Q. What if I am physically unable to exercise due to a medical condition?
There is virtually no medical condition that will keep you from doing any type of exercise. Even people with congestive heart failure -- who were long told not to exercise at all -- can benefit from moderate amounts of activity.
And people with limited mobility can often do water exercises, or do yoga or other exercises while seated in a chair (some "chair exercise" videos are now on the market). Of course, if you have any medical condition, check with your doctor before starting an exercise program.
If you have questions about your condition or are still not sure what exercise you can safely do, please consult your physician.
Q. What is interval training?
In interval training, you alternate between bursts of higher-intensity exercise and periods of less-intense exercise (or "active rest"). As you get more fit, you decrease the "rest" time and increase the high-intensity periods. You'll see big fitness gains if you train this way regularly.
For example, if you now run for 30 minutes at 6 mph, try this routine: Jog for five minutes to warm up. Then, increase your speed to 6.5 mph for one to two minutes (less if you can't go that long). Then, jog for a few minutes at your normal speed, then again at the faster speed, and so on until you reach your time limit. Your ratio of work to active rest would be 2:3 if you ran for two minutes at 6.5 mph, then jogged for three minutes at 6 mph.
You can also use your heart rate to set intervals. For example, if your heart rate hits 70% of your maximum when you jog at 6 mph, start at that speed. Then increase either your speed or elevation (if you're on a treadmill) to get your heart rate to 85% or 90% of maximum for one to three minutes. Then, go back to jogging at the 70% heart rate, and continue alternating.
As your fitness improves, your heart rate will be lower at the higher speeds, and then you can spend more time at those speeds. A good starting ratio of work to active rest is 1:3; you can always vary the ratios if they turn out to be too hard or too easy.
I recommend interval training just once a week to start, as it is more intense than you may be used to. Once you get a feel for it, you can do it more often.