DEAR DR. DONOHUE: The rules for exercise change so often that I’m completely confused. Do you have suggestions on what constitutes the best exercise program for a middle-aged male who wants to live longer and healthier? – R.T.

ANSWER: The age of fitness dawned in the 1960s with the introduction of aerobics – continuous exercise of large muscles for a specified period of time without any rest. The large muscles were usually the legs, and the suggested exercise was running or jogging. At that time, we were told to exercise for 20 to 60 minutes, three times a week, at a heart rate that was 70 percent to 80 percent of maximum heart rate. Maximum heart rate is obtained by subtracting age from 220.

In the 1970s, the advice changed a bit. Then we were told to exercise at a heart rate 60 percent to 90 percent of maximum heart rate, three times a week, for 20 to 30 minutes.

The latest change makes exercise more in line with reality. Now we’re told to get 150 minutes of moderate-intensity exercise a week. Those 150 minutes can be a little more than 20 minutes every day, or 30 minutes in five days, or 50 minutes for three days. Furthermore, the exercise sessions can be broken down into 10-minute segments with a rest between, or they can be done at different times of the day.

Moderate intensity is walking at a pace that raises your heart rate and breathing rate some but doesn’t make you pant. If you want it defined more precisely, it is walking at a rate of 3.5 miles an hour, one mile in 17 minutes.

Exercise doesn’t have to be formal walking. It can be anything that is physical activity. Raking the lawn and directing a power lawn mower are considered moderate-intensity exercise. Dusting is just a little less than moderate intensity. Heavy cleaning is above moderate intensity.

If you’re up to vigorous exercise, then you need only 75 minutes of it in a week’s time.

DEAR DR. DONOHUE: I am a 67-year-old male trying to stay ahead of the curve by doing cardio and strength exercise at my local YMCA. After several months of treadmills, stationary bikes and the StairMaster, I felt renewed energy. However, I now have a leg problem, and I venture to say it’s the infamous hamstring muscle. It forces me to hobble at times. What is the best remedy for it? – Anon.

ANSWER: The hamstring muscles are the muscles on the back of the thigh. They bend the knee. The front thigh muscles, the quads, are 1.5 times stronger than the hamstrings, and that inequality contributes to hamstring injury.

The most common hamstring injury is a pulled muscle. A pulled muscle indicates some muscle fibers have been torn. The tear can be minor, or it can produce a gap in the muscle that can be felt.

Most hamstring pulls come from sudden accelerations in sports like basketball, football and tennis. There is immediate pain in the back of the thigh, and the injured person often hears a popping noise or experiences a tearing sensation. The usual treatment of a hamstring pull is the RICE regimen: Rest, Ice, Compression with an elastic wrap, and Elevation of the leg. I would enlarge on this, but I’m not sure I’m doing you any good. Things other than a hamstring pull can produce pain in the same area – stress fractures of the thigh bone, sciatica or back disk pressure on nerves serving the thigh area.

The best advice I can give you is have your leg examined by a doctor. I doubt the hamstring diagnosis.

DEAR DR. DONOHUE: I have a problem with my legs when I go for my mile and a half walk. I get severe pain in my left calf, which lasts about half a mile and then disappears. What is the cause? – M.P.

ANSWER: If the pain comes while walking, and if it comes on at the same distance and if it disappears with rest, the cause might well be a blockage in one of the arteries that serve the leg muscles. This is peripheral artery disease. It also might be sciatica, spinal stenosis or a protruding back disk.

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.