Why Your Muscles Get Weaker as You Get Older
Loss of muscle stem cells is the main driving force behind muscle decline in old age in mice, University of Rochester Medical Center researchers have found.
Their finding challenges the current prevailing theory that age-related muscle decline is primarily caused by loss of motor neurons. Study authors hope to develop a drug or therapy that can slow muscle stem cell loss and muscle decline in the future.
As early as your mid-30s, the size and strength of your muscles begins to decline. The changes are subtle to start — activities that once came easily are not so easy now – but by your 70s or 80s, this decline can leave you frail and reliant on others even for simple daily tasks. While the speed of decline varies from person to person and may be slowed by diet and exercise, virtually no one completely escapes the decline.
“Even an elite trained athlete, who has high absolute muscle strength will still experience a decline with age,” said study author Joe Chakkalakal, Ph.D., assistant professor of Orthopedics in the Center for Musculoskeletal Research at URMC.
Chakkalakal has been investigating exactly how muscle loss occurs in aging mice in order to figure out how humans might avoid it.
In a study, published today in eLife, Chakkalakal and lead author Wenxuan Liu, Ph.D., recent graduate of the Biomedical Genetics Department at URMC, define a new role for stem cells in the lifelong maintenance of muscle.
All adults have a pool of stem cells that reside in muscle tissue that respond to exercise or injury – pumping out new muscle cells to repair or grow your muscles. While it was already known that muscle stem cells die off as you age, Chakkalakal’s study is the first to suggest that this is the main driving factor behind muscle loss.
To better understand the role of stem cells in age-related muscle decline, Chakkalakal and his team depleted muscle stem cells in mice without disrupting motor neurons, nerve cells that control muscle. The loss of stem cells sped up muscle decline in the mice, starting in middle, rather than old age. Mice that were genetically altered to prevent muscle stem cell loss maintained healthier muscles at older ages than age-matched control mice.
At the same time, Chakkalakal and his team did not find evidence to support motor neuron loss in aging mice. Very few muscle fibers had completely lost connection with their corresponding motor neurons, which questions the long-held and popular “Denervation/Re-innervation” theory. According to the theory, age-related muscle decline is primarily driven by motor neurons dying or losing connection with the muscle, which then causes the muscle cells to atrophy and die.
“I think we’ve shown a formal demonstration that even for aging sedentary individuals, your stem cells are doing something,” said Chakkalakal. “They do play a role in the normal maintenance of your muscle throughout life.”
Chakkalakal is building on this discovery and searching for a drug target that will allow him to maintain the muscle stem cell pool and stave off muscle degeneration as long as possible and he hopes this discovery will help move the field forward.
More Bike Commuting, More Crashes
& Skyrocketing Medical Bills
A recent study covering published in the journal Injury Prevention stated that, “Each year, the total costs associated with non-fatal adult bicycle trauma increased by an average of $789 million,” and that “Medical costs increased by 137% from $885 million in 1997 to $2.1 billion in 2013.”
Meanwhile, the “number of adult cycling injuries increased by approximately 6,500 annually,” and the authors added, “Over the last 15 years in the USA, the incidence of hospital admissions due to bicycle crashes increased by 120%.”
Since medical costs and emergency transportation expenses alone don’t tell the whole story of the total price tag of these accidents, other expenses were factored in, like the cost of missed work, rehab and physical therapy, and quality-of-life costs. Those are calculated using data from jury verdicts stemming from injuries to bicycle victims. The adjacent chart shows the total average cost per case – excluding inflation – of non-fatal bike injuries in the U.S. (credit: study’s authors Thomas W. Gaither, et al).
Researchers studied adult bicycle injuries from 1997 to 2013, utilizing non-fatal data from the National Electronic Injury Surveillance System and fatal incidence data from the National Vital Statistics System from 1999-2013.
One of their primary findings was that, “Costs associated with adult bicycle crashes exceeded $24 billion in 2013,” the authors wrote, which is “approximately double the medical and indirect costs of occupational injuries in the USA.”
In other words, as a nation we basically are spending twice as much money getting injured on the way to work then we are getting hurt while actually at work.
One main reason why the costs have risen so dramatically is because since so many more people are riding as a means of transportation, more and more bike accidents involve collisions with motor vehicles. That’s compared to “non-street” incidents occurring in more forgiving environs, like parks and wooded areas.
In 2014, 67 percent of injuries took place on a street; in 1997, only 46 percent did. These street accidents are also usually more serious, since they’re occurring at higher speeds.
“Street crashes represent an increasing proportion of total costs compared with non-street incidents. These crashes often involve motor vehicles, which increase velocity of crash impact and consequently injury severity,” the study reports. “Streets might also predispose to more injuries due to the coexisting environment with urban areas, increased population density or the presence of more unyielding street furniture,” like parking meters, anchored garbage cans, newspapers boxes and street lamps.
Another primary finding of the study was demographic in nature. The researchers learned that as injury-related costs soared, so have the number of bike riders aged 45 and up. “In 2013, 53.9% of total costs were due to riders 45 and older,” they wrote, “up from 26.0% in 1997.”
In addition, in the study’s final year, male riders were involved in more than three out of four accidents (77%).
While we consider the rising costs and risks involved in pushing for more bike riding in America, it might be wise for urban planners to focus more intently on injury prevention methods that can be incorporated into overall safety awareness campaigns, and if possible, the streetscape itself. – Erik Lief, American Council on Science & Health