Water is a wonderful environment to perform exercise. Most people who have splashed around, floated or swam in a pool of water are keenly aware of the buoyancy and resistance to movement that water provides. Buoyancy and drag forces, combined with water pressure, are some of the unique characteristics of water environment that make it such a useful and popular setting for exercise and rehabilitation.
When standing chest-deep in water, a person “weighs” only about 30% of what they would weigh on dry land. This is due to buoyancy, which is an upward force that acts in the opposite direction of gravity. Standing in waist-deep water, a person would “weigh” 50% of their dry land weight. The “unloading’ affects of water allows individuals who have limited weight bearing (for example due to a leg fracture or knee surgery) to walk or even run in the pool.
The buoyancy of water also causes decreased activation of muscles, especially those used in balance. This is one of the reasons that muscle activity while walking in water is only about 70% of that observed on dry land. The amount of muscle activation can be increased simply by increasing the speed of the activity (such as walking). This increased muscle activity is due to the drag force of water, which acts acts in opposition to all movements in all directions. Just a little increase in speed can cause muscles to work a lot harder.
A person tends to walk more slowly in water (vs. dry land) due to drag forces, yet take significantly longer strides due to buoyancy. One of the benefits of longer strides is that muscles are worked in a greater range than with dry land walking. Buoyancy also makes it easier for a person to walk normally, providing an opportunity to strengthen muscles in preparation for normal dry land walking.
The direction a person walks in water also affects muscle activation differently than dry land walking. Walking backward causes the quad and spine muscles to work harder, while reducing stress to the hip, knee and ankle joints. Walking forward in water results in greater hamstring and abdominal muscle activation, along with reduced stress to the hip, knee and ankle joints.
There are also benefits of exercising the upper limbs in water. The buoyancy of water allows the arm muscles to relax, providing a gravity-free exercise environment. As long as movements are slow the amount of stress to the shoulder joint is about equal to that of passive movement on dry land. Increasing the speed of arm movement can result in muscle activation equal to or greater than similar exercises on dry land. Compressive forces on upper extemity joints, like the lower extremity, are 30 to 80% less compared to dry land exercises.
Another benefit of exercising in the pool is water pressure. Just as wrapping a knee with a compressive bandage can help reduce swelling (edema), the increased force of water pressure (compared to air pressure at sea level) can help prevent and reduce swelling.
Overall, water is an extremely beneficial environment to utilize for exercise. It can allow early exercise (including weight bearing), decreased joint compression, increased range of motion, reduced joint swelling, and a wide range of muscle activation. Also, and perhaps most importantly, water can provide a pain-free environment for exercise.