Wearable Electronic Device

The custom device used in this study was based around a dsPIC33 microcontroller (Microchip Inc, Chandler, AZ, USA) running at 40 million instructions per second. The device had two amplifiers suitable for EMG recording (band pass 30 Hz–2 kHz, gain digitally adjustable 198–2239×); amplified EMG was sampled by the analog-to-digital convertor of the microcontroller with 12 bit resolution at 1 k samples/s/channel (3.3V full scale deflection). Note that ideally with this sampling rate, a 500 Hz low pass filter should have been used (the Nyquist limit). However, we verified with separate recordings at higher sampling rate that typically around 0.5% of the total power of a surface EMG recording lies in the 500–2000 Hz range; any aliasing would therefore have been negligible. Two nerve stimulators provided isolated constant-current stimuli, with currents up to 20 mA, and 220 V compliance. The current was adjusted manually using a miniature potentiometer; pulse width could be digitally controlled, but was fixed here at 150 μs. The stimulators could be powered down when not in use, which reduced system power consumption and extended battery life. After turning on, there was a 10 ms delay before the stimulators became able to deliver a stimulus. Data storage used secure digital (SD) card flash memory, formatted using the FAT32 system allowing files to be downloaded easily to a personal computer fitted with SD card reader. The user interface incorporated a six line 36 mm × 26 mm liquid crystal display (LCD), which could show graphical plots (e.g., EMG traces) as well as text. A four-direction joystick allowed navigation of menus for configuration. The device measured 143 mm × 68 mm × 25 mm; power was supplied by three AA alkaline batteries, which gave a runtime of around 10 h. Programs running on the device were written in C, using the MPLAB development environment and cross-compilers (Microchip Inc., Chandler, AZ, USA) running on a standard Windows-based PC.