DEAR DR. DONOHUE: I’m a running back for my school and have played on the varsity squad since my freshman year. I’m a senior this year. My question has to do with cramps that happen early in the season, when we’re practicing. I get terrible cramps in my calves. I am OK when the actual season starts. What causes cramps? Is there any way to stop them from happening? — R.J.

ANSWER: Readers should know that I am talking here about exercise-associated muscle cramps, not the cramps that happen to older people at nighttime, usually when in bed.

Every athlete has suffered from a sudden, painful cramp of the muscles, most often the leg muscles and almost always the calves or the front and back thigh muscles.

The cause isn’t known for sure. Some are convinced that dehydration and loss of sodium and potassium are to blame. Hot, humid weather, which promotes sweating, is often but not always part of the cramp picture. Athletes who sweat heavily do lose lots of water and minerals. You can tell if you’re a sodium loser if you see white deposits on your jersey when it dries. Staying hydrated might put an end to your cramps. An hour before practice, drink a quart of water, or better, a sports drink containing sodium and potassium. During practice, take a break every 15 to 20 minutes for more drinking. Weigh yourself before and after practice. If you lose more than 2 percent of your weight, you are seriously dehydrated.

Others believe cramps come because of muscle fatigue that leads to an avalanche of nerve signals that bombard muscles and cause a painful muscle contraction. Stretching before your practice sessions could prevent this from happening. For calf stretching, stand on the bottom step of a stairway with your heels projecting off the stairs. Rise as high as you can on your toes, then descend as low as you can, with heels below the stair. Do this as many times as you can.

Let me know the results, will you?

DEAR DR. DONOHUE: Please answer this as soon as you can. I am told that biking causes impotence. I bike long distances every day and have been doing so for many years. I don’t have this problem, and I don’t want to get it. The source of this information was reliable. I will stop biking if this is true. Is it? — W.W.

ANSWER: It’s possible. Sitting for long times on a hard bicycle seat compresses nerves and blood vessels needed for an erection. If this process is in its early stages, the biker gets a warning with numbness of the genitals. The numbness leaves soon after finishing a biking session.

Even if full-blown erectile dysfunction occurs, taking a rest from biking restores potency. However, the biker also needs to change the bike seat.

DEAR DR. DONOHUE: Is long-distance running good or necessary for soccer players?

My brother and I play soccer. He loves to run long distances. I don’t. He says I’ll never have the stamina for soccer if I don’t join him. What do you say? — K.N.

ANSWER: Long-distance running benefits all athletes and all people. It improves endurance by improving heart pumping action, by lowering blood pressure, by keeping weight in check and by improving breathing.

However, a soccer player doesn’t have to concentrate on it to be good at soccer. Soccer is a game of fast running for short periods intermixed with jogging or even walking. Sprint exercises are more important for this game. Running fast for 15 seconds and then slowing down to a jog for 30 seconds is a better way of increasing the kind of running needed for this game.

You can increase the period of sprinting but keep the ratio between sprinting and jogging at 1-to-2; for 20 seconds of sprinting, jog for 40 seconds, for example.

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.