A swimming machine is a resistance swimming apparatus, often self-contained, enabling the swimmer to swim in place. This may be accomplished either by accelerating the water past the swimmer or by supporting the swimmer, either in water or on dry land. The first type, known as a countercurrent swimming machine, usually consists of a water tank at least twice as long and about one and a half times as wide as an average person with the limbs extended. The swimmer swims unrestrained against an adjustable stream of water set in motion by means of mechanical devices, such as jets, propellers or paddle wheels.
Countercurrent swimming machines made their appearance in the 1970s, initially in the form of pump-driven jetted streams, which caused a certain amount of turbulence and an un-natural swimming environment. They were followed up in the 1980s by propeller- and paddle-wheel driven machines. These provided a smoother stream of water.
Two types of exercise machines make up the second group. Hybrid systems - self-contained micropools similar to the counter-current type but using a flexible tether to keep the swimmer in place are one type. These systems, being human powered, need neither machinery or electricity but have to be carefully designed to suppress wave formation. The second type allows a person to remain on dry land while simulating certain swimming strokes. Machines of the latter type 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. However, the higher effort required by such machines, in the absence of the metabolic effects of immersing the body in water, makes these devices more effective than true swimming if one's purpose is to achieve weight reduction. Similar in purpose, but not qualifying as swimming machines since they require access to a swimming pool, are various tether systems.
These systems depend on one or more pumps. Discharge rates of 13 L/s (200 US gal/min) and more are possible, from motors of three or four horsepower (2 or 3 kW); power requirements are determined from pump curves, where the pump is selected to maximise volumetric flow, as pressure loss is relatively low as the water does not need to be lifted, but only overcome swimmer drag and other pressure losses within the system. One of the earliest models on the market — introduced in 1973 — was the Badujet which is available only in the form of a bare propulsion system, to be installed into either an existing or newly built pool.
Also in this category are a number of swim spas, usually fiberglass shells equipped with several pool pumps to set the water in motion. Seen as more convenient since they come pre-assembled, the quality of the swim has been criticized by indoor swimmers as being somewhat turbulent, as the strength of the current comes from the speed and pressure of the discharged water, rather than its volume. Contrary to this, triathletes and other sea swimmers have praised the system due to the turbulence created by the jets mimicking the behavior of the sea, improving stamina and general fitness.
Swim Spas, as the name suggests, are a combination of a spa (or hot tub) and an exercise pool. Single-zone models are typically a fibreglass pressure-driven exercise pool which has swim jets at one end, and one or more spa seats fitted with massage jets at the other.
In the 1980s Monarch Spas developed the dual-zone swim spa so that pumps and other equipment needed for the pool, could also be used to power a separate spa. Today, the advantage of the modern dual-zone system is that the two pools can be at different temperatures using different chemicals - the Hot Tub (using bromine), is hot enough for relaxation and massage, while the swim zone is cool enough for strenuous exercise (using chlorine).
In 1994 HydroWorx patented its first therapeutic pressure-driven pool, the HydroWorx 2000. The new design utilized an underwater treadmill along with pressure-driven jets for rehabilitation. Dr. Paul Hetrick, founder of HydroWorx, got the idea after witnessing a horse trainer at Penn National Race Course using an underwater industrial-strength treadmill to rehabilitate injured horses. Later versions incorporate an underwater monitoring system that projects an image of the patient onto a monitor so therapists can adjust treatments in real-time.
In the 1980s a new type of machine made its appearance. In an attempt to correct problems of turbulence and resulting discomfort from swimming against a jet of water, systems were devised to set the water in motion in a smoother fashion. The first, in 1984, was the SwimEx, developed by Stan Charren together with two MIT-trained engineers. This machine, consisting of a fiberglass pool with the machinery housed in an adjacent compartment, sets the water in motion by means of a paddlewheel, thus generating a steady stream of water as wide as the swimming pool itself.
In the late 1980s the Endless Pool was developed by James Murdock, in an attempt to build a less-expensive alternative to the SwimEx. This machine places the water-moving equipment - a large propeller encased in a stainless steel box and powered by a remote hydraulic pump, and its stainless steel water circulation tunnels - inside the body of a vinyl-lined metal pool. Its stream of water is narrower than that of the SwimEx, though the swimming experience is comparable. Other companies have copied this system since it was introduced.
Around the same time, the Swim Gym, a propeller-driven propulsion system became available commercially. This machine is encased within a large (10" diameter) PVC tee which is then incorporated into the concrete wall of a swimming pool. It delivers a current equivalent to that produced by Endless Pools.
In 2008, SmartPools Sdn Bhd  Malaysia launched its Laminar Propulsion system using state of the art drive train technology capable of moving up to 30,000 litres of water per minute at low pressure to create a non-turbulent, bubble-free, smooth flow and fully speed-adjustable swimming treadmill suitable for beginners to serious athletes.
A number of "still-water" mini-pools have been built over the years, designed to be used in conjunction with various resistance-swimming tether systems. These human-powered devices combine the self-contained aspect of counter-current swimming machines with the low cost and simplicity and freedom of movement of tether systems used in athletic training. They have major cost and energy-use advantages over mechanical swimming machines. They are valuable for aerobic exercise, endurance and strength training, and for stroke practice. However, they cannot replicate open water conditions, in which the water courses at speed past the swimmer, so that for competition training their use has to be combined with open-water practice. One example of such a device is the Swimergy Swim System, which also makes use of wave-reduction technology.
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- "Horse Training". HydroWorx. n.d. Retrieved 2009-08-20.
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- "SwimEx". SwimEx. Retrieved 2011-06-12.
- "Endlesspools". Endlesspools. Retrieved 2011-06-12.