Biomechanics
Part of Ergonomics ● Etymology – Ergon (ἔργον): work – Nomos (νόμος): law ● Definitions – “The scientific study of the efficiency of man in his working environment.” ( OED) – “... a science whose main purpose is to design workspaces and tools that help people do their jobs and use their tools and other products comfortably and safely.” (Oregon OSHA)
Musculoskeletal Disorders • Musculoskeletal Disorder: pain or loss of function due to overexertion or repetitive motion, especially – Low back (e.g., related to manual material handling) – Upper extremities (e.g., related to typing, assembly) • Hands • Wrists • Elbows • Shoulders • Magnitude of the Problem (US) 1 – 70 million physician office visits annually – 130 million total health care encounters • outpatient • hospital • emergency room – Potential for long-term disability (but most return to work within 31 days) – Annual cost: $45 - $54 billion • workers’ compensation costs • lost wages • lost productivity 1 National Research Council (2001). Musculoskeletal Disorders and the Workplace: Low Back and Upper Extremities , Washington: National Academies Press. 3
General Postural Considerations Requirements ● Weight of body segments transmitted with minimal strain ● Forces to external objects counteracted Recommendations ● Provide for postural changes ● Sitting preferable to standing – exceptions: large hand forces, body movement ● Provide comfortable chair 4
Manual Material Handling and Low Back Pain 5
Skeletal System Source (Left): Wikimedia Commons, http://en.wikipedia.org/wiki/File:Human_skeleton_front_en.svg, accessed 19 Jan 10. 6 Source (Right): Wikimedia Commons, http://en.wikipedia.org/wiki/File:Human_skeleton_back_en.svg, accessed 19 Jan 10.
Vertebral Column/Spine Source (Left): Wikimedia Commons, http://en.wikipedia.org/wiki/File:Illu_vertebral_column.jpg, accessed 19 Jan 10. 7 Source (Right): Konz & Johnson, Work Design , 6 th edition.
Vertebrae/Disk, Oblique View 8 Source: Wikimedia Commons, http://en.wikipedia.org/wiki/File:ACDF_oblique_annotated_english.png, accessed 19 Jan 10.
Vertebral Movement ● Facet joints – Small bony joints covered with soft tissue that help movement and keep structure. 9
Vertebra Cross-Section (cervical vertebra) 10 Source: Wikimedia Commons, http://en.wikipedia.org/wiki/File:Cervical_vertebra_english.png, accessed 19 Jan 10.
Musculoskeletal Disorders of the Back ● Extremely prevalent and costly – 2 nd most common neurological problem in US* – 75,000,000 Americans have back problems – 6,500,000 Americans in bed each day with back pain – 80% working adults develop back pain during career – $7,400 per back claim – $50 billion/year spent in treatment* ● Include: – Low-back pain – Low-back impairment: reduced ability – Low-back disability: lost time due to injury – Low-back compensation: reimbursement for medical expenses, lost wages * Healthline (http://www.healthline.com) / National Institute of Neurological Disorders and Stroke 11
Musculoskeletal Disorders of the Back ● Causes* – Strains: muscles, ligaments stretched, torn from over- activity – Disc injury: herniated disc, ruptured disc compresses nerve root, causes pain; usually lasts more than 72 hrs – Sciatica: compression of sciatic nerve, pain in legs feet – Spinal Stenosis: narrowing of spaces in spin due to disc degeneration, usually due to aging – Other Causes ● arthritis ● fibromyalgia: long-term joint, muscle, tendon pain ● etc. * Healthline (http://www.healthline.com) 12
Herniated Disc “Slipped” Disc 13
Ruptured Disc 14
Low-Back Pain Risk Factors ● Task demand ● Individual physical factors, e.g., factors, e.g., – Weight – Posture – Physique – Gender – Speed ● Psychological factors, e.g., – Repetition ● Environmental – Depression factors, e.g., – Anxiety – Workplace design – Job dissatisfaction – Slippery floors – Distractions 15
Garg & Moore’s (1992) Risk Factors ● Personal Risk Factors ● Job Risk Factors – Age [+] – Heavy physical work [+] – Gender [x] – Lifting [+] – Body size [x] – Bending, stretching, reaching [+] – Physical fitness [x] – Twisting [+] – Lumbar mobility [x] – Pushing and pulling [+] – Strength [x] – Prolonged sitting, standing – Medical history [+] [+] – Years of employment [x] – Accidents [+] – Smoking [+/-] – Whole body vibration [+] – Psychosocial [+] – Structural abnormalities [+] 16
Manual Material Handling Recommendations (1) ● Lifting Method: squat vs. stoop vs. free style ● Job Design: – reduce manual materials handling (MMH) – decrease weight – use 2 or more people where possible – Push or pull vs. lift & carry – minimize carrying distance – stacking height < shoulder height 17
Manual Material Handling Recommendations (2) ● Job Design (continued): – heavy objects at knuckle height – reduce lifting frequency – provide rest periods – use job rotation – provide handles ● Worker Selection ● Training ● Supports and Braces??? 18
NIOSH Lifting Equation: Background ● Created by OSHA, 1970 ● Purpose: – Rank alternatives – Identify and correct problems – Not to rate absolute risk ● Goals – Biomechanical: ≤ 350 kg on L5-S1 – Physiological: ≤ 9.5 kcal/min 19
NIOSH Lifting Equation: Does Not Apply If > 1-2 steps ● One-handed lifting ● Seated/kneeling ● Restricted workspace ● Hot/cold/contaminated objects ● Unexpected events (e.g., slips, falls) ● Unstable load ● Carrying, pushing, pulling ● Using tools (e.g., wheelbarrow) ● Slippery floor ● Unfavorable environment (e.g., hot, cold) ● 20
NIOSH Lifting Equation RWL = LC × HM × VM × DM × FM × AM × CM ● LC = Load constant ● HM = Horizontal multiplier ● VM = Vertical multiplier ● DM = Distance multiplier ● FM = Frequency multiplier ● AM = Asymmetry multiplier ● CM = Coupling multiplier 21
Multiplier Formulas ● Horizontal multiplier HM = BIL / H – BIL = Body interference limit – H = Horizontal location – Large HM (small H) better (load close to body) ● Vertical multiplier VM = 1 – VC × | V – KH | – VC = Vertical constant = 0.0075 in. – V = Vertical location – KH = Knuckle height (for typical lifter = 30 in.) – Large VM (V close to KH) better (load closer to optimal lifting height) 22
Multiplier Formulas (cont.) ● Distance multiplier DM = .82 + DC / D – DC = Distance constant = 1.8 in. – D = Vertical travel distance 23
Multiplier Formulas (cont.) ● Frequency multiplier ● Lifting frequency = mean number of lifts in a 15-minute period ● Lifting duration /session in hours may be: – Short = .001 h to ≤ 1 h with recovery time of ≥ 1.2 × duration – Moderate = >1 h ≤ 2 h with recovery time of ≥ .3 × duration – Long = >2 h but ≤ 8 h 24
Multiplier Formulas (cont.) ● Asymmetry multiplier AM = 1 – 0.0032 × A – A = Angle of symmetry (≤ 135°) – 30% penalty for 90° angle ● Coupling multiplier – See Table 11.3 ● Depends on: – Height of initial and final hand–container coupling – Whether coupling is good, fair, or poor 25
NIOSH Lifting Equation Example from Waters et al (1994) 26
Analysis 27
Analysis After Redesign 28
Applications Manual For the Revised NIOSH Lifting Equation http://www.cdc.gov/niosh/docs/94-110/pdfs/94-110.pdf 29
Biomechanics and Hand Tools
Upper Extremity Notes : Skeleton is facing observer. Skeleton's left hand (on observer's right) is supinated (palm toward observer). Skeleton's right hand (on observer's left) is pronated (back toward observer). 31 Source: Wikimedia Commons, http://en.wikipedia.org/wiki/File:Human_arm_bones_diagram.svg, accessed 19 Jan 10.
Wrist Cross-Section 32 Source: Wikimedia Commons, http://en.wikipedia.org/wiki/File:Gray422.png, accessed 19 Jan 10.
Cumulative Trauma Disorders Associated With Hand Tool Use ● tendonitis – irritation & swelling of tendons ● tenosynovitis – inflammation of tendons & sheaths ● carpal tunnel syndrome – compression of median nerve caused by swelling ● trigger finger – finger cannot be extended ● vibration-induced white finger – loss of circulation, numbness caused by vibration 33
Hand/Wrist MSD Risk Factors NIOSH (1997). Musculoskeletal Disorders and Workplace Factors, Cincinnati, OH: Author. http://www.cdc.gov/niosh/docs/97-141/pdfs/97-141.pdf ● Verified (NIOSH study) Repetition – Force – Hand/wrist vibration – Combinations of risk factors – ● Possible Awkward (non-neutral) postures – High work demand (e.g., pace) – ● Cited but not verified (in NIOSH study) Cold – Direct pressure on nervers – Poor physical condition – ● Confounding factors Individual variability – ● Anthropometry ● Methods 34
Risk Factors Considerations ● More factors means more risk. ● Reduced factors means reduced risk. ● Presence of factor(s) does not imply CTD. ● Critical levels of exposure not known. 35
Rapid Upper Limb Assessment (RULA) Technique 36
Hand Tool Guidelines
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