Swimming: Swimming is an excellent cross-training activity for running because it's not weight-bearing, so it gives your joints (which take a lot of stress when you're running) a break. It allows you to build strength and endurance, and also improve flexibility. It's a great balance for running because you'll really work your upper body, while giving your leg muscles a breather. Swimming is especially recommended for people who are prone to running injuries or are recovering from an injury. Some runners also find it very relaxing and meditative.
Water Running: Water running is a great alternative for injured runners or as a substitute for an easy running day. It's also a smart way to get in your runs during hot and humid weather. While you can run in the water without flotation aids (vests, belts, etc), you’ll find the workout to be easier with them.
Cycling or Spinning: Cycling and spin classes are also great low-impact ways to boost your cardiovascular fitness and strength, especially your quads and glutes.
Elliptical: You'll get a total body cardiovascular workout on the elliptical machine. Their oval-like (ellipse) motion provides the user with the feel of classic cross-country skiing, stair climbing, and walking all in combination. You can program the elliptical to move in either a forward or backward motion, so you can work all the major muscles in your legs. Because the muscles used on the elliptical are similar to those you use when running, the machine is a good low-impact alternative when an injury prevents you from running.
Walking: Walking is a good activity to substitute for an easy running day, especially if you're recovering from a long run or speed workout. With certain injuries, you may be able to walk pain-free, and speed-walking is a good way to maintain cardiovascular fitness while you're recovering.
Rowing: An excellent cardiovascular, low-impact activity, rowing strengthens the hips, buttocks, and upper body. Just make sure you learn proper the rowing technique to maximize the benefits of this activity and avoid injury.
Strength (or Weight) Training: Strength training allows runners to improve the strength in their running muscles, create balance between unbalanced muscle groups, and focus on keeping their legs strong during injury recovery. You can do either resistance training, where you use your own weight for resistance (pushups, for example), or weight training, where you use weights (free or machine) for resistance (leg press, for example). Strength training is an excellent opportunity to strengthen your core, which helps runners avoid fatigue and maintain their form.
Yoga: Yoga offers some of the same benefits as strength training, since you'll use your body weight as resistance to strengthen your muscles. You'll also improve your flexibility since it involves a lot of stretching. Many runners find yoga a great way to relax after a long run or tough workout.
Cross-Country Skiing: With cross-country skiing, you'll get a great cardiovascular workout and focus on many of the same muscle groups as running. You'll skip all of that the pounding on the road, so it's a great cross-training activity for runners with injuries. You'll also work on your flexibility, as the gliding motion stretches your hamstrings, calves and lower-back muscles. And if there's snow on the ground, you can always use an indoor ski machine, which provides a very similar workout.
Ice or Inline Skating: Inline or ice skating is also another no-impact sport (as long as you don't fall!) and it's a great activity if you're recovering from shin splints, Achilles tendonitis or knee injuries. You'll really work your quadriceps, buttocks and lower-back muscles.