Heard any bad jokes lately?

I have. But my joke doesn't have a punch line. It's a saying that, if accepted as the truth, is surely no joking matter.

"Forty is the new 30."

It's supposed to indicate that people are not only living longer but also aging better. For too many middle-aged Americans, however, this is totally untrue.

For them, a more accurate saying is "Fifty is the new 75."

Research released earlier this year by the Rand Corporation, a nonprofit group working with National Interview Survey data, showed that more middle-aged Americans are less mobile than ever. For instance, when responses from 50-to-64-year-olds were compared in a 10-year period ending in 2007, more reported a difficulty standing for two hours at the end of the survey than the beginning.

By 2007, 40 percent of surveyed 50-to-64-year-olds a time that some consider the prime-of-life indicated a difficulty with one or more physical functions.

Like standing. Or walking. Or breathing.

Linda Martin, the study's lead author, noted in a news release that this trend means significant problems for these people as they continue to age, problems that will diminish their quality of life and cost them and society a substantial amount of money to treat.

Becoming immobile just about the time you retire is something you should want to avoid at any cost. So how can you keep that from happening?

Stave off sarcopenia now. Start lifting weights.

Sarcopenia is the technical term for the natural loss of muscle mass that results from disuse and aging and almost always factors into the aforementioned problems with physical function. For sedentary individuals, it typically begins around age 30 at the rate of about a half percent loss per year. By 45, the rate is 1 percent for all those but the most devoted weightlifters.

This loss can be mitigated, however, through a combination of healthy eating and lifting weights.

With this combination, anyone including previously untrained people in their 80s can increase muscle mass.

Here's why maintaining muscle mass is so important as you age:

Lifting weights improves bone density

For years, osteoporosis has been a feared age-related disease and for good reason. If you're severely afflicted, a stumble and fall can mean more than a broken leg or a fractured hip. It can mean spending the remainder of your days bedridden.

While osteoporosis normally comes from a lack of calcium from a poor diet (often early in life) and an overall lack of exercise, sarcopenia makes it worse.

Will Brink, in "Sarcopenia, the undiagnosed epidemic," found at BrinkZone.com, says, "Muscles generate the mechanical strength that keep our bones healthy." But Brink adds a caveat that research supports: "Aerobics does not build muscle and is only mildly effective at preserving the lean body mass you already have."

A better form of preservation and the only way to add muscle from middle age on is by progressive resistance, which is best accomplished with free weights though weight machines exercise bands can be effective, especially for those over 60.

Lifting weights burns calories in more than the expected way.

It's expected that any exercise will burn calories during the workout and even for a bit after (though experts are at odds at just how significant the "after burn" is), yet there's a second way in which lifting weights burns calories.

Muscles need calories

The more muscles you have, the more calories you can eat while maintaining your present weight.

To illustrate, let's take two overweight males of the same weight, put them on the same diet, and have them exercise for 45 minutes four times per week. In each case, the expected will occur. Both will lose weight, regardless of the type of exercise performed.

Initially.

But if one walks or bikes or even runs, muscle mass does not increase. Sometimes the original amount is maintained, but more likely an overall weight reduction produces a reduction in muscle mass.

While a beginning runner may notice more muscle in the calves and quads, he or she usually loses more in the arms, back, and chest. Something similar happens to beginning walkers and cyclists.

As a result, the dieter who does aerobics eventually reaches a weight-loss plateau quicker than the dieter who is lifting weights.

Yet when the lifter does hit a weight-loss plateau, it's not necessarily bad. That's because the plateau is the result of additional muscle mass negating the loss of body fat.

The plateaus appears only on the scale. That's because any time you swap 10 pounds of fat for 10 pounds of muscle, your appearance changes for the better.

And so will your health.

But all the lifting in the world won't amount to much muscle gain without a decent diet.

While middle-aged Americans typically get more than enough protein to maintain and build muscle, a number of studies have shown protein deficiencies in Americans who are older. Without protein, muscles will not grow.

And without a sufficient amount of carbohydrates senior citizens often don't eat as much as they should partially because of a diminishing sense of taste muscle is often catabolized and used as energy. In times of trauma, such as a broken bone from a fall, muscle mass will also be used if protein is lacking in your diet.