My Nordic Track Exp 1000 X runs in reverse.
Your treadmill runs on a permanent magnet DC motor. If it is running backwards someone has swapped the pos and neg leads where it connects to the motor control board MC-60. OR
the brushes were removed from the motor, and installed into the WRONG locations. there is a plus sign and a neg sign on the end cap of the motor, make sure the RED wire is going to the pos sign,.and so forth.
to get to the motor you'll need to remove the 4 phillips head screws FROM the bottom of the motor enclosure: lay the unit on it's side to get to them. 2 in the front,.and 1 on each side. If it doesn't come off easy,.you've missed a screw.
Lay the unit back down, and start digging in.
Lastly, you can verify the MC-60 is outputting the wrong polarity voltage with a voltmeter. Disconnec the motor and connect your voltmeter on a 20 volt DC range ( at least). plug your unit in, and turn it on at 3 mph. you should see positive voltage applied to the read lead.
It's very unlikely that the control board would swap the polarity, so, my bet is,...the wires are switched.
good luck. Read More At : NordicTrack Treadmill, A2550...