Buoyancy in Water

This Property of Water Can Ease Rehabilitation by Reducing the Effects of Gravity

Buoyancy is defined as the upward thrust acting in the opposite direction to the force of gravity. The more deeply immersed you are in water, the less you weigh; when immersed to hip height in water, you weigh only 50% of your weight on land. The buoyant property of water is used in rehabilitation therapy to assist in movement, as resistance to movement, and to support movement on the water’s surface.


Center Of Buoyancy Vs. Center Of Gravity

What are These Centers, and Where are They Located?



The center of buoyancy is defined as the center of all buoyancy force movements. The human center of buoyancy is in the mid-chest. The center of gravity is a point at which all force movements are in equilibrium. The human center of gravity is located in the pelvic area, although the exact location can shift when the body is in different positions.

How to Use Buoyancy to Your Advantage

When both centers are aligned vertically, we can perform flotation. When the points are not aligned vertically, a rotational force results. This rotational force can help you to maintain an upright, head-out posture during aquatic therapy. These same forces also affect your limbs and become a vector continuum as the limb moves through water.

  • In chest-deep water, buoyancy decreases the amount of shock transmitted through your bones, joints, and ligaments compared with land activities. In deep-water activities, there is no impact, and therefore no shock to the body.
  • Buoyancy decreases the effects of gravity; your body weight is reduced by 75–90% with chest to neck level immersion. An individual who weighs 70 kg on land will weigh about 7 kg in neck deep water. Obese persons, injured athletes, fibromyalgia and arthritis sufferers, and those with fragile bones, as well as pre and postnatal women, will exercise with relative ease in the water.
  • Your range of motion is increased by buoyancy; this improves joint mobility and contributes to dynamic flexibility.
  • The force of buoyancy assists in venous return. On land, the downward force of gravity pushes blood toward the feet. The cardiovascular system works against gravity to return blood to the heart. In the water, the up-thrust of buoyancy counteracts the downward pull of gravity thus assisting with venous return. This will contribute to lower exercise heart rates during thermal water activities!

Archimedes' Principle

The Mathematics and Science of Buoyancy

The Buoyant Force is a very characteristic force that acts upon all submerged bodies. This is how Archimedes' Principle explains buoyancy:
“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 following mathematical equation is based on Archimedes' Principle:
Submerged body buoyancy = displaced liquid weight minus body weight.
Therefore, we may conclude that:

  • The body will float if the buoyancy is positive
  • The body will be suspended if the buoyancy is neutral
  • The body will sink if the buoyancy is negative

Freshwater vs. Seawater: How Well Can You Float?

Liquid buoyant force depends on its density, which equals its weight per unit volume:

  • Fresh water density = 28.3kg / 0.03m³
  • Seawater density = 29kg / 0.03m³


A body immersed in seawater will, therefore, be buoyed up by a greater force than a body immersed in fresh water, so it is easier to float in seawater than in fresh water.

So How Does All of This Affect Your Hydrotherapy Experience?

In simpler terms, buoyancy lets participants experience a partial floating feeling when they enter the pool. When a person is in water, the water will naturally want to push the person up. This is particularly helpful to someone who does not want to place their entire body weight on an affected joint or body part. The deeper the person goes in the water, the less weight they will have to support with their own body.

How Buoyancy Reduces Stress on Joints and Connective Tissues

The buoyancy of water can reduce your “weight” by about 90 percent depending on body composition, size, muscular structure and depth of water. For exercisers with lower body or spinal injuries, pregnant exercisers, people with fibromyalgia, and participants with physical limitations, the buoyancy of the water is a welcome benefit. The buoyancy also adds a challenge to exercisers to maintain control of their body position while in the water.

Buoyancy Can Also be Used to Aid or Resist Aquatic Exercise Movements

Exercises that move towards the surface of the water, such as biceps curls or hip flexion, are assisted by buoyancy, making the exercise easier. Exercises that move away from the surface of the water, such as triceps extensions, are resisted by the buoyancy, which makes the exercise more difficult