Put down the magazine at the end of this paragraph and walk around. First walk normally, with your arms swinging. Then try it with your arms folded. Finally, do it with your arms and legs on each side in phase, swinging forwards and back at the same time – a “tick-tocking” motion. Ignore any strange looks from passers-by. After all, you are doing this in the name of physics.

If you have carried out this little experiment, then you will probably have noticed that the second and third ways of walking are more difficult. But why? The answer – as it is for many important questions in biomechanics – is more complicated than you might think. Much biophysics research focuses on how physicists are helping biologists to study the workings of cells (see Physics World‘s July 2009 special issue). In many ways this is not surprising. Sophisticated biophysical techniques are often required to probe the workings of things operating at micro- and nano-scales, and metabolic processes can even be influenced by quantum effects.

In contrast, one might assume that we already know all about how the organs and whole bodies of organisms – particularly Homo sapiens – work. Movements and forces at the macroscopic scale are, after all, relatively straightforward to measure and subject only to the laws of classical physics. Moreover, the anatomy of our bodies has been mapped for hundreds of years compared with the 10 years for which we have had comprehensive guides to the human genome. Surprisingly, however, research by biomechanics – people who combine expertise in both biology and physics – continues to uncover and fill hitherto undreamed-of areas of our ignorance about ourselves.

The key to understanding how our bodies work is to alter our viewpoint: to see the world with new eyes and thereby ask and answer new and awkward questions. Why, for example, do we swing our arms back and forth while we walk? Why do our teeth have notched blades, and why do we chew our food anyway? Why don’t our nails break into the quick? And why exactly do we have fingerprints? These are just some of the questions that have recently been raised by biomechanics. The answers to these questions may appear obvious or even trivial, but further thought and experiment is revealing that our world is far more fascinating than we could have dreamed.

Why do we swing our arms?

The basic process of walking has been understood for some time. At each step, we vault over the standing leg, our bodies moving like an inverted pendulum with the stationary foot as a pivot. The amount of energy required to move is minimized because there is a continual interconversion of kinetic and gravitational potential energy: we slow down as the body rises at the middle of each step, then speed up again as the body falls towards the end of the step. A small amount of energy is inevitably lost through sound and heat when we put each foot down on the ground, but basically we carry on moving more or less continuously, requiring only a small push-off from our feet to keep going.

So where do the arms come into the picture? Surely swinging them just uses up energy unnecessarily? Once Steven Collins and colleagues at the departments of medical and mechanical engineering at the University of Michigan in the US posed this question (most recently in 2009), they were able, fairly readily, to come up with alternative hypotheses to explain it. It could be, they reasoned, that the arm-swing helps to reduce vertical movements of a person’s centre of mass, and hence minimizes the forces on the feet. Alternatively, the arm-swing might help counter the inertial effects of the legs that would otherwise cause torques about the body’s vertical axis.

Testing these two hypotheses involved getting people to walk in three different ways, just as I suggested at the beginning of this article: with arms swinging normally; with them held or bound to stop them swinging; and with arms swinging in the same phase as the legs, or “tick-tocking”. Collins’ group filmed 10 subjects walking, while also measuring their oxygen consumption and the forces produced by their feet as they walked over specially designed “force plates” sunk into the floor. These plates use electronic strain gauges to measure instantaneous forces in all three planes, as well as the torque or twisting moment about the vertical axis.

The oxygen measurements showed that the subjects used about 10% more energy to walk without swinging their arms than when they swung them normally, while tick-tocking increased energy consumption by 26%. The next step was to explain why. Filming revealed no changes in the movements of the legs or body, but the force-plate records showed that while the forces involved did not change, the torques about the body’s inertial axis were twice as high with no arm swing and three times as high during tick-tocking than during normal swinging. It is clear, therefore, that swinging our arms reduces the torques we need to apply to counter the inertia of our legs, and so reduces both the energy needed to walk and the twisting forces on our knees.

Why do we have notched teeth?

The mechanics of eating is another area where asking the right questions has transformed our outlook. People have long talked about our “cutting” incisors, “stabbing” canines and “grinding” molar teeth. But these terms are vague and tell us nothing about the relationship between tooth shape and the sorts of food they can break up. When it comes to advancing our understanding of teeth, it is much more helpful to consider the mechanical and fracture properties of different foods, as Peter Lucas of the department of anthropology at George Washington University demonstrated in his book Dental Function Morphology (2004, Cambridge University Press). Incisors, for example, should be good at driving cracks through soft but tough foods such as meat and vegetables, but would be blunted by bones or nuts. These stiff but brittle foods are much easier to break by using blunt-cusped molars, which allow such foods to be loaded in bending and so be snapped. Biological materials that are both stiff and tough, such as wood, should be impossible to break down; indeed, most animals seldom use them as food.

These explanations seem to make sense, but why do so many cutting teeth, including our own premolars, have notched blades? Once again, alternative hypotheses can be put forward. A notched blade might help to trap the food, preventing it from being flattened as it is cut, or perhaps it could give the tooth an oblique slicing action. Both effects could reduce the energy needed to cut the food.

To test these possibilities, in 2009 palaeontologist Philip Anderson from the department of Earth sciences at Bristol University in the UK carried out experiments on two foods: salmon and asparagus. He measured the energy needed to cut through salmon (very deformable) and asparagus (much less so) using sharp blades held in four different arrangements (see “Munch, slice, trap” figure) In the first, two sharp blades were held parallel to each other. In the second, the top blade was angled at 30°, producing a slicing cut. In the third, the blades were parallel but the food could not become flattened as it was held at the sides by two vertical “traps” made of the backs of blades. In the fourth test, the top blade was notched, providing both a slicing and trapping action.

Anderson found that using the notched blades greatly reduced the energy required to cut both foods. In salmon, this was partly because the food was prevented from deforming: the side traps were just as efficient as slanting blades at reducing the energy required. For asparagus, in contrast, using an inclined blade reduced the energy just as much as using the notched blade, thus showing that the energy reduction was due to the slanting action of the cut alone. So the next time you tuck into a nice meal of salmon and asparagus, it might be worth investigating whether your own notched premolars are better at cutting up these foods than your incisors.

Why do we chew our food?

But why do we chop up our food anyway? Most textbooks say that it is to increase the surface area and so speed up digestion, and to make chunks of food small enough to swallow without getting stuck in the oesophagus. These assumptions were challenged in the mid-1990s by Jon Prinz and Lucas, who were then working at the University of Hong Kong’s anatomy department. They pointed out that when mammals swallow, food passes over the airway on its way to the stomach, so there is a potentially fatal risk of choking if stray food particles go down the wrong way. They suggested, therefore, that chewing allows us to press our food into a firm blob – technically known as a bolus – at the top of our mouth with our tongue. It can then be safely swallowed.

To test this idea, they got volunteers to eat diced carrots and nuts, counting the number of times the subjects chewed the food before swallowing it. They then modelled the cohesive strength of the bolus after different numbers of chews, by calculating the viscous forces needed to separate the ever-smaller particles. They found that for both foods, the bolus strength initially increased with the number of chews, because the smaller particles had a greater surface area, meaning that the bolus was held together by increased viscous forces. As chewing progressed, however, more saliva was pressed into the bolus, which eventually increased the distance between the particles, thus reducing viscous forces and weakening the bolus. Left to chew their food naturally, unencumbered by prying researchers, people swallowed the bolus at around the time when it was strongest – more support for Prinz and Lucas’s theory.

Why don’t our fingernails break into the quick?

My research group at Manchester University in the UK has shown that a similar approach of just asking the right questions can also revolutionize our understanding of the design of our fingertips. All of us either bite our fingernails or experience them breaking, yet the broken nails almost never tear into the quick. Instead, they break straight across, self-trimming the nail. But why? When I first posed the question 10 years ago, it seemed to be a new one; here, again, was a phenomenon that everybody has experienced but no-one had investigated further.

I set the problem to a group of second-year biology undergraduates – who rapidly came up with the answer. It turns out that the main, central part of the nail has all its keratin fibres arranged parallel to the “half-moon” at the nail base (see “Hangnail prevention” figure), so cracks travelling towards the base of the nail are instead deflected around the nail edge. We subsequently carried out tests to investigate how much energy it takes to cut nail clippings in different directions. These tests, which used scissors and nail clippers mounted in a universal mechanical-testing machine, showed that it takes twice as much energy to cut nails inwards towards the base of the nail as opposed to sideways around their edge.

However, if a nail were only made up of fibres oriented parallel to the half-moon, then it would keep on breaking all the time. To prevent this, the nails also have thin upper and lower layers in which the fibres are oriented in all directions. These layers gave the nail bending strength, and since they wrap around the edge of the nail, they also help prevent cracks from forming in the first place. There is only one design flaw in this clever sandwich construction: since the outer edge has no middle layer, cracks there can run in any direction. This is why nail fractures can run inwards at the very edge, causing the side of our nail to bleed – often painfully.

Why do we have fingerprints?

Now to the other side of our fingers. We all know that everyone’s fingerprints are different and consequently that they are useful in crime detection. It has also long been assumed – at least by the writers of medical textbooks – that fingerprints help us grip to objects by increasing the friction coefficient of our fingers.

Unfortunately, tribology – the science of interacting surfaces in relative motion – has shown that having a rough surface does not increase the friction of soft materials such as rubber and skin. That is because friction in rubber-like materials is not caused – as it is in stiff materials – by the jamming of rough, pointy structures. Instead, these softer materials deform easily, flowing into irregularities; friction between rubber and other materials is therefore due to short-distance molecular attraction or Van der Waals’ forces. What this means is that friction increases with the contact area, not with the normal force as it does in stiff materials.

To test whether our fingers behave like rubber, my group measured the friction of fingertips against an acrylic glass sheet while changing the normal force and contact area independently. To do this, the finger was held at different angles and the friction against sheets of different widths was measured. We found that friction increased with contact area, showing that our fingers do behave like rubber. Since fingerprints actually reduce the contact area, they must reduce friction too.

So why, then, do we have fingerprints? We are currently testing several alternative hypotheses. It could be that prints do increase friction against rough surfaces – just not against smooth ones like glass. More intriguingly, they might act like the treads on tyres, which remove water and so increase friction under wet conditions. Prints might also make the skin more flexible and so help prevent it blistering.

Initial tests have indicated that friction between fingers and surfaces actually falls as surface roughness increases. This casts doubt on the first hypothesis, although it is possible that smooth fingers might be even worse. Other work by Thibault Andre of the physical-medicine unit at the Catholic University of Louvain in Belgium has shown that grip is maximized at intermediate skin-moisture levels. This suggests that water removal might indeed play a role. However, the fact that we tend to get blisters mostly in areas of our hands that lack prints suggests that the antiblister effect is also important. Only time and more experiments will tell.

More awkward questions

As these examples have shown, it is rapidly becoming clear to those of us who work on the boundary between physics and biology that we have a lot more to find out about ourselves than one might think. But there is even more to be learned from other organisms: we are still trying to glean more information from how geckos walk on walls or how snakes slide over the ground, for instance. Both of these animals are being extensively studied, largely because we could use the knowledge to, for example, create glueless “gecko tape” or more freely sliding artificial joints. Ever since the advent of Velcro, which copied the gripping action of hooked plant seeds, the ever-expanding field of biomimetics has been seeking new inspiration from the natural world.

But, of course, applications are not the only reason to do such research: the pure joy of finding out is also a considerable prize. And unlike many areas of physics, this sort of research need not be expensive or mathematically difficult. Indeed, much of it can be done by any open-minded physicist. All you need is an enquiring mind, a bit of ingenuity and the courage to ask awkward questions.