Treadmill frames are made of one of two different metals – steel or aluminum. Steel is stronger, but heavier, than aluminum. Stronger is better in a treadmill because flexibility can give the unit a spongy feel that can increase the risk of damage to the feet, knee, and hip joints. Aluminum is used in less expensive treadmills that those that are designed to be the most portable and easily stored.
Welded or Bolted?
Better quality treadmills also have frames that are welded together, rather than bolted. Welds are often actually stronger than the steel itself. The bolts holding frames together can eventually loosen, which can cause misalignment of the belt and other components. Welded steel frames generally come with a lifetime warranty standard.