Reebok treadmill When I run the treadmill suddenly stops / jerks which could lead to serious knee injuries.It is not a complete stop but a definite ****. Less weight on the running belt seems to soften the jerking.RegardsJames Johnson

Hi James
Unplug unit remove motor shroud then plug unit back in , press start and begin walking on the belt normally and try to stall the belt to make it slip ( planting foot to stop the belt).
Be careful during this test. If the problem is in the drive belt, either the drive belt will stop when stalled or the front roller will stop when stalled. Here is the procedure for adjusting the drive belt.
Find the motor mounts. Many motors in Icon treadmills have a main pivot joint in which a bolt runs the full length of the motor mount. This bolt will require loosening to provide maximum adjustability. Some models have a single jack bolt and others have an auto tensioner. Single jack bolts are the easiest to adjust.
Find the set bolt(s) on the motor. This is typically run through only one side of the motor mount or may be on both sides but it does not run the full length of the mounting. Loosen this bolt(s) but do not remove. Jack bolt models will have flat mounts and may contain up to four bolts.
Look for a bolt that will tension the motor. If it has this bolt, it typically runs in the same direction parallel with the drive and walking belts. If your treadmill is not equipped with this bolt, skip to Step 4. If your treadmill is equipped with this bolt (very few are) the repair will only require one person. Using 2 7/16" wrenches, make a few turns to this bolt, tighten the bolts and test your treadmill for slipping. If belt continues to slip, and if their is too much tension ( you should be able comfortably twist the drive belt 90 degrees ) then replace drive belt.
If the motor, drive belt, and front roller continue to turn when the walking belt is stalled, the problem is with the walking belt slipping on the front roller, here the procedure adjusting the walking belt.
If the walking belt slips when walked on, first remove the key and UNPLUG THE POWER CORD. Using the allen wrench, turn both rear roller bolts clockwise, 1/4 of a turn.When the walking belt is correctly tightened, you should be able to lift each side of the walking belt 3 to 4 inches off the walking platform. Be careful to keep the walking belt centered. Plug in the power cord, insert the key and carefully walk on the treadmill for a few minutes. Repeat until the walking belt is properly tightened
If the drive belt continues to turn but the front roller stalls, you may have a broken front roller pulley. If this is the problem, you will notice it because the plastic will usually make a loud squealing sound on the metal front roller. The front roller assembly will need to be replaced and the drive belt must be replaced also. We suggest replacing the drive belt as well because it is a low cost item and will most likely, have suffered unusual wear from the problem. Another possibility is that the drive belt has worn down. In some cases, belt dressing may help correct the problem. If not, the drive belt must be replaced.