When a body is immersed completely or partially in water it experiences an upward thrust equal to the weight of the water it displaces. It appears that your body is lighter in water than on land.
A body in water has two opposing force buoyancy and gravity. The force of gravity relates to a body’s density - the more compact or dense a body is the less buoyant it is.
The centre of buoyancy is the chest area and the centre of gravity is the hip area. If the body is suspended in water i.e. deep water running, it rotates around the centre of buoyancy rather than the centre of gravity. Centre of buoyancy and gravity must be kept in a vertical line for equilibrium and alignment.
Buoyancy is determined by:
Body Type (Lean body mass sinks, Body Fat floats)
Body Fat distribution (Upper body = vertical floater, Lower body = horizontal floater
Air in Lungs (more air in lungs = more floatation)
Buoyancy also affects movement in water:
Buoyancy Assisted = a move in the same direction of buoyancy i.e. arm lift or jump up.
Buoyancy Supported = water supports the body or extremity i.e. a person standing in water and experiencing buoyancy support.
Buoyancy Resisted = a move in the opposite direction to buoyancy force i.e. returning limbs from flexion.
So when a limb moves towards the surface = buoyancy assisted
Stays at the surface = buoyancy supported
And returning = buoyancy resisted.
Buoyancy gives water a cushioning effect. There is a 90% apparent weight loss in shoulder depth water; participants are able to exercise with less biomechanical stress during each foot strike. This is important if participants are suffering from injuries, arthritis etc. as it reduces the stress on their joints.
The water slows down a movement returning to the pool bottom cushioning the impact stress.