Table 1.

Summary of water physics principles

Water property	Definition	Properties	Clinical significance
Buoyancy	Upward force that opposes gravity, and has a direct relationship with the immersion depth, movement speed, body composition, and gender36,37	Archimedes principle states that, as the body submerges, it displaces water, and this displacement creates a floating force (buoyancy) equivalent to the water volume that has been displaced38	Buoyancy can be used to assist or to resist movements, to provide bodyweight offloading, and to help improve muscle activation and range of movement.27,35,39 Also, buoyancy may assist in reduction of the perceived fatigue40
Hydrostatic pressure	Pressure exerted by the fluid on submersed objects	Pressure exerted by water on a submersed object is equal on all surfaces of the object, depending on the submersion depth40	Fluids are driven from the extremities toward the central cavity,38 compressthe thorax, and increase respiratory load.41 This property also provides support during movement performance underwater, improving static and dynamic balance,42–46 including in women with FMS31
Hydrodynamic drag forces	Force that acts in an opposite direction to the line of the movement,47 which is affected by the size and shape of the object41,47	Drag force is a function of the velocity squared, which means that doubling the speed quadruples the drag force41,47	As the movement speed through water increases, resistance to motion increases27 If a person stops movement, the resistance drops almost immediately to 0, allowing improved control of exercises considering the patient’s comfort35,47
Thermal conduction	Water conducts temperature 25 times faster than air and exchanges heat with the submerged object	The aquatic environment is stable to retain cold or heat.41 The rate of temperature change depends on the mass and specific heat of the object	A submerged body adapts to the aquatic environment, quickly exchanging heat and achieving thermal balance.41 Temperatures of 26–28 °C (80–84° F) are comfortably cool for exercising, while therapeutic pools are heated to between 30 and 32 °C (86 and 90° F)27

Abbreviation: FMS, fibromyalgia syndrome.