A Good Belt and Deck – The deck or bed (the surface underneath the belt) affects how hard or soft the ride feels. High-quality decks are usually coated with phenolic wax, which reduces friction against the belt --This will not only provide a smoother ride, but it will also extend the life of your treadmill considerably.
Usually, the wider the belt, the safer you are running on it. You need at least 17 inches to run comfortably, but better models will provide you with 20 or more. When it comes to length, don't settle for less than 50 inches; if you're over six feet tall, you need 60 inches.
A Sturdy Motor – Look for a motor that offers at least 1.5HP "continuous duty" (CD) power. Larger motors usually last longer, are quieter, and make the treadmill feel smoother. Some manufacturers rate treadmills according to peak HP, the maximum power the machine can produce before failing. This is not a valuable indicator to home users, and tends to be confusing. Stick to the CD indicators.
First-Class Shock Absorption – A good shock absorption system will provide you with a firm deck that does not wobble but still feels softer than running on concrete. This is mostly for your safety rather than for the longevity of the machine. Obese users and those with joint problems need the most cushioning, while walkers can do with a standard belt.
Stability - The amount of stability required in a treadmill varies according to the height and weight of the user. If two people of very different physical characteristics are going to use the machine, it makes sense to invest in one of the enhanced models, which can accommodate a wider range of users. Welded steel frames with a rust-resistant coating are the sturdiest and they usually come with a lifetime warranty.