Cross-training refers to participation in physical activities that are different from your main sport. Any activity you choose when cross-training should improve your cardio fitness, strengthen your muscles, increase your flexibility or help speed recovery from an injury. By adding variety to your workouts, it should also prevent boredom and burnout.

You can substitute approximately 25% of your weekly mileage workouts with cross-training activities to supplement your running routine. Some activities are complementary to running, meaning they use similar muscles as you do when running but in different ways, while other activities can be selected to build strength and flexibility in muscles that you don’t often use to prevent muscle imbalances.

For example, to strengthen muscles that benefit runners, choose activities such as biking, tennis, elliptical training and cross-country skiing. To build strength in areas of the body that may be lacking as a runner, choose activities such as swimming or weight-training.

No matter what type of activity you select, a well-rounded running routine should include cross-training to prevent overuse injuries while keeping you aerobically fit. Cross-training can also improve muscle strength and flexibility in areas you don’t typically use as a runner, making you less prone to injury when you do run. It’s important to note, however, that cross-training is not something you do when your running schedule says it’s a rest day. Rest is rest – and your body needs that, too.

There are many fitness activities you can choose from when you’re looking to cross-train. Here are three great options worth trying:

Swimming

Swimming is a great cardio workout that is low-impact and has a relatively low risk of injury. You can build your endurance and get a solid aerobic workout without the pounding forces that impact your lower body when you run.

Working out in water also helps build strength and flexibility. Additionally, swimming works your upper body, specifically your shoulders and arms. These muscles are typically weak in runners.

Biking

Riding a bike works muscles in your lower body just like running, but without all the impact that comes with running. It also provides you with a great aerobic workout that builds cardiovascular endurance. There is less risk of injury when cycling as compared to running because it’s a non-impact activity.

Cycling is also beneficial for runners because it works the quadriceps, a muscle group that is not worked effectively when running. A lack of strength in the quadriceps muscles can lead to knee problems in runners such as patella tendinitis.

Tennis

A game of tennis involves quick changes in direction and high-intensity bursts of speed. Running, on the other hand, only requires forward motion. The lateral movements of tennis help to improve agility and muscle elasticity. Strengthening the muscles needed to move side-to-side helps stabilize the larger muscles in the legs used for forward motion and can reduce the chance of injury when running.

Be aware, however, that tennis is a high impact sport. The quick starts and stops, as well as sudden side-to-side movements, can cause injury.

Sources:

http://www.active.com/cycling/articles/the-best-cross-training-for-runners

http://www.runnersworld.com/running-tips/16-cross-training-activities-to-try

http://www.runnersworld.com/tag/cross-training

http://running.competitor.com/2014/01/training/smart-marathon-training-which-kinds-of-cross-training-are-best-for-runners_48779

http://www.runningplanet.com/top-ten-cross-training-workouts-for-distance-runners.html