DEAR DR. DONOHUE: I know that in any weightlifting program you’re supposed to keep doing more and more work, but how do you go about doing that? There is a limit to what a human being can do, right? – P.T.

ANSWER: There is a limit to what humans can do. There’s a limit to what all creatures can do. A cheetah can run at a pace of 70 miles an hour, but even if it practices day in and day out, it’s not going to run much faster than that.

For a body to make progress and for muscles to grow larger and stronger, they have to be constantly challenged. You can do that in a number of ways. You can vary the amount of weight you lift, the number of times you lift the weight, the amount of rest you take between sets of lifts, and the amount of time you take to lift the weight up and the amount of time it takes to return the weight to its starting position. You can also vary the kinds of exercise you do.

Take the amount of time taken to lift the weight up and lower it down. It usually takes two seconds to raise the weight as high as possible. It should take four seconds to lower it. A controlled, slow downward return of the weight to the starting position is more important to muscle growth and strength than is the upward lift. When lowering the weight, muscles are lengthening. That’s an “eccentric” contraction, which is a greater stimulus for muscle growth than is a “concentric” contraction, muscle shortening during the upward lift. Vary the up and down times.

Another important variable is the amount of weight lifted. See how much weight you can lift once. That’s your one-repetition maximum, 1 RM. A standard program is to take 60 percent to 80 percent of that weight and lift it eight times in three consecutive sets with a rest between each set. When you can lift 12 times, add more weight and start over. Or you can lift 85 percent of your 1 RM, but lift it only three to five times. Or you can take 30 percent to 50 percent of your 1 RM, and make 12 to 15 lifts.

The variations you introduce into your program give you enough material to challenge your muscles over a lifetime of exercising.

DEAR DR. DONOHUE: I take a beta blocker for essential tremor. It is my understanding that a beta blocker slows the heart rate. If I do cardio (heart) exercise and am expected to raise my heart rate, does the beta blocker affect that goal? Does a beta blocker have a long-term effect on the heart in terms of health if the heart cannot be made to beat fast? – D.M.

ANSWER: You’re correct. One of beta blocker medicines’ effects is heart slowing. That makes it impossible for people on a beta blocker to use heart rate as a criterion of exercise intensity.

So forget heart rate as a measure of intensity. All you have to do is honestly evaluate how hard you consider you are exercising on a scale that ranges from extremely light to extremely hard. Aim for an intensity you would say is somewhat hard. That indicates you are exercising at a heart rate of a 120 beats a minute. That’s as hard as you need to get in order to exercise your heart.

Long-term effects of beta blockers on the heart rate have no effect on heart health other than good.

For readers, heart rate and pulse are the same thing.

DEAR DR. DONOHUE: I am 34 and began walking to lose weight. I lost the weight, but I have continued to walk. Where I walk, there are many people, much older than I, who whiz by me. I have tried to run, but I get too winded to continue far. What can I do? – D.J.

ANSWER: You’ve changed your goals. Now you want to be a runner. Start the change by mixing in a little running with your walking. Run for a short distance, a distance that doesn’t make you breathless. Then shift back to walking.

As the weeks go by, increase the time you run and decrease the walking time. You’ll be running the whole route within a year.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.