  Human limb motion tracking and recognition plays an important role in medical
rehabilitation training, lower limb assistance, prosthetics design for
amputees, feedback control for assistive robots, etc. Lightweight wearable
sensors, including inertial sensors, surface electromyography sensors, and
flexible strain/pressure, are promising to become the next-generation human
motion capture devices. Herein, we present a wireless wearable device
consisting of a sixteen-channel flexible sponge-based pressure sensor array to
recognize various human lower limb motions by detecting contours on the human
skin caused by calf gastrocnemius muscle actions. Each sensing element is a
round porous structure of thin carbon nanotube/polydimethylsiloxane
nanocomposites with a diameter of 4 mm and thickness of about 400 {\mu}m. Ten
human subjects were recruited to perform ten different lower limb motions while
wearing the developed device. The motion classification result with the support
vector machine method shows a macro-recall of about 97.3% for all ten motions
tested. This work demonstrates a portable wearable muscle activity detection
device with a lower limb motion recognition application, which can be
potentially used in assistive robot control, healthcare, sports monitoring,
etc.
