Inexpensive motorized treadmills can be very loud and motorized treadmills can be dangerous. The track is made of a plastic fiber. A good running surface is critical to avoid damage to your feet. Your foot follows a circular motion as it is planted and lifted in a running stride. That circular motion stops when running on a smooth non-giving surface like pavement or the mat of a treadmill. The circular motion then occurs between your joints and they wear out. But the worst thing about a treadmill track is that unlike gravel it can grab your foot. If by accident the tip of your foot hits the track it is immediately pulled out from under you. It is a terrible thing to be thrown down and ejected off the end of a treadmill.
If you are an elderly person, the only safe thing you can do on a treadmill is walk. Running on a motorized treadmill is just too dangerous if you don’t have good balance. You should stand erect when you run. If you hold on to the bar on the treadmill, you lean too far forward. People that run any distance on a treadmill don’t hold on. Actually your feet come out from under you much more easily when holding on. This is because you are leaning forward. In any case, you think about your balance when you run on a treadmill. It takes a certain amount of mental and physical energy to maintain your balance.
Elliptical exercise can offer an ALMOST perfect aerobic exercise over a thirty minute period. It is perfect for older people or younger people who are balancing weights (anaerobics) with aerobics. The elliptical delivers no impact to your bones. But the elliptical does hold your feet in a constant position. This means that for some the limited twisting that does occur in the motion can be felt in the knees and hips. The intensity of elliptical exercise is somewhere between bicycling and running.
Unlike a treadmill, you are very stable on an elliptical. On some units you can hold on to a bar in front of you or to the sides.