Drag is the term used to identify the resistance of water to swimmers' movements through it. That resistance is caused by a difference in pressure between the water in front of and behind swimmers. Objects tend to be pushed from areas of high pressure toward areas of low pressure. Consequently, if the pressure of the water in front of swimmers is greater than the pressure behind, their speed will be decreased unless they can overcome the added pressure by stroking with greater force. The reduction in speed will be in direct proportion to the magnitude of the difference in water pressure in front of and behind the body.
Drag force is always exerted in a direction opposite the direction of movement. In other words, it is a force that opposes the movement of an object. People usually think drag as a negative force that impedes forward progress. It is certainly true that drag forces can decrease swimming speed when the resistance of the water impedes swimmers' forward progress through it. However, drag can also be propulsive. Swimmers can accelerate their bodies forward by pushing the limbs backward against the resistance of the water, just as runners propel their bodies forward by pushing backward against the ground. The major difference, of course, is that water, being a fluid, gives way when limbs push against it while ground does not. Consequently, swimming propulsion is not nearly so efficient as land propulsion. The body will not accelerate as fast or as far when swimmers push back against the water as will the body of a runner.
To sum it up, there are two types of drag forces in swimming. Resistive drag (read here how to reduce resistive drag) that holds swimmers back and reduce their swimming speed, and propulsive drag that helps swimmers accelerate their bodies forward.
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