Although the human body is mostly water, the body’s density is slightly less than that of water and averages a specific gravity of 0.974, with men averaging higher density than women. Lean body mass,which includes bone, muscle, connective tissue, and organs, has a typical density near 1.1, whereas fat mass, which includes both essential body fat plus fat in excess of essential needs, has a density of about 0.9 
. Highly fit and muscular men tend toward specific gravities greater than one, whereas an unfit or obese man might be considerably less. Consequently, the human body displaces a volume of water weighing slightly more than the body, forcing the body upward by a force equal to the volume of the water displaced, as discovered by Archimedes (287-212 BC).
Pressure is directly proportional to both the liquid density and to the immersion depth when the fluid is incompressible. Water exerts a pressure of 22.4 mm Hg/ft of water depth, which translates to 1 mm Hg/1.36 cm (0.54 in.) of water depth. Thus a human body immersed to a depth of 48 inches is subjected to a force equal to 88.9 mm Hg, slightly greater than normal diastolic blood pressure. Hydrostatic pressure is the force that aids resolution of edema in an injured body part. Hydrostatic pressure effects begin immediately on immersion, causing plastic deformation of the body over a short period. Blood displaces cephalad, right atrial pressure begins to rise, pleural surface pressure rises, the chest wall compresses, and the diaphragm is displaced cephalad.
A human with specific gravity of 0.97 reaches floating equilibrium when 97% of his or her total body volume is submerged. As the body is gradually immersed, water is displaced, creating the force of buoyancy, progressively of floading immersed joints. With neck-depth immersion, only about 15 lb of compressive force (the approximate weight of the head) is exerted on the spine, hips, and knees. A person immersed to the symphysis pubis has effectively offloaded 40% of his or her body weight, and when further immersed to the umbilicus, approximately 50%. Xiphoid immersion of floads body weight by 60% or more, depending on whether the arms are overhead or beside the trunk. Buoyancy may be of great therapeutic utility. For example, a fractured pelvis may not become mechanically stable under full body loading for a period of many weeks. With water immersion, gravitational forces may be partially or completely offset so that only muscle torque forces act on the fracture site, allowing active assisted range-of-motion activities, gentle strength building, and even gait training. Similarly, a lower extremity patient with weight-bearing restrictions may be placed in an aquatic depth where it is nearly impossible to exceed those restrictions.
Viscosity refers to the magnitude of internal friction specific to a fluid during motion. A limb moving relative to water is subjected to the resistive effects of the fluid called drag force and turbulence when present. Under turbulent flow conditions, this resistance increases as a log function of velocity. Viscous resistance increases as more force is exerted against it, but that resistance drops to 0 almost immediately on cessation of force because there is only a small amount of inertial moment as viscosity effectively counteracts inertial momentum. Thus, when a person rehabilitating in water feels pain and stops movement, the force drops precipitously as water viscosity damps movement almost instantaneously. This allows enhanced control of strengthening activities within the envelope of patient comfort 
Water’s heat capacity is 1,000 times greater than an equivalent volume of air. The therapeutic utility of water depends greatly on both its ability to retain heat and its ability to transfer heat energy. Water is an efficient conductor, transferring heat 25 times faster than air.
This thermal conductive property, in combination with the high specific heat of water, makes the use of water in rehabilitation very versatile because water retains heat or cold while delivering it easily to the immersed body part. Water may be used therapeutically over a wide range of temperatures. Cold plunge tanks are often used in athletic training at temperatures of 10°–15°C to produce a decrease in muscle pain and speed recovery from overuse injury, although there are some contradictory studies regarding this [6-8]. Most public and competitive pools operate in the range of 27°–29°C, which is often too cool for general rehabilitative populations, because these populations are usually less active in the water. Typical therapy pools operate in the range of 33.5°–35.5°C, temperatures that permit lengthy immersion durations and exercise activities sufficient to produce therapeutic effects without chilling or overheating. Hot tubs are usually maintained at 37.5°–41°C, although the latter temperature is rarely comfortable for more than a few minutes, and even the lower typical temperature does not allow for active exercise.
Heat transfer begins immediately on immersion, and as the heat capacity of the human body is less than that of water (0.83 versus 1.00), the body equilibrates faster than water does.