DO THIS OUTSIDE. Unplug Treadmill. Loosen the belt and place a 2 X 2 across under each end to raise it off the board. Using mineral spirits and paper towels, moisten a towel with mineral spirits and clean underneath belt and the board surface. Dry with another clean paper towel. Mark the top of belts where cleaned, remove the 2 x 2's and move belt, by hand, to a new position. Replace 2 x 2's and repeat until the entire belt is clean underneath. Inspect the board under the belt. If it is worn through the black surface, obtain 2 boards of PLASTIC floating floor overlayment. You might have to buy a whole box or store may have two loose pieces. Make sure they are PLASTIC and not damaged. If the top has a wood grain embossed on it , turn it over to the smooth back. Cut them about 8 inches shorter than top of belt between rollers. Round off each end of the PLASTIC boards and sand smooth. Snap them together and slide them under the belt about 4 inches from each end, Along each side, and about 3/4 inch from side, install 4 flat head #8 recessed wood screws. Re tighten belt to the proper tension and your treadmill belt movement will be almost like new. Do not apply any lubrication under as it will gum up causing the belt to grab the board under and stall. Mine did this and I repaired it as described above and it is like new.
Retired Engineer, April 2013