IEEE Spectrum November, 2013 - 50

a considerable amount of sensory feedback, is far more difficult.
Choosing the right electrode configurations for standing requires
both a tremendous amount of intuition and plenty of trial and
error. "That's the challenge: to create the electrical field that's
going to give you the desired behavior," says Harkema.

on a Wednesday in February of this year, Shillcox arrived at the
Frazier Rehab Institute in downtown Louisville for one of his
first stimulation sessions. The array and pulse generator had
been implanted a few weeks before. He wore Nike sneakers and
black gym shorts, revealing thin legs atrophied from lack of use.
Shillcox joined Harkema and her team in a large room equipped
with custom rehabilitation equipment. He wheeled himself to
a three-sided stand Harkema had made out of metal pipes that
she'd bolted to a piece of plywood. Researchers taped 14 sensors
to Shillcox's legs. Using electromyography (EMG), these sensors would measure the electrical activity produced by his muscles and indicate how Shillcox was responding to the stimulation.
Two trainers hoisted Shillcox from his wheelchair onto his feet
and into the stand. Then they took their positions to keep him
upright-one in front of Shillcox with both hands pushing against
his knees and the other behind, steadying his hips. Shillcox held
onto the stand with his hands, and a bungee cord supported him
from behind.
That day, Harkema planned to test new stimulation configurations to see whether one of them would allow Shillcox to stand
on his own. She took a seat in front of a screen displaying the
EMG signals while two other researchers helped monitor the data
from other screens. To start the session, Harkema called out the
electrode settings: "1+, 2+, 3+, 9+, 14+, 12+, 13+, 6+, 7-, 8-, 4-, 10-."
This configuration used 12 of the 16 electrodes, 8 of them as anodes
(positively charged) and 4 of them as cathodes (negatively charged).
Harkema instructed her team to set the pulsation frequency at
30 Hz and the initial intensity at 1 V and to ramp up by a tenth
of a volt at a time. "Left independent," a trainer called out when

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the stimulation reached 1.5 V. Shillcox bore his weight on his left
leg without assistance for about 30 seconds.
Harkema jotted in her lab book and instructed the team to
turn off electrode 10, the one targeting Shillcox's lower leg.
"Going to zero," a researcher called out. He powered down the
system, punched in the new electrode configuration without
electrode 10, and ramped it up again. At 2.6 V, Shillcox's knees
buckled. "It shot me out," Shillcox said. The electrodes hadn't
sent the signal to the legs to stand straight but had twitched his
knees forward instead. The stimulation pattern and parameters
weren't quite right.
Harkema tried more configurations, but each time Shillcox
felt nothing until Harkema hit a particular voltage threshold,
at which point Shillcox's knees would give way. After 75 minutes,
on the 10th and last try, Harkema removed the bungee supporting
Shillcox from behind. The muscle activity on the EMG monitors
skyrocketed. He'd been balancing so perfectly with the bungee
cord that he hadn't been getting enough external sensory information to activate his muscles, Harkema concluded, so there
had been little input flowing back to the lower spinal cord. She
instructed her team to devote the next few sessions to the last
electrode pattern of the day, but without the bungee.
The technological limitations of the stimulation system make
these trials unnecessarily difficult. Each time Harkema changes
the configuration of electrodes, she has to turn off the electric
field they generate and start over at 0 V. It's a safety feature of
this off-the-shelf stimulator, but it destroys the body's neural
momentum. "You can get really close, and you think the person
is almost standing independently, and if you could just shift the
field a little you would have it. But you can't. You have to go to
zero. And then everything starts over," says Harkema. The limitation makes it especially difficult to induce a stepping motion
in her patients. "It's a left-to-right problem. If we get the right leg
to step, the left is doing nothing," she says.
It doesn't help that there are something like 4.3 x 107 possible
electrode patterns she can try and that each can be tried with a

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Table of Contents for the Digital Edition of IEEE Spectrum November, 2013