Through understanding the principles of aquatic physics and applying them to human physiology, clinicians can design aquatic programs to help a broad range of people achieve fitness and function. This article creates a foundation for understanding those profound physiologic changes.

IMMEDIATELY AFTER a person is immersed, water begins to exert pressure on the body. Because the pressure exerted is greater than venous pressure, the blood is placed upward, first into the thighs, then into the abdominal cavity vessels, and finally into the great vessels of the chest cavity and into the heart. Venous return is enhanced by the shift of blood from the periphery to the trunk vessels to the thorax to the heart.

Central venous pressure rises with immersion to the chest and increases until the body is completely immersed. Cardiac volume increases by nearly one-third with immersion to the neck. Because the heart is not static, it responds by increasing the force of contraction, an effect known as Starlings law.

Water-based exercise often has been said to be less effective than land based exercise for improving cardiovascular fitness, largely because the failure to achieve similar pulse rates. Since the heart's ultimate purpose is to pump blood, its best measure of performance is the amount of blood pumped per unit time. Called cardiac output, this is the product of the volume of blood pumped with each contraction (stroke volume) times pulse rate per unit time. The heart muscle must work harder during exercise to increase output by increasing rate than by increasing stroke volume.

Thus, as cardiovascular conditioning occurs, cardiac output increases are achieved with greater stroke volumes per beat but smaller increases in heart rate. The pulse rate of the conditioned athlete, for instance, is lower than that of an unfit individual for any given cardiac output by approximately 5 L/min. Submersion to neck depth increases cardiac output 32 percent at rest. Because of the increased stroke volume, a resting pulse of 86 bpm produces a cardiac output of 8.6 L/min., and is already producing cardiac exercise. Therefore, the claim that water exercise is not aerobically efficient is faulty. In fact, it may be the ideal cardiovascular conditioning medium.

TWO RECENT STUDIES have validated aquatic environments in cardiovascular rehabilitation following infarct and ischemic heart disease. Both investigators took the bold step of actively rehabilitating patients with cardiac disease in an aquatic environment.

While water helps patients with diseased hearts, it also helps those with joint disease. Studies have found that oxygen consumption (VO 2) is three times greater at a given walking speed in water than on land. Thus, when one looks at the reverse effect of this fact, only half to one-third of the speed is required to achieve the same metabolic intensity as on land. This effect may help patients with joint disease since lower joint forces are present with slower speed.

Water is equally beneficial for patients with hypertension. When people are immersed to the neck, for instance, systemic vascular resistance decreases by 30 percent, with peripheral venous tone diminishing by 30 percent. Total peripheral resistance lowers during the first hour of immersion and persists for a time thereafter. This drop is related to temperature, with higher temperatures producing greater drops. This decreases end-diastolic pressures and cardiac after load. While systolic pressures do increase with increasing workload, they appear to be approximately 20 percent less in water than on land. Thus, water is a safe and beneficial environment for patients who are hypertensive.

The combined effects of these changes increases the total work of breathing by 60 percent. Thus, for an athlete used to land-based exercise, water-based exercise is a significant workload challenge to the respiratory apparatus. If water training time is sufficient, this challenge can improve the respiratory system's efficiency.

Water immersion positively affects the musculoskeletal system as well, particularly with vasoconstriction. On land for instance, sympathetic vasoconstriction tightens the resistance vessels of the skeletal muscle to resist blood pooling. But water, immersion pressure removes the biologic need for vasoconstriction, thus increasing muscle blood flow. In fact, resting muscle blood flow increases by 225 percent during neck immersion."

Aquatic immersion creates many effects upon renal blood flow and the renal regulatory systems. For instance, the flow of blood to the kidneys increases immediately upon immersion, which produces an increase urine and potassium excretion (see chart). Sodium excretion also increases as a function of depth due to the shifting of circulating central blood volume.

MANY EFFECTS have been anecdotally noted about using aquatic environments for health maintenance and restoration. Predominant among these are the relaxation effect of water, and the effect it has on pain perception.

A relaxation effect is produced by a central process not understood and likely produced within the reticular activating system deep within the brain. Mood state has been found to improve following land exercise but has not been as well studied in an aquatic environment. Similarly, anxiety and depression are reduced following land exercise; research to test these effects following aquatic exercise has not been done.

Although some controversy exists about the optimal training program for athletes who need joint off-loading during a recovery period, it is known that aquatic exercise can indeed increase conditioning in that population. In fact, water running equals land running in its effect upon maintaining VO 2 max when training intensities and frequencies are matched."

Similarly, when aquatic exercise is compared with land-based equivalent exercise in its effect upon maximum gains in VO 2 in unfit individuals, aquatic exercise achieves equivalent results; when water temperature is low, the gains achieved are accompanied by lower heart rate water-based exercise programs may be used to sustain or increase aerobic conditioning in athletes who need joint off-loading.

Nonetheless, aquatic exercise programs may restore fitness in patients who are obese because of the protective effects against heavy joint loading. A program that starts in water and moves to land as tolerance builds may be the most effective method of achieving conditioning and weight loss.

This article and its attached references provide a basis for clinical dialogue about the effects of water immersion. Tremendous potential public health benefits can be achieved through programs targeted at the most significant physiologic consequences of aging, hypertension, cardiovascular disease arthritis and other joint pathology obesity and deconditioning.

This truly is the time to get in the swim of things.