Although literally conceived as a
motorized suit of armor reminiscent of medieval knights, it has come to
represent a true technological-biological fusion as the most complicated
neuroprosthetic ever imagined. The breadth and scope of sci-fi exoskeletal armor
is nicely captured in the sweeping and grand scene near the end of the 2013
Marvel Studios production ‘Iron Man 3’. When scientists want to produce a
movement, complex commands related to motor planning and organization send
signals to the motor output areas of the brain. These commands then travel down
the spinal cord to the appropriate level. That is, higher up for arm movements
and lower down for legs. At the spinal cord level the cells controlling the
muscles that need to be activated are found. From the spinal cord the commands
go to the muscles needed to produce the movement. All of this relaying takes
time and introduces control delays that would make armored superhero fights
difficult.
Because of these delays, the
ultimate objective should be to create neuroprosthetics controlled by brain
commands. This reduces all the transmission delays found in using commands
downstream in the spinal cord or at the muscle level. But it also currently
requires inserting electrodes into the nervous system. Instead, a good starting
point for now is to use the commands from the brain that are relayed and
detected as electrical activity (electromyography, EMG) in muscle. These EMG
signals can be detected quite readily with electrodes placed on the skin over
the muscles of interest. The EMG activity is a pretty faithful proxy for what
your nervous system is trying to get your muscles to do. It’s kind of like a
biological form of ‘wire tapping’ to ‘listen’ in to the commands sent to
muscle. Many different neuroprosthetics have been developed to use EMG control
signals in order to guide the activity of the motors in the prosthetic itself.
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