Science & Environment

Dead arms test importance of clenched fists

X-ray showing a fractured hand bone Image copyright Science Photo Library
Image caption A broken metacarpal, in the palm of the hand, is sometimes called a "boxer's fracture"

In macabre experiments which saw dead, severed arms swing punches on a large pendulum, US scientists have measured the extent to which a clenched fist shields fragile bones in the hand.

Consequently, a fist can strike with twice the force of an open-handed slap.

Prof David Carrier from the University of Utah said this supports his argument that fighting, as well as dexterity, drove the evolution of the human hand.

Other researchers are sceptical about Prof Carrier's "pugilism hypothesis".

Writing in the Journal of Experimental Biology, he suggests that our hands, with short fingers and meaty thumbs compared to other primates, evolved to satisfy two needs: "These are the proportions that improved manual dexterity while at the same time making it possible for the hand to be used as a club during fighting."

This is an argument Prof Carrier has previously put forward in a study of the force delivered by athletes hitting punching bags.

That work attracted criticism, partly because the data primarily showed that a clenched fist is a very effective weapon; it remained unclear whether the "buttressing" afforded by the thumb - the key advantage of a human fist - could actually save inner hand bones from breaking.

After all, the "metacarpals" of the palm are frequent casualties in human fist fights: the so-called "boxer's fracture".

"The real question was whether or not the buttressed fist provides protection," Prof Carrier told BBC News.

The latest experiments, while a little gruesome, allowed his team to measure the strain on specific bones.

"It is a little macabre and strange, but there was no other way to really get this data. You can't implant strain gauges on living subjects."

Image copyright David Carrier/University of Utah
Image caption A clenched fist lessens the strain on bones in the palm, compared to a loose fist or a slap

They took arms from cadavers, attached them to wooden boards and tied fishing wire to the tendons of the forearm. This allowed them to control the hand - clenching the fist, for example - by tightening those wires on guitar pegs.

The whole board was then mounted onto a large pendulum, which could swing the hand into a padded dumbbell.

Instruments attached to bones inside the hand, and to the dumbbell, measured the forces involved. And sure enough, strain on the metacarpals was greatly reduced when a clenched fist, rather than a loose fist or an open palm, slammed into the pad.

'Side-effect'

"This is relatively strong evidence that there is a performance advantage," Prof Carrier said.

"Whether or not natural selection ever acted on that advantage is something we can't answer directly. But at the same time, given this evidence, you can't argue that selection on aggressive fighting behaviour was not relevant."

So alongside the more widely accepted idea of dexterity as an evolutionary driver, this fist-fighting advantage "has to be included on the list of possible factors that could have influenced the evolution of the hand".

Image copyright Andre Mossman/University of Utah
Image caption The team contracted muscles in the arms using guitar pegs, and threw punches using a large pendulum

Dr Tracy Kivell, a palaeoanthropologist at the University of Kent who specialises in the wrist and hand, said it was a credit to Prof Carrier's team that they had found a way to test part of the hypothesis. But she disagrees with his assertion that fist fighting "must be relevant".

"Overall, it's not a hypothesis that I would support," she said.

"I think the way they've done it is creative and seems to be biomechanically sound. And the fact that the metacarpals experience less load when they're in a fist - I don't disagree with those results. But that's probably a side-effect of having hand proportions that have evolved for other reasons."

Dr Kivell said that "a much more realistic idea" was the established one, of our hands evolving to manipulate tools. "You can address that hypothesis from multiple directions and it makes sense," she said - noting that fossil evidence of tools goes back more than three millions years, and aligns with changes to hominid anatomy."

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