Even a simple bike trainer may be changed to modify the resistance by just moving gears, which may be all that is required. Some trainers are even electronic, allowing you to replicate a race and its circumstances. These are obviously designed particularly for competitive riders. For less serious users, a trainer with fixed resistance works perfectly well. The main advantage of using a gear-based trainer is that it allows the rider to choose how hard the exercise will be. If they want to limit their effort to a certain level, then changing down a chain ring will make the unit easier to ride. If they want to go all out for an entire workout, then changing up a chain ring will make the unit harder to ride. This is not possible on an electronic trainer because there is no way to adjust the resistance during a session. Gear-based trainers are common in sports such as road cycling and mountain biking where having control over the intensity of your workouts is important. They work by simply adding or removing teeth from a set of sprockets that are attached to the bicycle frame. The more teeth there are on one sprocket, the harder the exercise will be. To change the resistance, the rider just has to move the chain from one sprocket to another. This is useful if you want to work at a specific power output for example before a race or training camp.