Aquatic Therapy – Scientific Foundations and Clinical Rehabilitation Applications

Clinical Review by Bruce E. Becker, MD, MS

The aquatic environment has broad rehabilitative potential, extending from the treatment of acute injuries through health maintenance in the face of chronic diseases, yet it remains an underused modality. There is an extensive research base supporting aquatic therapy, both within the basic science literature and clinical literature. This article describes the many physiologic changes that occur during immersion as applied to a range of common rehabilitative issues and problems. Because of its wide margin of therapeutic safety and clinical adaptability, aquatic therapy is a very useful tool in the rehabilitative toolbox. Through a better understanding of the applied physiology, the practitioner may structure appropriate therapeutic programs for a diverse patient population.

Since the earliest recorded history, water has always been believed to promote healing and has therefore been widely used in the management of medical ailments. Through observation and centuries of trial and error, and scientific methodology, traditions of healing through aquatic treatments have evolved.

This review will detail the current scientific understanding of the many physiologic changes that occur during aquatic immersion. Aquatic immersion has profound biological effects, extending across essentially all homeostatic systems. These effects are both immediate and delayed and allow water to be used with therapeutic efficacy for a great variety of rehabilitative problems.

Aquatic therapies are beneficial in the management of patients with musculoskeletal problems, neurological problems, cardiopulmonary pathology, and other conditions. In addition, the margin of therapeutic safety is wider than that of almost any other treatment milieu.

Knowledge of these biological effects can aid the skilled rehabilitative clinician to create an optimal treatment plan, through appropriate modification of aquatic activities, immersion temperatures, and treatment duration.

Historically, the field of Physical Medicine viewed hydrotherapy as a central treatment methodology. In 1911, Charles Leroy Lowman, the founder of the Orthopaedic Hospital in Los Angeles, which later became Rancho Los Amigos, began using therapeutic tubs to treat spastic patients and those with cerebral palsy after a visit to the Spaulding School for Crippled Children in Chicago, where he observed paralyzed patients exercising in a wooden tank. On returning to California, he transformed the hospital’s lily pond into 2 therapeutic pools [1].

At Warm Springs, Georgia, Leroy Hubbard developed his famous tank, and in 1924, Warm Springs received its most famous aquatic patient, Franklin D. Roosevelt. A wealth of information, research, and articles on spa therapy and pool treatments appeared in professional journals during the 1930s.

At Hot Springs, Arkansas, a warm swimming pool was installed for special underwater physical therapy exercises and pool therapy treatments with chronic arthritic patients [2].

By 1937, Dr. Charles Leroy Lowman published his Technique of Underwater Gymnastics: A Study in Practical Application, in which he detailed aquatic therapy methods for specific underwater exercises that “carefully regulated dosage, character, frequency, and duration for remedying bodily deformities and restoring muscle function” [3].

During the 1950s, the National Foundation for Infantile Paralysis supported the corrective swimming pools, and hydrogymnastics of Charles L. Lowman and the therapeutic use of pools and tanks for the treatment of poliomyelitis.

In 1962, Dr. Sidney Licht and a group of physiatrists organized the American Society of Medical Hydrology and Climatology, which historically met at the annual meeting of the American Academy of Physical Medicine and Rehabilitation.

Nearly all the biological effects of immersion are related to the fundamental principles of hydrodynamics. These principles should be understood to make the medical application process more rational. The essential physical properties of water that effects physiologic change are density and specific gravity, hydrostatic pressure, buoyancy, viscosity, and thermodynamics.

Density

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 exces of essential needs, has a density of about 0.9 [4] . 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).

Hydrostatic Pressure

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.

Buoyancy

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

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 [5].

Thermodynamics

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 (Figure 1). 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.

 Aquatic Therapy Applications in Rehabilitation

Figure 3 details techniques appropriate for various populations seen in physiatry. As research demonstrates, immersing the body in water produces many physiologic effects that have been used therapeutically over centuries of medical history.

Aquatic exercise and rehabilitation remains vastly underused despite its recent increase in popularity. The health benefits of aquatic exercise have been shown to equal or surpass other forms of exercise including walking and running in studies of the Cooper Clinic database of more than 30,000 men and women [171,199]. These studies assessed overall health benefits of aquatic exercise with land-based walking and running, finding health effects comparable to both land activities, with the potential added value of aquatic activities broader range of clinical applicability in specific populations. Review of the Cooper Clinic database of more than 40,000 men showed exercise swimmers to have less than half the mortality risk of sedentary men, and, surprisingly, approximately half the mortality risk of exercise walkers and runners [200]. All these effects are good reasons to use the aquatic environment in training and rehabilitation.

Aquatic facilities are widely available, and public acceptance is already high, so there are tremendous potential public health benefits to be achieved through programs targeted at the most costly chronic diseases: hypertension, cardiovascular disease, arthritis, and other musculoskeletal pathology, obesity, and deconditioning. Aquatic programs for achieving fitness and restoring function may be designed for a broad range of individuals through an understanding of the fundamental principles of aquatic physics and the application of those principles to human physiology. There are unique attributes to aquatic therapy that seem to both pres