When scientists think of a human walking, they don't see fleshy limbs striding along; they imagine an upside-down pendulum, pivoting where the foot hits the ground, swinging in an inverted arc as our front leg swings our body forward. For all its simplicity, this system does a pretty good job of predicting how we recycle energy while we saunter. But everyone knows that walking requires effort. Our bodies are full of muscles that drive our motions and act as natural brakes, in addition to other soft tissues that absorb energy, which we must replenish. Yet it wasn't clear how these shock absorbing structures – such as our soft organs and the pad of fat that cushions the heel – absorb energy over the course of a stride, or how much energy our bodies must provide to replenish that which is lost. So, Tim van der Zee and Arthur Kuo, both from the University of Calgary, Canada, investigated how nine fit and healthy volunteers performed a range of walks – from a regular stroll to a range of bizarre lopes – on a treadmill as the forces they were exerting were measured, to find out how the shock absorbers in our bodies contribute to walking.
Initially, van der Zee analysed 3D movies of the volunteers’ movements, filmed by colleagues at the University of Michigan, USA. Then, he calculated how much energy different portions of the body were consuming as the muscles drove the movement, and also how much energy they absorbed when the heel hit the ground, across the range of different walks. ‘We applied a relatively new analysis to the 26 recorded walks of each volunteer, calculating contributions from both rigid and squishy structures’, says van der Zee, who found that the amount of energy absorbed by the volunteers’ bodies increased as they walked faster. Most importantly, van der Zee and Kuo realised that the walkers’ soft tissues – including our internal organs and the heel fat pad – were acting as the main shock absorbers as the volunteers’ feet hit the treadmill. ‘Soft tissues account for most (∼63%) of the energy absorption after the heel hits the treadmill’, says van der Zee, adding that the rest of the energy is probably absorbed by springy tendons – which recycle some of their stored energy into the next stride – and muscles. These shock absorbers also accounted for the majority of the energy absorbed by the body over the course of the entire stride, not just the instant of impact.
In addition, the duo fine-tuned the ‘inverted pendulum’ way of thinking about how people walk and incorporated how these shock absorbers affect our movement, discovering that they play a crucial role in keeping us moving forward economically, as well as protecting our joints from injury. ‘Energy absorption by squishy structures like the heel pad is an important feature of human walking, observable for a large variety of walks’, says van der Zee, adding that these shock absorbing structures seem to hold the key to a pleasant and comfortable stroll.
