Fitness professionals need to understand water’s unique properties in order to design effective training programs for the pool. Exercises that work on land do not translate directly to water.
Factors affecting results include not only water’s distinct properties but also training depth and temperature. Here’s a brief review of the physiological effects of vertical immersion in water:
Buoyancy. Water exerts an upward thrust on immersed objects equal to the weight of fluid displaced. Depending on a person’s body composition, shape, bone density and immersion depth, this force varies. At xiphoid level, gravity decreases approximately 60%. In deep water, total body weight may drop by 85%—90%.
Hydrostatic pressure. Immersion exerts pressure on all surface areas below water, making lung expansion and breathing more difficult. This pressure enhances venous return and stimulates the lymphatic system. Cardiac volume increases up to 30% with neck–level immersion, leading to a 35% mean stroke–volume increase at rest. In neutral temperatures (33.5°—35.5°C), the heart rate response is typically 12%—15% less; in warm water (36°—38°C), the response is higher. At chest depth, the total work of breathing increases by approximately 60%, especially during inhalation.
Viscosity. Water is approximately 800 times “thicker” than air, and movement creates resistance. Drag forces, movement speed, turbulence, lever length and equipment can all be used to manipulate resistance levels. Resistance increases exponentially as more force is applied. Resistance to an average speed in water is approximately 12—15 times that of the same movement in air. Varying movement speed and surface area in water facilitates progressions as skill and fitness levels improve.
Thermodynamics. Water transfers heat 25 times faster than air. This affects heart rate and oxygen consumption. Warm–water (30°—35.5°C) training and land training produce similar heart rate responses. Cycling or running in cool to cold water (18°—25°C) results in heart rates that are 10—15 beats per minute lower than on land. For higher–intensity workouts, water temperature needs to be considered.
Sources: Alberton et al. 2015; Nagle, Sanders & Franklin 2015; Sanders 2000.