Cross-training won't necessarily guarantee top running performance. After all, some of the best runners in the world don't bother with it; it's tough to imagine a Stairmaster in one of the Kenyan villages that have produced so many champion runners. All the same, low-impact cross training can give your legs a welcome rest from pavement-pounding running -- a good idea if you're recovering from an injury or in the post-race phase of your training cycle.
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Since running tends to neglect some muscle groups while strengthening others, certain cross-training routines can correct muscle imbalances that might otherwise result in injury. While running exercises the muscles in the back of your legs, for example, biking does the exact opposite, and can keep your legs from getting out of whack and promote total fitness from the waist down.
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Perhaps even more important is the simple fact of variety. Psychologically, the monotony of a relentless training program in a single sport can be draining. Some runners become such a slave to their running schedule that their favorite sport begins to seem more like work than than the fun it's supposed to be. Cross-training can break things up and add some spice to your routine.
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Of course running is always the best training for the runner, but it's also wise to remember that running is not the be-all and end-all of fitness. It's perhaps the best aerobic exercise around, and it's great for certain leg muscles, but running doesn't help other leg muscles at all and leaves the upper body untouched. Other sports can improve muscular fitness in those neglected areas while also maintaining your aerobic fitness. While you can pursue a weight-training program to address the muscle groups of your choice, you can also target specific muscle groups by choosing the right cross-training exercises:
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