard • Mouse and other input devices • Notebooks and hand-held PCs • Rest breaks • Psychosocial factors IE 552

  19. Mouse and Other Input Devices IE 552

  20. Mouse and Other Input Devices • Clear speed-accuracy tradeoff • Touch screen, light pen: fast but inaccurate • Cursor keys, joystick: unacceptable • Mouse ranks high in all categories (trackball is a close second) • Thus the ubiquitous mouse, but problems! IE 552

  21. Mouse - Problems • 60° ulnar deviation, 45° lateral shoulder rotation, wrist extension, (Karlqvist, Hagberg, 1994) • Over gripping, 2.8 x min force (Park, 1999) • CT pressures at injury level (30 mm Hg, Keir, Bach, 1999) IE 552

  22. Mouse – Remedies, Alternatives • Use of drag-lock feature • Upright handle redesign • power grip • thumb activation • 23%↑ force • ↓ pain intensity (Aarås and Ro, 1999) Renaissance Mouse IE 552

  23. Mouse - Alternatives • Trackball (Burgess-Limerick, 1999) • ↓ ulnar deviation • ↑ wrist extension • Touchpad(Akamatsu, MacKenzie, 2002) • 22% ↓force • 19% slower • Trackpoint (Batra, Dykstra, 1998) • ↑ forearm loading • ↓ performance IE 552

  24. Key Factors in Office Ergonomics • Seated posture • Visual display terminal • Keyboard • Mouse and other input devices • Notebooks and hand-held PCs • Rest breaks • Psychosocial factors IE 552

  25. Notebooks and Hand-held PCs • 34% of US 2000 PC market (Sommerich, 2000) • Reduced size and weight • Smaller keys and screen • Lack of mouse (unless added) • ↓ keying speed (Yoshitake, 1995) • ↑ neck flexion, ↑ shoulder flexion • Worse with hand-held PCs IE 552

  26. Rest Breaks • Provides relief for frequent motions • Also relief for color adaptation and ocular accommodation • Short rest pauses (1-5 min) (Graf, 1960) • Active rather than passive • Irregular, spontaneous intervals • At least every hour • Perhaps even micropauses • Use them!! IE 552

  27. Psychosocial Factors • Work environment factors • High workload (lack of rest?) • Monotonous work (counterintuitive?) • Low job control • Individual factors • Gender – females at ↑ risk • Age – older workers at ↑ risk • Pain sensitivity – chronic pain-spasm reflex • As ergonomists, little knowledge or control IE 552

  28. CONCLUSIONS – What can we do to prevent WRMSDs in the office environment? • 50% psychosocial – difficult to change • 50% job-related/physical – can change • frequency: • give and use rest breaks • limit to 8-hour days (no 12-hr) • limit pace - no incentives/keystroke counts • tradeoff with productivity! • force: use minimum (redesign/training) • posture: keep neutral (redesign/training) IE 552

  29. Writers’ Cramp – Why? • Problems with quills: • Overgripping of thin shaft • Co-contractions of forearm flexors and extensors • Focal dystonia (treat with lidocaine) • For the digitally challenged: • Oversized pens • Large (13.6mm) flared grip area • ↓ flexor pollicis EMG (Udo,Otani, 2000) Dr. Grip IE 552

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