You should be doing aerobic exercise for at least half an hour five times a week, and lifting weights twice a week, say the American Heart Association and the American College of Sports Medicine.

The two groups issued Monday revised guidelines for physical activity.

All healthy adults aged 18 to 65 need moderate-intensity aerobic physical activity for at least 30 minutes on five days each week, or 20 minutes of intense aerobic activity at least three days a week, the AHA and the ACSM said.

Healthy adults also should do 8-12 repetitions of 8-10 resistance exercises that use all the major muscle groups, according to the new guidelines. The exercises should be performed on two nonconsecutive days each week.

The recommendations sharpen slightly those made in 1995 by the Center for Disease Control and the ACSM on the types and amounts of physical activity needed by healthy adults.

The principal changes made in the guidelines were the inclusion of specific muscle-strengthening recommendations, and the explicit incorporation of vigorous-intensity physical activity.