Strength training sculpts the contours of your body and strengthens the bones within. By building lean body mass, it boosts your metabolism and your energy levels, making you resistant to the slow down that occurs with age. A well-designed exercise program that includes weight training will impact your weight, health, fitness and well-being for decades to come.

At 20: A 20-year-old woman who does not lift weights will lose about 6 pounds of muscle and gain 5 pounds of fat by age 50. Subtle changes begin to occur in your body composition that are not reflected on the bathroom scale. Even though you maintain your weight perfectly over time, if you do not lift weights your lean body mass begins to decline and your body fat increases.

Body composition refers to the quality of your weight (what proportion of your weight is fat) as opposed to the quantity of your weight (total pounds). The scale cannot differentiate between fat pounds and muscle pounds so someone who appears to be thin can qualitatively be measured as being “over fat.”

“Skinny fat” is also called “normal weight obesity.” As many as 10 percent of the population may be skinny fat according to a recent study done by the Mayo clinic. High percent body fat changes your cholesterol and blood sugar, putting you at risk for heart disease and diabetes.

BMI, Body Mass Index, which is commonly used as a measure of healthy weight, fails to differentiate between high body fat and lean muscle weight. Generally, the quality of your weight is more important to your overall health than your scale weight.

At 30: Strengthening the muscles benefits the bones as well. Now is the time to put “bone in the bank” to fortify against the natural loss of bone that occurs gradually with age. By age 25-30 you’ve achieved your peak bone mass, the highest bone content you’ll have in your lifetime. Although bone continues to renew itself, from this time on you will experience a natural decline in bone density which accelerates at the time of menopause before leveling off again.

The best bone-building exercises maximize the load to the bones with a progressive (i.e. gradual intensification) program of weight bearing aerobic exercise and weight lifting. Assuming your joints are healthy, you should aim for high impact aerobic exercise (defined as activities in which both feet are off the ground, such as running, jumping rope or gymnastics) and high intensity weight training, using the heaviest weights you can lift in good form for 8-12 repetitions.

At 40: Turning 40 is a wake-up call, as many women begin to notice changes in their bodies that sound the alarm. You may be perplexed by creeping weight gain and stubborn belly fat. Around age 40, most women start to lose bone and muscle causing a decrease in metabolism of about 3 percent every decade.

Metabolism refers to the cellular processes that convert food into energy. A person with a “fast metabolism” utilizes calories more quickly, sometimes making it easier to resist excess pounds.

Your resting metabolism (RMR) is the energy it takes to simply sustain your basic bodily functions. It accounts for 60-75 percent of your total daily calorie burn and depends on several factors: your build, weight, genetics, hormone levels (e.g. thyroid and insulin) and muscle mass.

Two women of the same age, height and weight can have very different RMRs, which is why some can eat whatever they want without putting on a pound, while others seem to gain weight just be looking at food.

Strength training keeps you lean by building muscles. Lean body mass is metabolically more active than fat, burning more calories as you breathe, digest food, even as you sleep. Each pound of muscle that you gain raises your metabolic rate by about 50 calories per day, so if you were to gain six pounds of muscle instead of losing it (as in the example of the 20-year-old above), you would burn 300 more calories each day.

At 50: What causes midlife belly? The average weight gain during perimenopause is 10 pounds, and there is a natural tendency to store fat in the abdominal area. The combination of age, hormones and stress all contribute to belly fat. With age, a woman’s level of estrogen declines and the male hormone, testosterone, becomes more prominent. This causes fat to migrate to the gut from other parts of the body. Stress reaction has a similar effect on fat distribution as it releases another hormone, cortisol, which also encourages fat storage in the belly.

Apples vs. Pears Waist circumference is a good measure of central obesity. Visceral fat, deep inside the abdomen and surrounding the organs, is linked to heart disease, diabetes and other health problems. To measure: first thing in the morning, at the narrowest part of your waist. Breathe naturally – don’t suck in your stomach. Anything above 35” (89cm) is high risk for women; above 39” (99cm) is high risk for men.

At 60: At around age 45-50, you start to lose about 1.5 percent of strength each year, or about 10 percent per decade. At 65 or 70 the loss speeds up to 3 percent per year or 30 percent per decade. The loss of muscle fibers causes you to slow down, lose strength and vigor. Specifically, the fast-twitch muscle fibers, which provide for rapid, high-intensity movement, shrink in size, causing not only a loss of muscle mass, but also a loss of power.

Is it possible to re-vitalize energy levels? Sarcopenia is the technical name for the natural loss of muscle strength with age. Exercise, and specifically strength training, is the best way to restore muscle, reviving your strength and energy levels. Only 10 percent of Americans even claim to be doing any strength training.

At 70: In the U.S. 2/3 of women over the age of 75 can’t lift 10 pounds. Strength training makes you stronger, more stable, more active and energetic. Weight training creates stability in the large muscles of the legs, helps balance and stability. It restores fast twitch fibers and mitochondria, the power engines of the cells, to stimulate muscle growth and repair. It helps maintain healthy joints and prevent falls and fractures.

Studies show that you are never too old to begin a weight-training program and that lifting weights can improve your quality of life even into your 80's and 90's. Strong people are more active and self sufficient.

Much of what we consider the aging process – the loss of strength, stamina, bone density, balance and flexibility – is actually due to inactivity. A program of regular, moderate physical activity that includes strength training will preserve a more youthful functional age and increase your capacity for life