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A volunteer cranking the flywheel to simulate throwing a punch. Photo credit: Jeremy Morris.

The differences between males and females can be extreme. Tiny triplewart seadevil males simply fuse themselves for life to their relatively large female partners while female rusty tussock moths are almost immobile egg carriers for their more elaborate males. Although the differences between male and female humans are far less dramatic, men can have up to 90% more upper body strength than women. ‘[The] difference between males and females often reveals how evolution has shaped the bodies of males and females in different ways’, says Jeremy Morris from Wofford College, USA, who was curious to find out why men are so much more powerful than women. The possibility that male hand-to-hand combat was a driving force in human evolution has long intrigued Morris's thesis advisor, David Carrier at the University of Utah, USA, so Morris decided to check out whether coming to blows could have driven men to build up.

But Morris, Carrier, Jenna Link and James Martin, also from the University of Utah, had to come up with an alternative way of testing how much power people pack in punches. ‘People that are not trained in martial arts are hesitant to punch a punching bag with much force because there is a risk of injury’, says Morris. However, after putting their heads together the team came up with the idea of sitting volunteers down and asking them to power their arms forward by simply cranking a flywheel. ‘The advantage of using arm cranking … is that we can get maximum performance from people that have little or no experience punching’, says Morris.

Recruiting almost 40 reasonably fit adult volunteers from friends and colleagues around the campus, Morris and Link asked them to warm up gently before driving the crank wheel as hard as possible over the top third of a full rotation to simulate a punching action. ‘We encouraged the participants, yelling “Go, go, go! Come on!” the same way that a physical trainer would at a gym to get the maximum performance’, recalls Morris. Calculating the amount of power each participant threw into spinning the heavy flywheel, the chaps came out streets ahead of the women, packing almost three times more power on average into their wheel spins (282 W) than the females (108 W). However, when the team tested how much effort the adults could put into pulling the wheel in reverse, the differences were less pronounced; the men only just doubled the amount of power exerted by the women. In addition, when the team compared how much more force the male volunteers were able to invest in overarm throwing – an alternative theory for the differences between the physical builds of men and women – the men only invested twice as much power as the women.

‘The results of this study support the hypothesis that evolution has selected for physical traits in males that make them better at inflicting damage by punching with the fist’, says Morris. And he is keen to find out how hard male and female trained athletes can throw a punch, to answer an important question: is punching the physical activity with the greatest disparity between men and women?