DEAR DR. DONOHUE: I remember reading something in your column about the rest interval between sets of lifting weights. I thought it was interesting then, but I wasn’t into weightlifting at that time. I forgot what you said.

 Now I am lifting weights and ask you to repeat that advice. Thank you. — B.R.

 ANSWER: The rest interval time between sets of weightlifting is important for at least two reasons. If the interval isn’t long enough, the muscles haven’t recovered enough energy to sustain a training effect, and too little rest leads to muscle breakdown. If the interval is too long, the muscles are not stimulated enough to encourage growth. Furthermore, if you’re exercising in a gym with shared equipment, a too-long rest irritates others.

 This question involves some arcane aspects of exercise physiology. This includes energy systems called ATP, adenosine triphosphate system, and PC, phosphocreatine system. Both are energy-producing molecules that supply muscles with their initial burst of power.

 A full five minutes of rest between weightlifting sets restores 99.9 percent of these energy molecules. A set is a series of consecutive lifts taken with no break. Generally, it is eight to 12 lifts. With two and a half minutes of rest, 95 percent of ATP and PC is regenerated. In one minute, 75 percent is back. A two-minute rest is considered adequate time.

 Other experts feel that a ratio of 3-to-1 is a good rest-to-exercise formula. For example, if lifting takes one minute, the suggested rest time before lifting again is three minutes. If lifting takes two minutes, then a six-minute rest is prescribed. That sounds too long to me. You’d spend most of your gym time sitting.

 Bodybuilders, striving for muscle growth and muscle size, adopt an exercise program with relatively short rest breaks between exercise sets. Strength athletes take longer rest periods. Muscle size and muscle strength are not the same.

 DEAR DR. DONOHUE: Does jumping rope burn three times the calories that jogging does? I have tried jumping rope, and I am winded after only a few minutes. I can jog for more than 30 minutes. I believe this is true about jumping rope. — M.J.

 ANSWER: In all exercise, calorie burning depends on the strenuousness of the exercise, how much work it requires. Jogging can be as demanding as rope jumping. It depends on the pace of each exercise.

 For a novice rope jumper, jumping rope is exhausting. The person has to learn how to balance the body, and many muscles are involved in doing that. Jogging is a more natural motion for people, and the chief factors involved in calorie burning include the jogging speed and the terrain — hill jogging versus straight-path jogging.

 Jogging at a rate of a mile in eight and a half minutes — a fairly brisk pace — is equivalent to jumping rope at a speed of 100 to 120 turns of the rope a minute.

 Starting a rope-jumping program is very tiring. Practice reduces the effort. However, even when a person has learned good jumping skills, it’s still a hard exercise. A beginner shouldn’t attempt more than a few minutes of jumping at first, and the speed of the rope turning ought to be about 60 to 70 turns a minute.

 DEAR DR. DONOHUE: Do you risk kidney and liver damage if you put a heating pad on your skin after you’ve rubbed it with ointment for sore muscles? I heard this from a person who has been an athletic trainer for many years. — K.V.

 ANSWER: I do see how it would.

 Many compounds used for sore muscles irritate the skin. The warmth you feel is skin irritation. Transmission of those sensations to the brain blocks the transmission of pain signals. You should not put a heating pad on irritated skin.

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.

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