Body Strength and Load Handling

1. Body Strength and Load Handling IENG 321 SDSMT Spring 2010

2. Manual handling is a hot topic because… 27% of all industrial injuries 670,000 injuries/yr in the United States 60% of all money spent on industrial injuries 93,000,000 lost workdays/yr

3. Approaches to Recommendations Biomechanical Forces and torques Esp. L4/L5 and L5/S1 Permitted load does not vary with frequency Physiological Energy requirements of tasks and effect to cardiovascular system Greater for occasional or frequent lifting?

4. Strength Factors Individual Related to gender, age, training, fitness: Muscle cross-sectional area Length of muscle Static or dynamic exertion Fatigue Neurophysiological excitation Situational The amount of strength one will or can apply Motivation, ego, will to succeed, fear of injury Skill/experience Body motion Ability to brace Posture Body segments used Coupling

5. Strength Exertion Static (Isometric) Controllable, easy to measure Rapid onset of fatigue Reasonably estimate slow motion Dynamic More difficult to measure More common Isoinertial testing Maximal Structural integrity of object (safety factor) > 95th percentils Minimal Ability to perform task 5th percentile

6. Strength Exertion

7. Hand Strength Grasping, holding, pressing, pushing, pulling,… Extrinsic Muscles Muscles in forearms, tendons cross carpal ligament Mostly gross motor control

8. Hand Strength

9. Hand Strength

10. Grips and Grasps

11. Hand Tool Design Provide friction Gloves may help, but generally increase required grip strength Mechanical interlocking Keep wrist straight Natural grasp centerline 60o-70o May need to supply left hand specific tooling ~ 10% population left handed

12. Whole Body Strength Pushing Pulling Lifting Lowering

13. Foot Strength Standing Little force Infrequent operation Support body weight on one foot Alternating feet can help maintain “muscle pump” Sitting Exert large force with sufficient support

14. Pushing/Pulling Factors Handles Poor hand/handle surface is the weakest link One hand vs. two hands Postural freedom vs. fatigue Body posture Standing vs. sitting vs. kneeling Application height Where to push or pull from? Direction Generally perpendicular to shoulders

15. Push/Pull Summary Two hands are usually better than one. Force capability goes down as it is exerted more often. Initial force capability is higher than sustained capability. Pushing capability is higher than pulling. Push at waist level; pull at thigh level.

16. Task Modifications Measure the force required to move all wheeled equipment; periodically check the forces. Force gauges Install vertical push/pull bars on carts. Push rather than pull loads. Spinal compressive loads Other safety concerns? Avoid muscle-powered pushing and pulling for ramps, long distances, and frequent moves. Use mechanical aids and momentum. Reduce horizontal transfer force by reducing friction (rolling contact).

17. Holding Problems Holding gives a static load combining body weight and object weight. Low-back pain arises from spine biomechanics. Solutions Reduce the magnitude and duration of the torque. Use balancers. Limit high loads to short durations.

18. Carrying Replace carrying with pushing or pulling. Minimize the moment arm of the load relative to the spine. Consider carrying large loads occasionally rather than light loads often. Use teamwork (common in medical facilities). Consider using balancers, manipulators, conveyors, or robots. Reduce lifting by raising the initial location. Avoid carrying objects up and down stairs. See Liberty Mutual Tables

19. NIOSH Lifting Guidelines 51 lbs is the maximum that can be lifted or lowered (load constant). Recommended weight limit (RWL) is load constant multiplied by various factors. Weight 90% of U.S. industrial workers can safely lift Lifting index = load weight/RWL Applicable when conditions are met!! Section 1.2, pg. 9

20. Basic NIOSH Lifting Formula RWL = LC × HM × VM × DM × FM × AM × CM LC = Load constant (51 lbs) 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 (10 in.) H = Horizontal location Maximum = 25 in. (functional reach) Measured = midpoint of hand to midpoint of line connecting ankles Estimated – Consider vertical location from floor and width of container < 10 in. = 10 + W/2 > 10 in. = 8 + W/2

22. Multiplier Formulas Vertical multiplier Significant control at origin only? Determines if destination of hands required. VM = 1 – VC × | V – KH | VC = Vertical constant (.0075 in.) V = Initial vertical location of knuckles Maximum = 70 in. KH = Knuckle height of typical lifter (30 in.) <30 in. = whole body lift >30 in. = upper body lift

23. Multiplier Formulas Distance multiplier DM = .82 + DC ÷ D DC = Distance constant (1.8 in.) D = Absolute value of vertical travel distance Asymmetry multiplier AM = 1 – .0032 × A A = Angle of symmetry

24. Multiplier Formulas Frequency multiplier See Table 13.9. 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

26. Multiplier Formulas Coupling multiplier Depends on: Height of initial/final hand location and container coupling Whether coupling is good, fair, or poor (see Table 13.11 or NIOSH guide for classification)

28. Example Load Weight =15 lb Ho = 12 in Vo = 33 in Hf = 12 in Vf = 22 in Ao = 5o Af = 6o Frequency = .5 lifts/min Duration = 3 h Recovery Time = 6 hours

29. Back Problems Wear and tear worse than muscle strain Low Back Pain (LBP) Reduces mobility and vitality Long absences from work Current main cause of early disability Major “At Risk” Jobs Nurses Laborers Farmers Baggage handlers Warehouse workers

30. Intervertebral Disc Strain

31. Basic Biomechanics

32. Intra-abdominal Pressure (IAP) Increase in pressure of abdominal cavity with contraction of abdominal muscles Helps stabilize trunk Reduces spinal loading Back Belts Thought to increase IAP and minimize risk of injury

33. Safe Manual Handling Material handling aids Proper techniques We will visit this again in Workstation Design

34. Guidelines for Manual Handling Bend the knees. Don’t slip or jerk. Don’t twist during the move. Use machines. Move small weights often. Get a good grip. Put a compact load in a convenient container. Keep the load close to the body. Work at knuckle height.