I like to both run and swim to stay in shape. But I have noticed as I swam more that my running was becoming more of a chore, my legs started feeling heavy until I got through the first couple of miles. Any suggestions?

— Mark Weiler, Austin, Texas

I'm assuming that by swimming more you were running less. Both running and swimming are great aerobic activities, the problem is that they lie pretty far apart on the sports specificity spectrum. Sort of like playing tennis to become a good golfer. The human body, if trained properly, can adapt to just about any level of physical stress, so you can jump around from sport to sport and continue to improve your basic level of fitness and health. But if the objective is to improve your performance in a particular activity, then you have to dedicate a minimum amount of training to that sport.

To keep your running legs fresh, you should run a minimum of two, preferably three times per week. Try to vary the intensity of each run, one longer slower (at 60 to 70 percent of your maximum heart rate) run each week, one moderate intensity (at 70 to 80 percent of your maximum heart rate) and one that gets you working (at 80-90 percent of your maximum heart rate), like intervals on the track or uphill repeats. If you're swimming because you just like being in the water, then consider water running one day instead of swimming, you'll get a wet, running specific workout. You may also want to do some aggressive kicking sets during your swimming--it's not the same as running but you'll be giving your legs a little more attention, which they may thank you for by being a little less "heavy" on land. Finally, consider adding a couple of days a week of strength training, with the focus split between upper and lower body--two to three sets of 15 to 25 reps of a few basic exercises may help add a little snap to your legs.