Liberty Mutual’s Manual Material Handling Tables is based on psychophysics. Psychophysics is the science that studies the relationship between the perception of the magnitude of a sensation and the magnitude of the stimulus. An example of this is how salty something tastes in relation to how much salt there is in it. The same idea has been applied by Liberty Mutual (and other research groups) to determine what weights and forces people find acceptable when performing lifting, lowering, pushing, pulling, and carrying tasks. Liberty Mutual is the most referenced source of psychophysically determined manual handling limits.
The metric that is derived from this research is the population percentile – the percent of the population that can be expected to do a specific task “without straining themselves, or without becoming unusually tired, weakened, overheated, or out of breath” (Snook and Ciriello, 1991). The LOWER this number is, the HIGHER the risk of injury. The goal is to design jobs that are acceptable to at least 90% of the female population. Tasks whose population percentile is less than 75% of the female population expose workers to much higher risk of injury (Snook, et al., 1978, Marras, et al., 1999). The 75th female population percentile is the criterion that the NIOSH Revised Lifting Equation is based on (Waters, et al., 1994).
The use of the female 75th percentile as a criterion does NOT mean that only females are more susceptible to back pain; on the contrary, BOTH males and females are at elevated risk for tasks where the population percentile is below 75%. While males, on average, have higher weight/force capability than females, it is the female preferred weights/forces that provide discernment to injury risk for both males and females. Here’s an analogy. Say you went to a picnic and the host served everyone potato salad. Let’s say all the men and most (74%) of the women thought it tasted great, but 26% of the women said, “something doesn’t taste right about that potato salad”. When everyone got home they ALL got sick. In this analogy, it was a small fraction of the population that was perceptive to the risk of getting sick, even though everyone was susceptible to getting sick. The 75th female population percentage is the level that is sensitive to identifying high risk tasks.
For certain jobs and industries, it is very difficult to design jobs that can be performed by 75% (or more) of the female work population. These Tables are very often used to perform what-if scenarios of various ergonomic interventions to help determine the most cost effective and practical solution that offers the highest degree of control. There is no right answer or wrong solution. Whatever solution offers the most practical, cost effective and highest degree of control possible is a good result.