Weightlifting increases muscle density. Underlying your breasts are your pectoral major and minor muscles. Breast tissue, in front of your pecs, contains no muscle at all, but rather is composed of connective tissue and mammary glands -- called fibroglandular tissue -- and fat. Exercising can increase the density of the muscles behind your breasts, but your actual breast tissue is not affected by exercise.
Factors Influencing Breast Density
Hormones, pregnancy and weight affect the density of your breasts. Pregnancy and lactation hormones increase breast density, as does weight gain. Hormonal changes accompanying menopause generally lead to reduction in breast density.
Breast Density Differences Among Women
Breast density is described as the comparison of fibroglandular tissue to fatty tissue. The Monroe County Medical Society assigns descriptors to breasts based upon percentage of fibroglandular tissue: fatty -- less than 25 percent fibroglandular tissue; scattered tissue -- 25 percent to 50 percent; heterogeneously dense -- 50 percent to 75 percent; and extremely dense breasts -- more than 75 percent.
Exercises for Pectoral Muscles
Although you cannot increase breast density through weightlifting or other exercise, you can give your breasts the appearance of increased size and density by bulking up your pecs. According to the American Council on Exercise, the barbell bench press increases chest muscle size more quickly than any other weightlifting exercise, but you need a spotter when performing barbell bench presses. When you exercise solo, the dumbbell bench press is safer and provides a good workout for your chest muscles, increasing pectoral muscle size when performed on a regular basis.