Engineers Create Ankle Exoskeleton to Aid Running

Running is great exercise but not everyone feels great doing it. In hopes of boosting physical activity and possibly creating a new mode of transportation engineers at Stanford University are studying devices that people could strap to their legs to make running easier.

In experiments with motor-powered systems that mimic such devices – called exoskeleton emulators – the researchers investigated two different modes of running assistance: motor-powered assistance and spring-based assistance.

The mere act of wearing an exoskeleton rig that was switched off increased the energy cost of running, making it 13 per cent harder than running without the exoskeleton. However, the experiments indicated that, if appropriately powered by a motor, the exoskeleton reduced the energy cost of running, making it 15 per cent easier than running without the exoskeleton and 25 per cent easier than running with the exoskeleton switched off.

In contrast, the study suggested that if the exoskeleton was powered to mimic a spring there was still an increase in energy demand, making it 11 per cent harder than running exoskeleton-free and only 2 per cent easier than the non-powered exoskeleton.

If future designs could reduce the energy cost of wearing the exoskeleton, runners may get a small benefit from spring-like assistance at the ankle, which is expected to be cheaper than motor-powered alternatives.

The frame of the ankle exoskeleton emulator straps around the user’s shin. It attaches to the shoe with a rope looped under the heel and a carbon fibre bar inserted into the sole, near the toe. Motors situated behind the treadmill (but not on the exoskeleton itself) produce the two modes of assistance – even though a spring-based exoskeleton would not actually use motors in the final product.

As the name implies, the spring-like mode mimics the influence of a spring running parallel to the calf, storing energy during the beginning of the step and unloading that energy as the toes push off. In powered mode, the motors tug a cable that runs through the back of the exoskeleton from the heel to the calf. With action similar to a bicycle brake cable, it pulls upward during toe-off to help extend the ankle at the end of a running step.

Eleven experienced runners tested the two assistance types while running on a treadmill. They also completed tests where they wore the hardware without any of the assistance mechanisms turned on.

Each runner had to become accustomed to the exoskeleton emulator prior to testing – and its operation was customised to accommodate their gait cycle and phases. During the actual tests, the researchers measured the runners’ energetic output through a mask that tracked how much oxygen they were breathing in and how much carbon dioxide they were breathing out. Tests for each type of assistance lasted six minutes and the researchers based their findings on the last three minutes of each exercise.

The energy savings the researchers observed indicate that a runner using the powered exoskeleton could boost their speed by as much as 10 per cent. That figure could be even higher if runners have additional time for training and optimisation. Given the considerable gains involved, the researchers think it should be possible to turn the powered skeleton into an effective untethered device.

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