Walking is a very economical way to get around. We continually recycle mechanical energy through the course of each stride. But no one had ever succeeded in measuring the metabolic energy used to move individual joints as we perambulate. Which is where Daniel Ferris's bionic boots step in. Initially designed to aid the rehabilitation of spinal injury patients, Ferris was curious to see if his neurally controlled robot boots could reduce the metabolic cost of walking by doing some of the ankle muscles' work. Teaming up with Gregory Sawicki, Ferris put a team of robot-boot clad students through their paces to see if their bionic walk was more economical(p. 1402).

Sawicki admits that wearing the boots is amazing, and adds `I'd buy a pair if I could, but they're not going to be in the stores any time soon'. Powered by a pneumatic artificial muscle behind the leg which helps push the foot off the ground, the boot's movements are controlled directly by electrical activity from one of the wearer's calf muscles; the robots are directly controlled by the wearer's own central nervous system. According to Sawicki,it only took a few sessions before the wearers got used to using their ankle assistants and he could start collecting data.

Filming each walker, so that he could be sure that wearing the ankle robots hadn't changed their gait, Sawicki first measured their metabolic rate as they sauntered along wearing the unpowered boots. Then, he activated the boots and remeasured their metabolic rate while recording the mechanical work done by the robot's pneumatic muscle as it assisted the walker's calf muscles to push the foot off.

Initially, when the walkers put on the boots, their metabolic rate increased because they had to adjust to walking with the artificial muscles'assistance. However, as the students got used to their bionic footwear their metabolic rate dropped by 10%. The boots had made walking easier. `You really notice when they're not there, your legs feel really heavy' says Sawicki.

Next, Sawicki used the robotic boots to measure the ankle's mechanical efficiency based on the work done by the pneumatic muscle, and realised it was almost 3 times greater than the team had expected. They suspect that instead of being powered by muscular contraction, the energy required to extend the ankle joint and push the foot off is supplied by the enormously springy Achilles' tendon, which stores energy from earlier in the stride cycle.

Having made the first measurements of mechanical efficiency in a human joint and found that we're even more efficient than they'd expected, Sawicki and Ferris are keen to learn more about individual muscles' contributions to walking by adapting the remarkable bionic boots to fit hips and knees.

Sawicki, G. S. and Ferris, D. P. (
). Mechanics and energetics of level walking with powered ankle exoskeletons.
J. Exp. Biol.