Poor calibration...one setting too weak and the next setting causes muscle spasm...
The settings are NOT well calibrated.  Bringing it to 3 bars of intensity was too weak I can barely feel it.  So I turned it up to 4 bars and that was too strong causing visible muscle contraction and spasm.  A setting that allowed an intensity between 3 and 4 bars would have been nice considering I paid 300 bucks for it.
Rating: 1 stars