Will sitting around at work all day kill me? Does running strengthen your knees or ruin them? Standing desks, yay or nay? And can I eat eggs now, or what? 

If you're the kind of person who wants to stay up to date on the latest health advice, following the headlines can be bewildering. New findings are constantly coming in and the way the media covers them can sometimes be either reductive or sensationalized. Sorting out truth from dubious trends can be difficult. 

Daniel Lieberman can help. A professor of evolutionary biology at Harvard, he studies how people moved and exercised in the past and what that can tell us about how to keep fit today. His latest book, Exercised, boils down these time-tested lessons with a focus on busting common myths that prevent people from being as healthy as they can be. 

If that sounds fascinating to you, run out and pick it up. But which of the many myths Lieberman outlines does he see as the most harmful and most wish to debunk? He shared his number-one worst myth with Big Think recently. 

Aging Americans versus aging hunter-gatherers 

Picture an older person. What do you see? If your mental image is of a slightly stooped individual shuffling slowly along on the way to their weekly shuffleboard game, then Lieberman would like you to stop right there. That might be how your average American senior looks, but it's not at all what you see if you visited with hunter-gatherer elders. 

"Americans, as we get older, know that strength declines rapidly. By the time people are in their 60s and 70s, they're pretty frail, but hunter-gatherers remain fairly physically active as they age because they're doing stuff. They have to lift things and carry things and do stuff that keep them strong. And the end result is that they maintain that strength," Lieberman explains. 

Age, he insists, doesn't naturally lead to frailty. Sure, none of us are as strong and swift at 63 as we were at 23, but the reason so many seniors in the developed world become so weak is, basically, because they believe loss of strength is inevitable and they give up on exercise. That belief is the myth Lieberman is most keen to bust. 

"I think the most pernicious, the most serious, the most problematic, the most concerning way in which we think about exercise in the Western world is that as people get older it's kind of normal to be less physically active," he tells Big Think. 

As a result of this false belief, many older Americans stop exercising, which leads them to become less fit, which in turns causes them to exercise even less. It's a vicious circle that leads to what scientists term sarcopenia, or loss of muscle mass and strength. 

Boosting brains and brawn

We tend to think of this loss of muscle mass as an inevitable part of getting older, but it's not. And you don't have to hang out with hunter gatherers to prove that's true, Lieberman insisted in a separate NPR interview. Chasing prey and digging up tubers isn't the only route to aging with strength--you can also hit the gym into your 80s, as did former Supreme Court Justice Ruth Bader Ginsburg

"She was celebrated for her vim and vigor, which meant that a lot of that came from the fact that she kept working out and as she got older, she went to the gym several times a week. Now, she didn't do crazy, pump iron stuff. She wasn't trying to be like Arnold Schwarzenegger. But she did a few rounds of weight training every week and that helped keep her marvelously active and vigorous up until her late 80s," LIerberman notes. 

It's worth noting that separate studies show that this same type of regular but modest weight training helps keep the brains of older adults functionally younger too. That means there are additional benefits for seniors beyond just keeping up the strength required to interpret the constitution (or run your business) into your ninth decade -- you'll stay sharper longer too.  

So if it all the conflicting advice gets too confusing, feel free to tune out shouting headlines about every new research finding (the basics of staying healthy remain simple and stable over time), but make sure you're not falling prey to this insidious myth first. If you expect to get weaker as you age, you probably will. Thankfully, the opposite is also true -- a modest commitment to keeping up your strength will prevent much of the age-related declines in fitness too many people see as inevitable.