Sketch courtesy from Riekes Material Handling Severity of

Sketch courtesy from Riekes Material Handling

Severity of the problem • Manual handling (lifting) is injury prone & expensive – BLS 2007: 140, 330 out of 1, 158, 870 or 12% of all non fatal injuries and illness cases in US private industries with days away from work occurred from exposure from overexertion in lifting. – Median days away from work per incident was 8 days – 26. 8% cases caused 31 or more days off. – Details: http: //stats. bls. gov/news. release/osh 2. nr 0. htm

Nature of Injury • Pain in shoulder, upper back, lower back and knee. • It is believed that cumulative trauma of soft tissue over time is the cause of injury and not from an acute trauma due to overload. • Lower back pain is a major problem associated with MH. Courtesy, US Department of Energy, Berkeley Lab

Approaches to investigation of cause of MMH injuries • Biomechanical approach • Physiological (or cardiovascular) approach • Psychological approach

Biomechanical approach • Computes torque/internal-forces due to body posture and load handled on critical body joints and compares those to joint strength. • Generally applicable for one time loading situation or worst case scenario of a task. • Can predict localized muscle fatigue. • Shortcoming: Does not take into account effect of duration and frequency of MMH task.

Physiological (or cardiovascular) approach • Considers metabolic energy requirement of the MMH task and systemic fatigue. – Goal is to keep metabolic rate less than 5 Kcal/min for an eight hrs task • Takes into account rate and duration of MH and dynamic effect of body movement. • However, injury may occur due to localized muscle or joint overload, which this method cannot isolate.

Psychological approach • Based on the assumption that human can inherently perceive the stress level and can determine his or her limits of MH. • Skilled handlers perform MH in laboratory settings with varying load and type of activity. Frequency and other MH factors (distance, height , size of the box etc. ) are kept constant for a given task. Based on the maximum load acceptable by the handler population, allowable load limits are determined in percentile form. • Supposed to take care of both biomechanical and physiological factors.

MH variables • Individual – Selection – strength testing • Technique – training – posture • Task – Most effective way to limit occupational injury is to design the MH task such that everybody can perform it with least risk of injury

MH task types • • Pulling/pushing holding carrying, and lifting

Pulling & pushing • Limits of pull and push forces for many combinations of handle height and frequencies are available for industrial population. Table 13. 1 and 13. 2. • Force capability goes down as it is exerted more often. • Pushing capability is higher than pulling. Pushing also produces less spine compressive force. • Push at waist level; pull at thigh level

Pulling/pushing task design • Use a force gage to measure the force • Reduction of friction coefficient may reduce the force. • Remove obstacles, larger wheels. • A vertical push-pull bar may allow height adjustment for both short and tall person. • Avoid muscle power for long distance, ramps and high frequency moves.

Holding • Holding causes static muscle contraction and fatiguing. • Often higher reach requirement causes undue muscle tension at the lumbar spine region. • Reduce the holding torque and reduce the duration of holding.

Carrying • Carrying induces internal static muscle tension in hand, arm, shoulder and trunk muscles. • Reduce the load and or reduce the moment arm. Body hugging back-pack design reduces the moment arm. Keep the load as close as possible to the spine. • Box with a handle may induce more lower back stress compared to a box without a handle.

Lifting • NIOSH lifting equation (1994) provides a formula to determine the Recommended Weight Limit (RWL) for a specific lifting task. • It starts with a load constant of 51 lbs (23 kg), which is the maximum load for an ideal lifting task situation. • This load constant is then multiplied by various factors (all are equal or less than 1) to obtain the RWL. – RWL= 51 x HM x VM x DM x FM x AM x CM lbs • If control of the load is necessary at the lift initiation and lift destination, then two RWLs are determined, one for the lift initiation and lift destination points. • Lifting Index (LI) = Actual Load weight during lifting / RWL, – if LI is >1, the task is not acceptable and design modification is needed to make the LI = 1 or less – LI < 1 should be acceptable to 75 percent females and 99 percent males.

Criteria used to develop niosh lifting equation • NIOSH lifting equation (1994) is based on – Biomechanical criterion of max spine compressive force 3400 N – Physiological (metabolic) criterion of 9. 5 kcal/minute (which is VO 2 max for 50 th percentile female of age 40) multiplied 70% (due to arm work), 50% for one hour, 40% for two hours, and 33% for eight hours. – Psychophysical criterion is based on a 34 cm wide box for a vertical displacement of 76 cm and lifting frequency of 4 lift/min.

Scope of NIOSH lifting equation • Applicable for two handed lifting task in free standing posture. Not applicable MH at seated or kneeling posture. Load must not be unstable. • Handling should not include too much carrying, not more than one or two steps. • performed in normal room ambient condition. • Other physical tasks are 10% or less. • For other conditions, specific biomechanical and physiological investigation will be needed to set the limit.

Factors in NIOSH Equation (US system of measures inch, lb) • Horizontal multiplier HM =10/H , where H is the projected distance from the handle to body centerline. – Closest to body (H<=10 inch) is optimum HM = 1, If H is more than 25 inch, HM = 0. • Vertical Multiplier VM = 1 -. 0075|V-30|, where V = objects vertical location (knuckle location) from floor. – Knuckle Height (V=30 inch) is optimum, any height other than this is penalized. – Penalty for both up and down from knuckle height.

Factors in NIOSH Equation (continued) • Distance multiplier (DM) =. 82 + 1. 8/D, where D is the vertical load movement distance. If D is less than 10 inches DM = 1, If D = 70 inches DM = 0. • Asymmetry multiplier AM = 1 - 0. 0032 A, where A is the angle in degrees from mid-sagittal plane. – For lifting in mid-sagittal plane AM = 1. – Ignore positive or negative angle. – Max value of A = 135 o.

Factors in NIOSH Equation (continued) • Frequency multiplier (FM) is based on lift frequency (lift/min) – – – • • • (1) over short (< 1 hr), moderate (< 2 hr), long (< 8 hr) duration. (2) have adequate recovery times after the lifting task (3) whether below or above knuckle height Refer to table 13. 9 to determine FM. Max frequency is 15 lifts/min and beyond this FM = 0. If the job is short term (<1 hr) and F < 0. 2 lift/min, FM = 1.

Factors in NIOSH Equation (continued) • • Coupling multiplier (CM) Depends on handle design, and vertical location (V) of load See table 13. 10 and 13. 11.