Pool workouts make such effective cross-training tools for runners -- and all athletes, really -- because they apply resistance to every motion. On land, your muscles work hard to raise your legs and arms, but gravity does most of the work on the way back down. Since water pressure is 12 times greater than air pressure, you'll work muscles in the pool that don't see much action on land. The result for runners is greater overall strength and fitness, which translates directly into increased stamina and speed.
Writer Marc Bloom says that water workouts provide the "resistance to stress the body and the liquid density to protect it" [source: Bloom]. There are no jarring movements in a pool. Everything happens in relatively slow motion. Since the body weighs only a fraction of what it does on land, there's less shock to absorb.
Running in hot weather is exhausting, even dangerous. After 15 minutes of summer running, your body temperature raises five degrees. Add humidity into the mix and your sweat can't evaporate quickly enough to cool you down, leading to heat exhaustion and even heatstroke [source: American Council on Exercise]. But things are always cool in the pool. The cushion of cold water means less sweat, less water loss and almost no risk of overheating. For many runners, the cool water also has an invigorating effect, especially on days when they don't feeling like working out [source: Wischnia].
Pool running is perfect for athletes recovering from a chronic or acute running-related injury like shin splints, stress fractures and plantar fasciitis [source: Wischnia]. This enables serious runners to stay in shape, while giving their sore bones time to fully mend. Runners should note, however, that some injuries can be aggravated by pool running [source: Bloom]. As a general rule, if your injury still bothers you in the pool, stay out of the pool and off your feet.
Since pool workouts are lower impact than street running, it offers an excellent supplement or running alternative for older runners who suffer from arthritis. It also works for pregnant runners.
To get the most out of your pool workout, you have to learn the right pool running technique, which we'll talk about next.