A colleague is looking into controlling 5 treadmillsusing an Arduino. Both speed and on/off. My thoughts were you could either hack the circuitry, with a lot of reverse engineering or go to directly controlling the motor. My thoughts were also that you would need to be really dedicated to this because either approach would be a real pain.
Other more hopeful thoughts or suggested approaches?
Dedication is helpful in a situation like this. 
I helped a student hack a treadmill last year and we just bypassed a lot of the native circuitry and directly controlled the motor. We figured out where the motor control signal was happening and just wired that up to a TIP 122 switching circuit and ran it off an Arduino PWM pin . As I recall, there was a slight delay in the change of signal to the actual speed of the treadmill. With this setup, you could run all 5 treadmills from one Arduino, since you’ve got 6 PWM pins to work with.
I’d second @noahcoleman’s approach. Depending on the qualities of the motor – his method requires a DC motor (which it probably is). Also, depending on the size of the motor, a TIP 122 might a MOSFET (IRFZ44N is what we have in resale) for driving much higher currents. TIP 122 maxes out at 100V @5A, whereas the IRFZ44N maxes out at 55V @ 49A. You should be able to get reasonable PWM duty cycles from either.
More options here http://www.saic.edu/media/saic/pdfs/campusresources/instructionalshops/Resale-Catalog-FA14.pdf.
Thanks @noahcoleman and @bakercp. I suggested that he contact people here if the project is built in Chicago. This is not, imho, something someone should attempt as a first electronics project
So I searched the particular model linked in the original post and found this page showing a guy’s diy walking desk. He didn’t even assemble the treadmill and just sent the pwm signal straight into the pin the native controls plug in to. So I’m thinking this would be an even easier hack than before.
Ha, did this a couple years ago. No need to take away the board used with the board! Just grab the signal wire. I did a project a while back with hamsters and treadmills (ha):