Resistive forces

When an object moves in the water, just as on land, several resistive forces are at work that must be overcome. These forces include the cohesive force, the bow force, and the drag force.

COHESIVE FORCE. There is a slight but easily overcome cohesive force that runs in a parallel direction to the water surface. This resistance is formed by the water molecules loosely binding together, creating a surface tension. Surface tension can be seen in still water, since the water remains motionless with the cohesive force intact unless disturbed.

	The bow force
BOW FORCE.A second force is the bow force, or the force that is generated at the front of the object during movement. When the object moves, the bow force causes an increase in the water pressure at the front of the object and a decrease in the water pressure at the rear of the object. This pressure change causes a movement of water from the high pressure area in the front to the low pressure area behind the object. As the water enters the low pressure area, it swirls into the low pressure zone and forms eddies, or small whirlpool turbulences. These eddies impede flow by creating a backward force, or drag force.
Drag force

DRAG FORCE. This third force, the drag force, is very important in aquatic therapy. The drag force on an object can be controlled by changing the shape of the object or the speed of its movement.

Frictional resistance can be decreased by making the object more streamlined. This change causes less bow force and less of a change in pressure between the front and rear of the object, resulting in less drag force. In a streamlined flow, the resistance is proportional to the velocity of the object. Therefore, to decrease the resistance for a weak athlete, exercises should be performed slowly in the most streamlined position possible.

On the other hand, if the object is not streamlined, a turbulent situation exists. In a turbulent situation, drag is a function of the velocity squared. Therefore by increasing the speed of movement 2 times, the resistance the object must overcome is increased 4 times. This provides a method to increase resistance progressively during aquatic rehabilitation. However, increases in speed also affect stability adversely. Sin­gle limb movements are generally not as affected as trunk exercise by this loss of stability. Considerable turbulence can be generated when the speed of movement is increased, causing the muscles to work harder to keep the movement going. This is especially true when changes of direction take place. Therefore, by simply changing the shape of a limb through the addition of rehabilitation equipment or increasing the speed of movement, the sports therapist can modify the athlete's workout intensity to match strength increases.