There are 2 x cat 5e cables running into that room, one to a printer and the other to a desktop in there. Have tried powering both off when the treadmill is on and it still happens
You would need to un-plug the cat 5 leads from printer and desktop as there could still be an earth ( system 0 volt ) loop via the cables.
The wire under carpet or floor board being crushed when tread mill occupied is a known cause of similar problems.