We are very good at knowing our physical capabilities. You know if you can lift a gallon of milk off the top shelf of your refrigerator. What we are not as good at is accurately estimating the weight of an object. For example, if that gallon of milk has been partially consumed, it is hard to tell if it weighs 5.5 or 6 pounds! Expand that to push or pull forces, do we really know how much force it takes to push a fully-loaded shopping cart at the grocery store? Worse yet, what if you get the cart with a wobbly wheel?
The same principles apply in the work environment. As an ergonomist, I have asked operators if they know the weight of a part or can push a cart full of material. Often, the response is something like “Yes, it probably only weighs 25 or 30 pounds”. Once verified, it is usually significantly more. One horror story, I was working at a plant and an operator estimated a part at 30 pounds, when it was actually 76!
The use of a force gauge, such as a push/pull gauge or dynamometer, is imperative to get accurate measurements. Doing so will provide a much more valid, accurate, and reliable ergonomics assessment.