Sketch courtesy from Riekes Material Handling

Sketch courtesy from Riekes Material Handling

Occupational MH tasks Occupational material handling (MH) can be classified into three broad types: – Pulling/Pushing – eg. Pushing (or pulling) medical carts, gurneys, food carts, boxes, heavy machinery etc. – Holding/Carrying – Holding tools and implements, carrying boxes, ladders, back packs, bags, parts etc. – Lifting/Lowering – Loading/unloading pallets, machines, order picking in warehouses etc. Occupational MH tasks mostly involve heavy physical exertion.

Nature of Injury from MH Apart from slip, trip and fall related to MH, a vast majority of injury occurs in the form of pain in shoulder, upper back, lower back and knee. Lower back pain is the most common problem associated with MH tasks, which is associated with high lower spinal loading. It is believed that cumulative trauma from MH gradually degenerates specific body tissues over time. A degenerated tissue subsequently manifests in the form of chronic pain. The pain from MH are generally not from an acute trauma from a single overload. Courtesy, US Department of Energy, Berkeley Lab

Three types of Investigation approaches to design MH tasks Biomechanical approach Physiological (or cardiovascular) approach Psychophysical approach

Biomechanical approach It computes torque and internal forces in critical body joints (specially lower back) that arise from the body posture in performing the MH task. The joint torque and forces are then compared to the joint strength to assess the permissible MH load (weight to be handled). Limitation: It cannot take into account the effect of duration and frequency of MH task. According to this approach the permissible load is the same if the MH task involves one repetition or hundreds of repetitions a day. It can predict localized muscle fatigue.

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

Psychophysical approach This approach is based on the theory of human sensation (pshycho), that is, we can perceive our internal stress levels (physical). In this approach subjects (often skilled handlers) perform an MH task in under controlled condition. Frequency and other MH factors (e.g. distance, height , size of the box etc.) are kept constant, while the MH load is varied during their task performance. Based on the load acceptable by the subjects, an allowable load limit is determined. A virtue of this approach is that it combines both biomechanical and physiological stresses from MH tasks.

MH Variables Individual – stronger the worker, lesser the risk of injury – Selection based on strength testing Technique – Training related to posture, hand orientation, foot posture etc. It may reduce injury risk, however human forgets. – The industries that adopted MH training produced no significant reduction of MH injury rate. 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 - ergonomic design approach.

Pulling & pushing Various limits of forces for pushing or pulling (Table 13.1, 13.2, 13.3 and 13.5) for many combinations of handle height and frequencies are available for industrial population. Main features: – Force capacity goes down when pulling/pushing frequency goes up. – Pushing capability is higher than pulling. – Pushing 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, choose larger wheel size to reduce push force. 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.

Carrying/Holding Carrying induces internal static muscle tension in hand, arm, shoulder and trunk muscles. Attach wheels or place it on a hand truck. Pulling or pushing is better than carrying or holding. If a load must be carried (rucksack, backpack, tools, etc) 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. Make the package so heavy, that it cannot be carried without a mechanical aid. Box with a handle may induce more lower back stress compared to a box without a handle.

Lifting Back injury from lifting increases significantly (both frequency and severity) when lifted objects are: – Heavy – Bulky – Lifted from the floor – Lifted frequently, and – Lifted asymmetrically (with torso twisted)

NIOSH Modified Lifting Equation (1994) It provides a formula to determine the Recommended Weight Limit (RWL) for a specific lifting task. RWL 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 These factors are determined, one for the lift initiation and one for the lift destination points, and two RWL’s are determined. 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.

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.

Guidelines for MH Use machine Minimize spinal torque – – – – – – Keep load close to the body Bend the knees Get a good grip, Don’t slip and jerk Work at knuckle height, don’t put load on the floor Don’t twist during the move Move small weights often Don’t lift above the shoulder. Select strong people based on tests