Negative RAW EMG signal

[awjoy_tud]

Hi,
I am doing research on EMG signal analysis using Olimex shield EKG/EMG AND STM32F4 discovery board. I am using 5 volt power supply. As I know raw EMG signal gives negative value also. But In my case I am getting only signal between 0-5v that means all are positive signal. So my question is ,

1. Am I missing something from raw EMG signal?
or
2. What can I do to get the negative raw EMG signal?

3. Supply voltage from STM32F4 not exactly 5 V its actually around 4.64 V so it will make any difference for output?

your advice is valuable for me. Thank you