In the void left by the anticlimactic World Cup exoskeleton adventure, other efforts to make the paralyzed walk again are recapturing the spotlight. Chief among them is Grégoire Courtine’s research group at the EPFL in Switzerland. Their latest breakthroughs, just published in Science Translational Medicine, suggest that a more grounded approach is to repower the locomotive effort at the level of the spinal cord. The researchers were able to get the paralyzed rats to walk on their hind two legs with the help of a treadmill and harness.
What makes this feat impressive is that the rats had no control over their own legs because their spinal cords were completely severed. The key advance, and what may eventually make similar feats possible in humans, is that manual adjustment of the electrical stimulation of the spinal cord has been successfully automated into a realtime feedback loop. This primarily means that stimulation parameters like pulse width, amplitude, and frequency are adjusted by a single algorithm that incorporates the full leg kinematics. Or, as Courtine would say, “We have complete control of the rat’s hind legs.”
While that statement is no doubt true, it is not meant to imply that the researchers have the ability to make the rat’s legs do whatever they want. Considering that other researchers have demonstrated that simple ball and stick models of legs can “walk” down an incline under the direction of nothing but gravity alone, I would like to suggest another challenge to meet before any measure of control over a spinal circuit might be claimed. That challenge would be not just to have the legs follow a powered walk on a treadmill, but rather the flip of the remote control switch would convert between a simple step and a two-legged hop.
The basic alternating step gait is as old or older then the backboned fish which have swiped their tails from side-to-side for eons unimaginable. That primitive spinal circuit in fish is still as much in us as adults as the gills and tail we have when in the womb. The elaborations on that key circuit which first flexed fins to push fish onto land at some point learned to cooperate in unison to lift the creature in synchronous defiance of gravity. Capturing the conversion point from asynchrony to synchrony would be the ideal milestone to mark our our accomplishment, and the foundation on which to build a more nuanced control platform.
For those who don’t feel it is essential to be able to rise first from the chair before walking across the floor, human trials for a device based on the Swiss protocols are scheduled to begin during summer 2015. With any luck, they will be a success. With a little more luck, the existing spinal circuits will survive the electrical hammering that already lets rats take upwards of 1000 steps, and the researchers will have learned how best to work with the wetware that paraplegics still have rather than forcing it to match their vague intuitions of it.