Muscle only turns into fat in very rare circumstances, and it never occurs in healthy people.
"They're two different types of cells," said Endress. "Muscle doesn't turn into fat and fat doesn't turn into muscle."
This myth probably originated from people seeing toned, muscular athletes develop some extra layers of padding with age.
What actually happens, Endress notes, is that when you stop working out, you rapidly lose muscle and gain fat, potentially giving the appearance that muscle is turning into fat.
"It's almost like a slang colloquialism that people use," he said.
Also, once people stop working out, they don't cut back on the calories they were eating before, but are no longer burning off.
Ho said that muscle can literally turn into fat, but, "You only see it in those extreme circumstances."
"If you're talking about healthy muscles that aren't torn or injured, then no. For a noninjured muscle, that's a myth. But it's true with serious injury, like with a complete rotator cuff tear being the best example."
The difference, Ho explained, is that with a complete tear, the muscle isn't just infrequently used, it isn't used at all, and it can be replaced by fatty tissue. But he noted that even then, it is unusual.
Fact or Myth? Eating Protein After a Workout Will Build Muscle
"That's definitely a myth," said Elisabetta Politi, a nutrition director at the Duke University Diet & Fitness Center. "What you need after a workout are carbohydrates."
When you work out, your body uses the glycogen stored in your muscles and liver. Glycogen is made from carbohydrates like glucose.
The exhaustion you feel after a workout, Politi said, is because of a lack of glycogen.
"The best way to be reenergized is to eat carbohydrates," she said.
Protein is helpful in toning existing muscle. However, Politi notes, the average American eats twice the necessary amount of protein already.
"If [you're] eating an average diet, [you're] probably getting plenty of protein," she said.
Fact or Myth? You Need To Stretch Before Exercising or You'll Be Injured
Answer: Probably a Myth
Had this article been written a few years ago, this might have ended up in the "Fact" column.
"We go back and forth on that," said Ho.
The most recent studies, he said, have shown that stretching will not help avoid injury.
"I think in people who have been injured or are injury-prone, those people would benefit from stretching," said Ho.
He said people who, for instance, have torn their calf muscle, should stretch both calves before exercising, but they should jog a little first.
"I think the key, what we do recommend, is that you warm up before stretching," said Ho.
Endress said that with his clients, he recommends some sort of slower-paced warm-up.
"Whatever activity you're doing, start off at a slower pace and gradually build up."
He suggested warming up for three to five minutes before starting, which would include aerobic exercise before weight training.
Stretching is also helpful after a workout, where it helps to maintain flexibility, which can be particularly important after weight training.
But alone, before exercise, it isn't warranted.
"Stretching in and of itself, for someone who has normal flexibility, has not been injured, may not reduce injury," said Ho, but he added that more research is needed before drawing a definite conclusion.
Fact or Myth? Running Destroys Your Joints
Answer: It Depends