Rushing through the underground or getting jostled in the street, minor bumps hardly knock us off course. But what strategies do we use to recover without stumbling? At Hof from the University of Groningen, The Netherlands, explains that our reflexes help us to regain our balance after a bump from behind, but it wasn't clear how we recover after a jostle from the side. Hof had built a mathematical model that suggested that we regain our balance within one step, but would this theory stand the test? Could walkers recover from a minor sideways collision without stumbling and in one graceful step as Hof's theory predicted ()?
Hof teamed up with students Marije Vermerris and Welmoed Gjaltema to test some walkers' reactions. Recruiting 10 fit undergraduate students to walk on a treadmill, the team tied a belt attached to a piston around each student's waist. After every 10 strides, the piston gently tugged or pushed the walker while the team measured the forces acting on the walker's feet. Subtly adjusting the piston's timing so that it jostled the student at different points during the stride cycle Hof also increased the strength of the push from 2.7 to 12.4 kg m s–1. Collecting force measurements for 5000 steps over a 2 h period as each student was jostled 400 times, Hof also filmed the first minutes of each walking experiment and was impressed by how well the students reacted to the disturbance. Their arms and trunks barely moved; ‘I measured the trunk movements with a gyroscope and they were so small that they were not interesting,’ says Hof.
Analysing the ground reaction forces as the students recovered from each bump, Hof saw the walkers use two strategies. In the first strategy they simply recovered by stepping wide of the position where they would normally have placed their feet. For example, if a walker that was standing on the right foot was jostled from the right, then the left foot was free to be placed wide to brace the walker. However, when the walker was standing on his left foot and pushed towards the left, the walker had to cross his right leg over his left before continuing on his way. What is more, the students were completely unaware of the dramatic step they took. ‘The subjects saw their own videos after doing the experiments and they said, “Did I do that strange stepping crossing over?” They didn't realise how complicated the things were that they were doing,’ says Hof.
However, the walkers only used the ‘step’ strategy when jostled early in a stride and had to resort to the second strategy, rolling the ankle, when bumped toward the end of each step. ‘The stepping strategy needs at least 300 ms (30%) before foot placement to be successful,’ explains Hof, but walkers can roll their ankles at any stage of a stride to recover. He also noticed that on the occasions when a walker did not recover sufficiently in a single stride, they rolled the ankle slightly to complete the correction.
But what does all this mean for Hof's theory? Well, the walkers behaved exactly as the theory predicted. By stepping and rolling their ankles the walkers were always able to track their moving centre of mass (which Hof refers to as the extrapolated centre of mass) with the centre of pressure exerted by their feet on the ground by shifting their weight from one foot to the other. Next Hof is keen to find out which muscles the walkers use when recovering and whether the reaction is a true reflex.
