Python + Arduino on/off the LED.

pythonArduino.ino

#define LED 9

void setup() {
    pinMode(LED, OUTPUT);
    Serial.begin(9600);
}

void loop() {
    if (Serial.available()) {
        int brightness = (int)Serial.parseInt();
        if (brightness > 255) {
            brightness = 255;
        else if (brightness < 0) {
            brightness = 0;
        }
        analogWrite(LED, brightness);
    }
}

PythonArduino.py

import serial # you need to install the pySerial :pyserial.sourceforge.net
import time

with open('RMS_EMG.txt') as f:
    lines = f.read().split('\n')

arduino = serial.Serial('/dev/tty.usbserial', 9600)
for line in lines:
    brightness = int(float(line.split(',')[-1]) * 255)
    arduino.write(bytes(str(brightness), 'ascii'))
    time.sleep(1/100.)