Published online 23 July 1998 | Nature | doi:10.1038/news980723-2

News

One small skip for mankind?

Despite the grand words about small steps and giant leaps, it turns out that astronauts from the Apollo missions favoured a form of skipping over all the other means of getting about on the moon. Children regularly skip, but adults don’t often choose this mode of travel. What is special about skipping? Why is it suited to low gravity and young children? Alberto Minetti of the Consiglio Nazionale delle Ricerce in Segrate, Italy, writing in the Proceedings of the Royal Society, has now analysed the mechanics of skipping, and describes it as “more than a behavioural peculiarity”.

Minetti persuaded five men to skip at various speeds on a treadmill and measured the body position, where the centre of mass fell, the radius of gyration, and how much mechanical work was done to accelerate and lift the body.

He compared skipping with other forms of locomotion, and finds that it seems to be the bipedal equivalent of galloping. Because there is a ‘flight’ phase, skipping ought to compare with running, trotting and galloping motions, rather than walking. The mechanical work done and stride frequencies are indeed comparable to these three gaits. Surprisingly the ‘apparent efficiency’, defined as the ratio of the amount of work done for the amount of metabolic energy expended, is very similar in walking and skipping, but there the similarity ends. Skipping clearly has a much higher energy expenditure.

But unlike other forms of locomotion, the stride frequency in skipping was constant whatever the speed. This puts it on a par only with galloping, as does the pendulum-like phase during flight, which allows energy saving.

Minetti concludes that skipping in humans and galloping or cantering in quadrupeds are related. The type of skipping favoured on the moon was a little unusual in that the same foot was kept forward throughout. Two people skipping in this way (‘unilaterally’) can mimic cantering by skipping about one-quarter of a period out of phase. Galloping would be nearer to half a period out of phase in the usual ‘bilateral’ skipping mode with a second flight period.

In man, running is a much more energy efficient form of locomotion than skipping for the same speeds, while galloping is most energy efficient in quadrupeds. But the difference in efficiency is not only explained by the extra limbs, but also in the elastic properties of the spine, which helps to store and release elastic energy.

Minetti also explains why this gait might be favoured by children. It seems to be a way to progress from walking fast without breaking into a run. Skipping seems to be an evolution of walking at unnaturally high speeds.

The analyses also show that the preference for a skipping gait on the moon was not surprising. Walking is impaired because of a mismatch between potential energy and horizontal energy, and when running or walking, the vertical component of the force may be too low to maintain adherence to the ground. On Earth, skipping is more energetic than running because of the greater length of time spent off the ground, and the vertical force produced has to counteract the body weight. On the moon the drawback associated with the high metabolic cost of skipping is overridden by the need for less work against gravity.

So children, horses and astronauts seem to share a gait with very different mechanical properties to either walking, running or trotting. Maybe astronauts will be happier to make a small canter for man and a giant gallop for mankind?