Q.I participate in aerobics four times a week and have recently added Nautilus training to my exercise program. I am female and 28 years old. My main objective is to tone the muscles and trim my hips and thighs. I have two questions. Why do my clothes feel tighter? I'm trying to lose size, not gain. Also, should I increase the number of repetitions I perform on the Nautilus equipment?

M.P.

Waldorf.

A. Don't be alarmed. You seem to be involved in a sound exercise program if it's being performed properly. The different feeling of your clothes could be attributed to a number of things:

* First, are you burning fewer calories? Even with your exercise routine, are you less active than you were before? Have you changed jobs to a more sedentary post?

* Second, have you increased your caloric itake recently? Are you eating more and adding some pork in those places where your clothes feel tighter? Have you added a morning doughnut? Or perhaps you weren't drinking those two cans of soda (150 calories each) before. Doublecheck for any changes in your normal eating habits.

* I'll assume those two reasons don't apply to you. A third possibility is a new awareness of the muscles you've been exercising. I'm willing to bet that before you started strength training, you were unaware of your body's many major muscle groups. By exercising them, you've become aware of where they are, what function they serve and the discomfort involved in working out.

You are now aware of the quadriceps on the front of the thigh. You've experienced the burning sensation while performing the leg extension exercise. You notice muscle fatigue and soreness.

My point is this. It's possible that your muscles are the same basic size now as they were before you began your strength program. Remember, not many women have the genetic ability to significantly increase muscle size. Because of your new awareness (both mentally and physically), it may simply be that you're more sensitive to your muscles -- and they're more sensitive to the feel of clothing.

* Fourth, and finally, it could be that your muscles have increased in size. The only way to know for sure if it's muscle gain and not fat would be to have yourself weighed underwater (which evaluates fat and muscle percentages), or have an extremely competent person measure you with skin calipers. To make a reliable comparison, you should have have been measured before you started lifting.

I was involved with a study at West Point to determine the effects of a very intense strength training program on women. All of the women in the study increased their strength significantly. However, as most studies in this area support, there was no significant increase in the size of the muscles. A few of the wome gained some weight, but it was fat, not muscle.

At any rate, so what if you did add a couple of pounds of muscle? Most women need to. Don't be afraid of your muscles. Be concerned about your bodyweight, but be more concerned with the percentage of fat. Women's fat percentage is much higher than men's, so adding three or four pounds of muscle would help the appearance of most women, if appearance is a concern. Realize that it's possible for you to lose some fat, gain muscle and not have your bodyweight change. If I were you, I'd be eager to swap a pound of fat for a pound of muscle.

I've observed many women who are slim, yet look soft and flabby because of poor muscle tone. By contrast, look at the musculature and body type of women gymnasts. Most watch what they eat and are also involved in strenuous lifting programs. You can see their well- toned muscles while they're exercising and performing routines. But they look very normal when they're relaxed. That's because most women have a thin layer of fat covering their muscles.

Society has accepted the lean, firm, athletic look. So evaluate yourself physically. See if the above information explains why your clothes seem to be tighter. Ask your friends if you're looking fatter. Are you developing that Neanderthal Man look? Or are your muscles basically the same and you're suffering from Mr. America phobia?

Your second question dealt with the number of repetitions you should perform while working out. I recommend about 12 reps, but up to 15 is okay if you're comfortable with that. Don't go any higher, however.

Continue your exercise program. Remember that we all have individual genetic differences in the way we store fat and add muscle. Continue your existing program. You'll feel better and look better because of it. Diana Ross was right when she sang, "I want muscles."