If you're looking for a way to run longer and faster than ever before, perhaps all you need is... a spring in your step.
A light resistance band, stretched between your feet, provides the bounce of an extra external tendon while saving you some energy in the long run, a new study has shown.
Designed and tested by mechanical engineer Elliot Hawkes while at Stanford University, the quirky hack could reduce the energy required for running by approximately 6.4 percent.
"It's surprising; it makes you feel light and kind of fast," says Hawkes, who has of course tested his own device.
"Pretty immediately your body figures it out."
The 19 participants who tested the hack in the study had no problems with it, either. Beforehand, they were told that the simple spring, or 'exotendon', would improve their running efficiency if they simply relaxed into it and tried to think about something else.
The runners were given time to get comfortable with the new set-up attached between their shoes before beginning a series of trials.
Not only did nobody trip during these runs, researchers found the energy they burned through dropped significantly, and it did so because of a complex leg-swinging mechanism.
"A runner, with a natural pace of 2.7 m/s and a marathon finishing time of 4:20, could expect a 6 percent improvement in economy with our device, theoretically leading to a decrease in their finishing time to around the 4-hour mark," the authors conclude.
Normally for humans, running is one of the most inefficient ways to get around. Of the calories burned, less than eight percent is used to swing our legs and maintain constant forward velocity. The rest is simply required to support and balance our bodyweight.
As such, most hacks have focused on reducing this much larger energy expense - redirecting the centre of mass and supporting our weight. The new device, however, is among the few that specifically targets leg-swinging.
Not only did the passive exotendon reduce the mechanical work needed to swing both legs - as tendons are wont to do - it also helped redirect the centre of mass, resulting in overall greater efficiency than expected.
"We show that the exotendon increases the energy optimal stride frequency, which runners then adopt," the authors write.
"At this new stride frequency, the mechanical work of redirecting the centre of mass during stance is reduced - shorter strides result in a reduction of biological joint moments and powers."
This suggests that the energy runners are saving from this hack is not derived solely from their swinging legs, but also due to offsetting the larger costs of balance, allowing you to 'bounce' when your heel makes contact with the ground.
"Naturally, people run at the 90 steps per minute," Hawkes explains.
"If you could take shorter, quicker steps, it would reduce the energy required to bounce, but it takes much more energy to swing your legs that fast, so you don't naturally do it. However, the band removes this cost for leg swing, meaning you can easily take 100 steps a minute, reducing the energy required to bounce."
It's important to note that this study only tested one relatively slow speed of running, so it's not clear whether this passive exotendon would help in all cases. But it certainly implies that the average recreational runner may be able to run faster and further with the device.
Even better, it's low-tech and easily made at home with affordable materials. In the study, the exotendons were made from natural latex rubber surgical tubing, with a loop and a carabiner on either end for the shoes.
The length of this tubing, the authors explain, should be about a quarter of the runner's leg length, measured from the hip to the ankle.
"Someone compared it to an electric bicycle, where you just go faster, so it's fun," says Hawkes. "Why not?"
The research was published in the Journal of Experimental Biology.