What forces can a body sustain?

In the last blog post on the [slaice] rope terminations I wrote that the forces being discussed were well above anything a human body could sustain without suffering injury. This statement of course begs clarification…

Quite a bit of research has gone into this matter, dating back to studies investigating injuries paratroopers were suffering back in WW2 when their parachutes deployed, seeking to better understand the mechanisms involved and prevent people from getting hurt. Also there is the research looking at injuries suffered by drives and passengers in car accidents, this is about impacts rather than falls, but of course there are parallels when it comes to the mechanisms involved.

In case of a fall, it is not necessarily the landing that will hurt you, but rather the deceleration.

Impact with a hard, immovable object (e.g. the ground and/ or structure) is one instance of deceleration, but falling into your climbing systems can equally lead to injuries caused by rapid deceleration.  Considering that we are all ultimately naught more than sacks of skin filled with bones and squishy bits and bobs, we do not suffer deceleration well – as the momentum of the squishy stuff wants to carry on moving it at the same speed. Sudden deceleration can therefore lead to significant internal injuries, such as haemorrhaging.

When using work positioning systems, European legislation requires that the Maximum Arrest Force (MAF) shall be limited below 6 kN (≈600 kg), for the US and Canada it is 8 kN (≈800 kg). During any activity liable to generate a MAF higher than that, the person shall be protected from a fall by use of a fall arrest system.

So far, so good. But of course this is not the whole story.

What is critical is how the force is exerted upon the body. A number of organisations and researchers have done work investigating this matter.

NCAP, of the crash test dummy fame, an organisation whose aim is to improve safety in cars, for instance, have shown significant spinal deformation as from side impact forces of 350 kg on upwards.

Also, the Mägdefrau report (“Human Bodies in Falls”) and Andrew Sulowski (“How Good is the 8kN Maximum Arrest Force Limit in Industrial Fall Arrest Systems?”) looked into the matter of how the direction of fall has an influence upon the severity of injuries sustained. When comparing X-, Y-, and Z-axis falls, both studies conclude that there are significant differences.

A +Z-axis fall could happen when a person is secured to a back up device on a ladder attached to a sternal attachment point on the harness. In this fall orientation, the resulting force compresses the spine. From a constructive point of view, the spine is well able to do so, with plenty of muscles and vertebrae to dampen the force.

A +Y-axis fall could happen as a result of attaching a lanyard to the same side D-rings of a harness (not recommended practice). The spine is less well equipped to deal with this kind of force – a shearing force across the spine. Yet there are large muscle groups to either side of the spine in the abdominal region which will help to dampen the force.

A +X-axis fall will occur when ventrally attached to a work positioning systems, e.g. when using a sit or work positioning harness. Our body struggles to handle this kind of fall, as there significant leverage exerted over the lower back in a whiplash motion in a direction lacking in structural stability (muscular or skeletal).

These mechanisms are reflected by the findings of both Mägdefrau and Sulowski.

As you can see in the chart above, whilst all six falls generated similar peak forces, falls one to four would have fractured a real person’s spine (a humanoid dummy was used for the tests, of course), falls five and six caused no visible damage. The difference between the falls? Falls one to four used a sit harness (resulting in a +X-axis fall), five and six a full body harness (resulting in a +Z-axis fall).

It is worth noting that the forces recored above are well below the 6 or 8 kN limit defined by legislation.

Sulowski’s findings are similar to those described in the Magdefrau report, where only in the instance of a +X-axis fall did the fall protection effectively limit the MAF to below 6 kN.

So what is the conclusion? Use fall arrest systems, attached sternally to our work positioning systems to ensure a +X-axis fall?

The answer to that is obviously no.

As we are working in work positioning systems these should by definition not generate a MAF, as the aim is to prevent a fall from occurring in the first place. I do however think these are interesting figures to have in the back of ones mind, as they allow you to put other values in perspective, for instance when discussing MBS for personal protective equipment which are double, tripple or even quadruple the forces discussed above. Incorporating robust safety margins and allowing for wear and tear is all very well, but we must remember that the human body is fragile and likely to be the weak link in the chain.

Well, that, and the anchor points we select in trees – but that is a whole different story.