The E-Fit device operates with low voltage. With the power running through the device, there is the possibility of muscle cramp, no different than cramping during other sporting or fitness activities. No other harm can be expected.
The E-Fit device is completely safe: it has 3 circuit breakers to protect the user from a fault in the device. As a precaution, the latter can cut the power in the device in case any malfunctioning is detected. Circuit breakers are activated sequentially, in case of malfunctioning one would activate, if it did not, then the next would start working and for a very rare case that none of the first two worked, then the third would.
The E-Fit device complies with the EU directive for medical devices.