The search for ways to prevent muscle loss as we age goes to the very fiber of the problem
Think of it as the muscle equivalent of osteoporosis. Just as our bones tend to become weaker and more brittle as we get older, our muscles are predisposed to wither with age. Starting as early as age 30, muscle mass begins to decline by about 1 percent—about a third of a pound—a year.
If you haven’t heard much talk of sarcopenia before, it’s likely you will soon. Physicians, caregivers and even drug companies “are realizing that muscle loss is something they really need to pay attention to,” says Professor Roger Fielding, N93, who studies muscle and its mysteries at the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts.
The flood of retiring baby boomers has lent some of that urgency. The fastest-growing segment of the older population is people over the age of 85, and more of them are living independently. Yet a significant number of older adults lack the muscle strength to get up out of a chair, climb a flight of stairs or walk a quarter mile.
Not only does that limit their ability to get around, it places them at an increased risk of falling, which can lead to hip fractures, long hospital stays and a cascade of health problems.
One estimate put the direct health costs of disabilities caused by sarcopenia at $18.5 billion in 2000. No wonder the president of the International Association of Gerontology and Geriatrics recently told the New York Times, “In the future, sarcopenia will be known as much as osteoporosis is now.”
Fielding, who directs the Nutrition, Exercise Physiology and Sarcopenia Laboratory at the HNRCA, estimates that roughly 10 to 20 percent of seniors are at risk of losing some of their independence because of sarcopenia. While some amount of muscle loss is inevitable and even acceptable as we age, he believes that many disabilities could be prevented if older people were able to hold on to their muscle mass, and more specifically, their muscle function.
Use It or Lose It
Our bodies are constantly breaking down old muscle and building new muscle. But as we get older, something seems to slow down the assembly process; even highly trained athletes are less able to develop new muscle as they age. By the time they are 80 years old, marathon runners and weight lifters see their peak levels of performance cut in half.
Fielding believes there may be a disordered response to the stimuli that induce muscle growth, and has been looking carefully at a group of signaling proteins that may be involved in the confusion. Muscle cells also seem to get fattier with age. Not only do they tend to store more lipids—which would make sense if they are less adept at burning fat for energy—they actually start to synthesize more of it. The Tufts researchers have even found genetic triggers for the process.
“It means that there is a very distinct program that is activated with aging that causes the muscle to have a greater ability to make more lipid,” Fielding says. He suspects that having all that fat sitting around could be wreaking havoc with the muscle-building system.
But muscle building is only part of the picture. Although muscle mass—the actual size of those biceps, quads and other skeletal muscles that move us through our lives—begins to decline in the third decade of life, our muscle strength actually holds up pretty well, until about age 50.
“When we are young we probably have more muscle than we really need,” Fielding explains. “There is probably enough reserve for a while.”
The bad news is that after age 50, strength really begins to ebb, so that over the course of a lifetime, the loss of muscle strength far outpaces the loss of muscle mass.
Not surprisingly, people who exercise do a better job of retaining both the size and strength of their muscles as they age; the adage “use it or lose it” has plenty of evidence behind it. And even if you do lose it, there is hope of getting some of it back.
In the mid-1980s, researchers at the HNRCA broke new ground when they showed that a small group of men, ages 60 to 72, improved their muscle strength by 120 percent when they did high-intensity strength training for 12 weeks.
In the 1990s, Miriam Nelson, N85, N87, now director of the John Hancock Research Center on Physical Activity, Nutrition and Obesity Prevention, followed a larger group of 50- to 70-year-old sedentary women for one year. The women lifted free weights twice a week for 40 minutes a session. By the end of the study, their strength had increased an average of 75 percent. Additional studies left researchers satisfied that starting even older people on a resistance-training regimen does increase their muscle mass and strength.
But several questions remain: Can preserving muscle prevent or delay the kind of functional losses that hamper a person’s ability to live independently? And if so, what specific exercises work best?
Strength vs. Power
Mass and strength, while important, don’t interest Fielding nearly as much as muscle power. Power takes into account not only how much force a muscle can muster, but how fast it can generate it.
Some studies have shown that older people who have problems getting around are more likely to have low muscle power than their younger or more mobile counterparts. In practical terms, an older person with poor muscle power might not be able to walk with confidence, or quickly prevent himself from falling down when he trips. “Loss of muscle performance seems to be a stronger predictor of loss of function, and possibly even disability, than loss of muscle mass per se,” Fielding says.
“You can measure the power at really slow speeds, when people are contracting their muscles very slowly, or you can dial it up and measure the power output at very fast speeds, so they are really kicking out very quickly,” Fielding says.
As expected, the older test subjects trailed behind the power output of their young or middle-aged counterparts at the slowest speeds. “But what’s really interesting,” Fielding says, “is if you ask them to move faster, the difference in power output is much more dramatic,” with the older test subjects producing almost no power at the highest speeds.
Does reduced muscle size explain the loss of power? Not the case. Even accounting for the seniors’ reduced muscle mass, which the scientists calculated from CT scans, the older subjects were disproportionately weak when they had to move quickly.
The researchers then took a look at muscle at the cellular level. Human muscles are made up of different kinds of fibers, known as fast twitch and slow twitch. Slow-twitch fibers are experts at slow, sustained efforts, such as holding a yoga pose, while fast-twitch fibers are called into play for quick, forceful actions, like kicking. Some muscles, like the tibialis anterior in the front of the leg, which stabilizes your ankle when you walk on uneven ground, are mostly fast-twitch fibers. Others, like the soleus muscle in the back of your calf, which helps support you when you are standing, are almost 100 percent slow twitch. Many muscles are a mixture of both, giving humans the dynamic range they need to stand in line at the checkout but also dodge a wayward shopping cart.
To see whether it is those fleet, fast-twitch fibers that might get weaker with age, the researchers turned to the microscopic equivalent of a Nautilus weight-training machine. Taking individual, millimeters-long muscle fibers—both fast- and slow-twitch—biopsied from older volunteers, they sutured the ends to plates in the tiny machine to measure force. They then flooded the fibers with calcium ions—the same way the body signals a muscle to work—and the fibers contracted, pulling on the plates. The researchers then repeated the test a decade later, using new muscle fibers from the same test subjects. The results were virtually identical.
“Over 10 years of aging, there was no change in either the slow-twitch or fast fibers in how much force they can generate,” Fielding says.
The Nerve of Some Muscles
If the muscle fibers themselves aren’t losing their pep, Fielding surmised, perhaps the problem is a communication breakdown. The brain transmits its commands to the muscles (pick up the fork, swat that fly) through neurons, cells that pass along messages through chemical and electrical signals.
Fast-twitch muscles are controlled by a specific kind of large motor neuron, while slow-twitch muscles get their cues from their own, smaller motor neurons. Both types of neurons are located in the spinal cord and send out missives to the muscles through long, hair-like extensions.
Fielding points out that the big motor neurons are more susceptible to the oxidative stress that injures cells with aging, and tend to die off faster than the small motor neurons. Thus, older people have fewer of the fast motor neurons than younger people do.
To see whether it is this lack of fast-twitch neurons that leads to disability, the HNRCA researchers again had test subjects perform knee extension exercises, but this time they also monitored the electrical impulses in the muscles with an electromyograph.
Younger test subjects showed a rise in electrical activity as they kicked out at higher and higher speeds, while the electrical impulses in the mobility-limited seniors stayed flat. It’s a clue that their fast-twitch muscles might not be receiving their messages.
The findings have led researchers to revisit the traditional strength-training routine—marked by drawn-out dumbbell raises and slow-mo leg curls. Even though slow repetitions with heavy weights can certainly increase strength dramatically in healthy older people, “whether that is the best exercise to improve function still isn’t clear,” Fielding says. “You certainly get more improvements in power if you do things faster.”
Could the antidote for slower muscles be faster exercises? Fielding’s lab conducted a small study of 57 women over age 65 with limited mobility; they performed high-speed leg-press and knee-extension exercises on an exercise machine three times a week for 12 months.
There were no robust changes in their muscle power. But the researchers are working on another version of the experiment, this time looking at the same exercises done quickly with high resistance and quickly with light resistance. “We think maybe it’s the high velocity and not the force that is necessarily important,” Fielding says. Research continues to hone in on the ideal intensity, speed and frequency of exercises to keep older people functioning longer.
Fielding’s lab is also exploring the synergies between strength training and nutrition, particularly when it comes to protein, an essential building block of muscle. Although most Americans eat more than the Recommended Daily Allowance of protein (0.8 grams per kilogram of body weight for adults), some data suggest that 20 to 25 percent of seniors consume less than they should—perhaps a side effect of the “tea-and-toast” diet some older people fall into because of diminished appetite, tooth loss or difficulty preparing meals.
The solution may not be as simple as telling seniors to drink more milk or eat more eggs. When it comes to building muscle, not all proteins are created equal. “There may be specific types of protein or specific amino acids in proteins that may have very strong pro-anabolic [muscle-building] potential in muscle,” Fielding says.
He points to substantial evidence in animal and human studies that branched-chain amino acids, which are abundant in whey protein, can actually stimulate muscle development, especially when they are consumed right after exercise.
With that in mind, the sarcopenia lab is studying several dozen men and women, ages 70 to 85, who are taking either a whey protein pill or a placebo after strength training. Researchers will be assessing muscle mass and function changes over six months.
Pharmaceuticals may also play a role in future treatment of sarcopenia. Drug companies that once paid little heed to muscle loss are investing in sarcopenia research, looking for a product that can serve the burgeoning consumer base of aging baby boomers. Fielding, for one, has welcomed their interest, noting that some elderly people, due to heart disease, arthritis or other illnesses, simply won’t be able to get all the help they need from exercise.
“It could be exercise in combination with the right [drug] therapy that could optimize outcomes,” he says. “I think that this very much needs to be a partnership.”
As part of the global ramp up in sarcopenia research, the National Institute on Aging is funding the largest and longest study to date on physical activity in older adults. The Lifestyle Interventions and Independence for Elders (LIFE) study, being conducted at Tufts and seven other institutions across the country, will follow 1,600 mostly sedentary people ages 70 to 89 for two to four years. In addition to determining the effects exercise and health education have on mobility, cognitive function and heart health, the study will look specifically at whether exercise can prevent falls and keep people from losing their ability to generally care for themselves.
In the meantime, Fielding is helping lead an academic-industry task force that is proposing a clinical definition of sarcopenia, one that will help physicians make diagnoses and develop treatment plans. It recommends testing whether a person can walk four meters, or about 13 feet, in four seconds, an assessment that can be done easily in any doctor’s office.
Ultimately, the impact of muscle loss is hard to sort out from the other illnesses that tend to come with aging. Chronic kidney disease, Type 2 diabetes and congestive heart failure can all speed up sarcopenia, while obesity and arthritis are themselves risk factors for loss of function.
But Fielding, who has worked closely with elderly volunteers throughout his career, needs no convincing about the pain and frustration sarcopenia causes.
“What I see is that there are a lot of older people for whom muscle weakness and loss of muscle mass really are a root cause of their lack of independence, their lack of mobility, their increased likelihood of becoming disabled,” he says. “I believe that.”
This story first appeared in the Summer 2011 issue of Tufts Nutrition magazine.
Julie Flaherty can be reached at firstname.lastname@example.org.