When we lift a load, we use muscles in the low back as part of a lever system. Unfortunately, because the muscles in our low back are only about two inches from the spine, the further a load is from our body, the more stress transfers to the low back and the amount of work required increases. Think about snow on the branch of a tree; the further the snow is from the trunk, the more the branch bends under its weight.
The National Institute for Occupational Safety and Health (NIOSH) developed a model that determines whether a lifting task is safe. The result of the NIOSH Lifting Equation is the Recommended Weight Limit (RWL), a weight that 99% of males and 75% of females should be able to lift safely. Under perfect circumstances — lifting an object with good handles only once in a certain posture — the highest possible RWL is 51 pounds. In calculating the RWL, other factors are also considered including horizontal distance, vertical location at the beginning of the lift (floor level is the least desirable), and whether there is twisting during the lift.
The NIOSH Lifting Equation makes it is easy to identify opportunities for design improvements. By working with the numbers, you can quantify the risk reduction that results from raising a load from the floor (0") to table height (30"), or decreasing the width of a table to minimize reach distance. The equation is equally useful for identifying ergonomic solutions, as well as ergonomic challenges.
Use the right tool for the job!
When I was about 10 years old I was sitting at the kitchen table trying (without much success) to strip the insulation from a 26 gauge wire with a big old and well worn set of linesman’s Kleins (heavy duty wire cutters for 8 & 10 gauge solid conductor steel wire). My father came by and saw my frustration, brought me a set of small gauge wire strippers, and lectured me: ”Don’t use a Blacksmith’s tools to do a Jeweler’s job!” We have forklifts, pallet jacks, carts, winches, pulleys, block-and-tackle, j-bars, and most importantly — other people, to help us. We are not Iron Man, Superman, or the Incredible Hulk, so don’t be shy, get help! It’s better to be called a ‘wimp’ than to be called at the hospital.
Caveat to the Equations: The NIOSH Lifting Equation does not apply if any of the following occur:
- Lifting/lowering with one hand
- Lifting/lowering for over 8 hours
- Lifting/lowering while seated or kneeling
- Lifting/lowering in a restricted workspace
- Lifting/lowering unstable objects
- Lifting/lowering while carrying, pushing or pulling
- Lifting/lowering with wheelbarrows or shovels
- Lifting/lowering with high-speed motion (faster than 30 inches/second)
- Lifting/lowering with unreasonable foot floor coupling (<0.4 coefficient of friction between the sole and the floor)
- Lifting/lowering in an unfavorable environment (i.e., temperature significantly outside of 66-79 degree F range; relative humidity outside 35-50% range
NIOSH Lifting Equation (on-line calculator): http://www.emcins.com/losscontrol/quick_links/employee_safety_health/ergonomicsNIOSH.aspx
UCLA Ergonomics Lab: http://www.ergonomics.ucla.edu/
Canadian OSH (with online metric calculator): http://www.ccohs.ca/oshanswers/ergonomics/niosh/