Prioritizing one sensor
You have to think of your program as having two distinc modes of
operation:


The "fixed servo" mode is entered when you press the button on the
app. In this mode the servo is kept at a fixed position. The mode
lasts for a predefined delay.
The normal (not fixed servo) mode is the default mode. In this mode
the servo is positioned according to the reading of the sensor.


So you need a boolean variable to remember whether the "fixed servo"
mode is active or not, and the code in loop() should take care of
providing the correct behavior depending on the mode, and updating that
variable when needed. For example:

void loop()
{
    static bool fixed = false;  // whether we are in "fixed servo" mode
    static uint32_t fixed_started;  // when the mode started
    uint32_t now = millis();

    // On button press, start "fixed servo" mode.
    if (Serial.available() &amp;&amp; Serial.read() == 'a') {
        servo.write(SERVO_LEFT);
        fixed = true;
        fixed_started = now;
    }

    // Get out of "fixed" mode after some delay.
    if (fixed &amp;&amp; now - fixed_started &gt;= FIXED_DELAY) {
        fixed = false;
    }

    // If not in "fixed" mode, react to the sensor.
    if (!fixed) {
        if (analogRead(EMG_PIN) &gt;= THRESHOLD) {
            servo.write(SERVO_LEFT);
        } else {
            servo.write(SERVO_RIGHT);
        }
    }
}