Did you know that trampoline was invented way back in the 1930's? Back then, a gymnast named George Nissen noticed that when trapeze artists fell from their bars, they simply landed in a springy emergency net. This net had a very elastic quality, and the trapeze artists could usually play off their mistake by bouncing up and into a trick as soon as they fell. Nissen envisioned an even more elastic platform that could be used for perfecting jumps, twists, and flips, and that's how the trampoline was born. For many years, the trampoline mats were made of canvas attached to a metal or wooden frame by a series of springs.
As time passed, it was discovered that these trampoline springs could present a safety hazard, especially for those who were first starting out on the structure. Metal springs create space when they're stretched, but if a finger, toe, or unsuspecting bit of skin were to fall against them while they were open, the resulting pinch would likely leave a bruise and might even draw blood. There's also a concern that a gymnast working on their double jump might lose their balance in the air, and injure his or herself if they were to land on the springs instead of the mat.
This design flaw of the original trampoline mats led inventors and engineers to change their thinking about how to provide buoyancy and resistance to the mat, while still helping gymnasts to keep all their appendages intact. What they came up with is true advancement in trampoline technology: instead of springs, they used heavy duty elastic bands that would provide the same level of springiness, without all the fear of injury while novice trampoline gymnasts were learning their new tricks in the air.
There are advantages and disadvantages to both trampoline mat designs, and which one you choose depends solely on what you're willing to pay for, and how the trampoline will be used. Keep in mind that the more frequently the trampoline is used by older, heavier individuals, the more wear and tear there will be on the springs. This could mean replacing the springs as often as each month, although springs aren't that expensive. While bands might seem like a more efficient alternative, you have to remember that they can't be detached from the mat. When the bands become stretched out, you'll have to deal with replacing the whole mat, and not just the springs.