Problem in reading analog raw emg data

I'm using a muscle sensor "MyoWare" to read muscle activity(known as EMG signal) ,when I try reading the rectified and filtered (i.e signal envelope) it reads just fine(it gives high values when the muscle is contracted and less values when relaxed) ,but when reading the raw EMG signal(it's centered around 2.5 volts so there are no negative portions) it just gives high values regardless of the muscle being contracted or relaxed .
note:the output of the sensor s connected to arduino analog input A0.The sensor is supplied with arduino 5 Volts output.I'm using arduino uno.
What could be the problem?any help is appreciated.
the code is as follows:

double sensorValue = 0;
void setup() {

Serial.begin(9600);

}

void loop() {

sensorValue = analogRead(A0);

Serial.println(sensorValue);

}

what is result display on Serial monitor ?.

I never use this sensor.
but, you can try to change to use external 5V for sensor.
And try reading again.

Hi esra, how are you powering the Arduino? Laptop USB port?

AdvancerTechnologies:
Hi esra, how are you powering the Arduino? Laptop USB port?

Whats the solution? I powered using laptop USB.

Is the laptops power cord plugged in when you use the sensor?

Can you post some pictures of how you've wired the sensor and where you're placing the electrodes?