About a year back, we bought a treadmill from Costco and it would run for a while and then trip the GFCI breaker. I thought it was the incline motor and replaced it. Didn’t fix the problem. So we returned the treadmill and bought a new different model. The problem returned after a while.
After searching the internet, I found that this is a common problem since the treadmills discharge the static through the ground wire. I was surprised that a consumer device escaped codes/regulations somehow.
The solution I settled upon was to replace the GFCI breaker with a regular breaker since there are no wet areas around the treadmill outlet and all the wet area outlets are GFCI protected.
However, I am looking for a better solution. Is there a code recommended breaker (Not something priced 100s) for treadmills ? Alternatively is there a GFCI compliant treadmill ?
Try a GFCI from a different manufacturer. Some are more susceptible to nuisance trips than others.
It’s not necessarily static. Treadmills have an irregular load, which makes for occasional surges in the motor current. Similarly, and for the same reason, anything with a relatively large motor can cause a GFCI to trip any time the motor starts. And again, some brands are more susceptible than others to these types of nuisance trips.
I don’t think it’s the way it discharges static, a GFCI doesn’t look at current flowing through the safety ground; it senses an imbalance between the current flowing through the hot and neutral wires.
IIRC the problem is that electric motors (especially induction motors) are known for tripping GFCIs. I think it’s due to the voltage and current being out of phase (depending on load) and the GFCI sensing that as an imbalance.
I don’t know if it applies to treadmills but often a GFCI breaker will not trip as easily as the wall outlets. If possible you could try upgrading the breaker on that line.
Before settled on the non-GFCI breaker, I did try changing to brand new GFCI breakers, twice. Still had the same problem.
Here is a Treadmill maker’s description of the problem 
https://www.nordictrackfitness.com.au/support/incline-trainers/GfciafciOutlets
they’re not quite correct though:
GFCI outlets are designed to flip their breakers if there is too much power flowing through the ground port.
like I said above, they trip when the current through the hot side is not the same as the current through the neutral. they trip because the leakage current might be going through you. that’s why it’s called a ground fault circuit interrupter.
I have the refrigerator (the compressor has a motor), the garbage disposal (has a motor), food processor (has a motor), bathroom exhaust blowers (has a motor), etc all on GFCI and they don’t trip the GFCI. If your theory is correct, then how do you explain that ?
The treadmill has a different type of motor under a different load and different circumstances.
In fact, depending on the type of motor, you may be generating electricity if you are pushing faster than it.
The loads of all those that you listed are usually much more steady, and use much less power, though I have had refrigerator compressors pop the GFCI before. If you hooked a treadmill up to a meter, you’d see the current spiking and dropping with each step you took on it.
They trip when a certain amount of current that should be flowing in the neutral conductor somehow finds its way to ground, which could conceivably be through the ground conductor.