MyoWare Muscle Sensor Kit
They are connected in parallel for extended capacity at a nominal 3.0V. One thing to note is that due to the size of the batteries, the remote cable header can't be passed through. If you need access to these connections, this Power Shield must be stacked above the board(s) that need access.
Battery power allows for a cleaner signal and also eliminates the possibility of creating a dangerous current path to the power grid.