In its most simple form, it consists of swimming while being held relatively stationary by a flexible restraining device, such as a harness or a foot restraint. This training method is used by a number of swim teams, such as the ones at Purdue University, University of Indianapolis, Florida State University and the University of Florida. Likewise, Special Forces Scuba Team members at Fort Bragg use a commercial restraining device in their personal training for open water swimming. Users often indicate that it takes a certain amount of time to learn to compensate for the feeling of swimming against a restraint. However, for those who have familiarized themselves with these systems, tethers enable the swimmer to swim at any speed, even flat-out sprints, and to vary the speed at will.
Tethered swimming, as this method is known, has been in use since the middle of the twentieth century, and has undergone a gradual evolution. Initially the swimmers were restrained by a rope, which was eventually supplanted by interlocked bicycle inner tubes or surgical tubing. Presently the systems in use employ either bungee cords or coiled lines to provide the shock-absorption necessary for a comfortable swim. Attachment methods also vary. Some devices restrain the swimmer by means of a belt, others wrap around the ankles, and still others connect to specially modified water shoes.
Another form of resistance swimming is that in which the swimmer is unrestrained and the water is set in motion by means of mechanical devices, such as jets, propellers or paddle wheels. Such an apparatus is also known as a countercurrent swimming machine, of which a number are presently available commercially. A prototype machine was used by the Columbia University swim team in the late eighties.
Swimming machines made their appearance in the 1970's, initially in the form of jetted streams.Some of these were criticized for causing turbulence and an un-natural swimming environment. They were followed up in the eighties and nineties by paddle-wheel and propeller-driven machines. These generated a smoother stream of water. Many users find them easy to swim in, but the machines are also criticized for being expensive, noisy, and wasteful of energy (consuming 6.5kW to 11kW of electricity just to set the water in motion).
Finally, there are exercise machines which allow a person to remain on dry land while simulating certain swimming strokes. These devices however can not compensate for the weight of the body and the limbs and thus deprive the user of the benefits of exercise in an aquatic environment.