Fall arrest forces, harnesses, and the human body
Edited by by Jim Johnson (D2000safety.com); photos by Terratribes
Knowing how a system works/affect to the human body is as important as how to operate it.
Whether you are an iron worker who has slipped off an I-beam or a rescuer whose fall is being arrested by a belay line, you will feel the arresting forces. They will travel from the anchor, through the connectors, and ultimately to your body.
Understanding these forces and their effects are essential aspects of both our fall protection and rescue classes.
While much attention is generally spent on understanding the impact of arresting forces on the components of the fall protection system, we need to keep in mind that these forces will also be applied to the human body. So what can possibly go wrong? Several things. Here are three.
• Harness Fit. Anyone who has actually fallen (or hung) in a full-body harness is keenly aware of one fact: The Fit Matters. If a harness is poorly adjusted - if there is slack in the straps or the leg or chest straps aren't positioned properly - the transmission of the arrest forces will not be as clean and elegant as the harness designers intended. They designed the harness to transmit most of the forces to your pelvis which is the largest and strongest bone in your body. But if the straps slide up or down as you fall, other parts of the body may be the ones absorbing the impact. Chest straps positioned below the breasts (whether male or female), or leg straps that slide up and apply force to the wearer's crotch can cause serious injuries.
• Contents of Your Pockets. Even a well-adjusted harness will slide to some degree. The question that the wearer needs to answer is: What will happen if the straps jam the contents of my pockets upwards? That ball point pen in your chest pocket or the Leatherman in your front pants pocket can ruin your day if they end up imbedded in your muscles. And in a fall there will a lot of force available to imbed those items. Cargos pockets on the sides of work pants shouldn't be an issue, but other pockets need to be emptied. The fact that some of the newer harnesses include small pouches for these types of items is a step in the right direction.
• Body Modifications. I recently had a student in one of my fall protection classes describe a fall that they took when they were knocked off an I-beam. They said that the equipment worked as designed, but they were injured by several of their body piercings. While we had always instructed students to wear their gear correctly and empty their pockets, I realized that as body piercings become more common, we need to ensure that users of this equipment realize the risks that these items can pose. Once they understand the risk, we can usually count on the users to make an informed decision.
I often tell students that fall arrest equipment is the only personal protective equipment that will probably injure you if you ever have to use it. The same can be said of automotive seat belts. When we are potentially applying over a thousand pounds of force to our bodies, we need to ensure that the straps which will transmit these forces from the anchor to our tissues are used in a manner in which the risk of injury is as low as possible.