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Updated: 15 October 2008

Aquatic Exercise snapshot

The Aquatic Environment - Qualities of Water


Gravity is the force on Earth which pulls all objects toward its center.  The force of gravity pulls down vertically and is concentrated at each object's center of gravity.  The center of gravity is that point where an object balances perfectly.  For an object to remain balanced, the center of gravity must be on a vertical line with the point of suspension, above or below it.

The location of the center of gravity in the human body varies slightly, depending on body shape, i.e., weight distribution.  (For example, in a male with a muscular upper body and small hips, the center of gravity is higher than in a female with narrow shoulders and wide hips, or, in an infant with a large head in relation to its body, it is higher than in an adult).  The lower the center of gravity, the more stable the balance  (which is why wide, low-suspended race cars “hugging” the road rarely tip over, and why tall SUVs, riding high, tip over easily when taking a turn too fast).

Buoyancy is a very characteristic force that acts upon all bodies submerged in a liquid.  Buoyancy acts in the opposite direction of gravity and causes an object to become lighter.  Buoyancy in the water exercise class therefore helps you to "take a load off" your joints.

Comparison of Location of Center of Gravity in Human Body Shapes
Location of
Center of Gravity in Human Body Shapes
Red X marks the Spot

Archimedes discovers the concept of buoyancy
This is how Archimedes' Principle explains buoyancy:
Archimedes, a Greek philosopher (circa 287 BC - 212 BC), noticed that the level of water in a tub rose when he sat in it.  (The "Eureka" story ... see image to left.)  From this observation and other experiments, he found that an object immersed in a fluid is buoyed up (lifted) by a force equal to the weight of the fluid it displaces.  We can observe displacement by filling a glass to its brim with water. If you push a ping-pong ball into the glass, the water runs over the side.  The volume of the amount of water that overflows the glass is the volume that has been displaced.  If you weigh the water that overflows and the ping-pong ball, you find that the water weighs more than the ping-pong ball.  If an object displaces an amount of water weighing more than it does, it floats, like the aforementioned ping-pong ball.  If an object displaces an amount of water weighing less than it does, it sinks.  If an object displaces an amount of water weighing the same as it does, it hovers.   Look at the image below, and the various levels of submersion of three identical bottles (size, shape, volume) made to have three different weights.
A body immersed in a liquid, either wholly or partially, is buoyed up by a force equal to the weight of the liquid displaced by the body.
  • The body will float if the buoyancy is positive (body weight smaller than weight of displaced liquid).
  • The body will be suspended if the buoyancy is neutral (body weight equals weight of displaced liquid).
  • The body will sink if the buoyancy is negative (body weight larger than weight of displaced liquid).
Buoyancy is related to the density, volume and shape of the immersed body.  A Hydrofit© buoy (“dumb bell” made from styro-foam)  -  or an empty bottle  -  will float on the surface of the water, while a much denser dumb bell of similar size made from iron  -  or the same bottle filled with sand -  will sink to the bottom.  Lung capacity affects the buoyancy of a person.  A person with full lungs displaces a greater volume of water and, according to Archimedes' Principle, is more buoyant than a person with deflated lungs.  Air in the lungs is less dense than water and so increases buoyancy.  Bone structure, bone weight and body fat are some of the other factors that have an effect on buoyancy in the human body and vary from person to person.  That is why some people float more easily than others.

Various levels of buoyancy depending on weight of the submerging body
The shape of an object determines how much of its surface (in relation to its volume) is exposed to the forces of water, in particular pressure and buoyancy:  If you lie flat on the surface of the water you will float, because you distribute your body weight (pressure onto the water) on a large surface and present a lot of body surface to buoyancy.  In a vertical position (“standing upright” in deep water) you will sink, because you place all your weight on a small area, and present only a little body surface to buoyancy.  You can try this with your kickboard – how much effort does it take to submerge your kickboard when it lies flat on the surface versus when you place it on its edge?

The more submerged an object is, the more affected it is by buoyancy.  The deeper you are in the water, the "lighter" you are - a human body submerged to waist level will experience only 40%-50% of its weight on land.  Standing in chest-deep water reduces weight bearing to 25-30% of body weight and 10% when immersed to the base of the neck.  (Jamison & Ogden, 1994)   This results in low impact across the spine and other weight bearing joint surfaces and is the main basis for the safety and ease of aquatic exercise.

Same as all weight forces can be added and presented as a single force applied to the center of gravity, buoyant forces correspond to the center of buoyancy.  Balance on and under water depends on the location of the center of gravity and that of buoyancy.  If they are situated on a vertical line and the center of buoyancy is above that of gravity, equilibrium will be stable.  Thus, a boat with a tall mast and a heavy load in its belly (low center of gravity) is stable on the water.  (See right boat in the image to the right.)  Increasing the load up top (raising center of gravity) and decreasing the "balast" by emptying the hull (lowering the center of buyoancy) may bring the center of buoyancy below the center of gravity, and the boat will become unstable on the water.  (See left boat in image on the right.)  This is also what happens when the melting ice on the submerged part of an iceberg causes the relationship between the berg's center of buoyancy and center of gravity to change:  when the iceberg becomes "top-heavy" it rolls over so the center of buoyancy is once again above the center of gravity.

In the human body, the center of buoyancy is located just above the center of gravity.  (See image to the right.)  This is the reason why a person at rest in the deep water will float mostly upright, with the upper body on or close to the surface and the pelvis and legs hanging down.  (See images to the left.)  For most people, the amount of buoyancy in a vertical position in the deep water does not cancel out the force of gravity, which is why for Aquatic Exercise classes in the deep water, buoyancy support is needed.

Floatation devices such as Aqua-Joggers® or Hydrofit© cuffs help to keep the body afloat, so the Aquaciser can concentrate on performing beneficial exercise moves instread of fighting to stay on the surface.  It is not recommended to exercise in the deep water without flotation devices, as the struggle to stay afloat will compromise good posture and correct performance of movement.

Some floatation devices may change the relationship between those centers.  Hydrofit© cuffs worn at the ankles not only increase the amount of buoyancy but lower the center of buoyancy to below the center of gravity.  (See image to the right.)  This makes it more challenging for the exerciser not only to keep balance, but also to stay vertical and keep the cuffed ankles (place of increased buoyancy) below the body (center of gravity).

Go to the next chapter:  Resistance
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