It's called a "thigh gap" and it's the latest disturbing body obsession trend that women with eating disorders want: an oddly bow-legged look that might have indicated a bad case of rickets in decades past.
The obsession with thigh gaps is popping up on blogs and social media, despite the fact experts say it is almost physically impossible to be healthy and have a thigh gap.
Eating disorder experts say it's very common for people with body image distortion problems to focus on one particular body part—which may explain another leg obsession, hot dog legs, that spawned Internet satires.
Rarely seen in the United States, in the developing world rickets are a painful illness associated with a vitamin D deficiency that results in soft bones.
Pittsburgh orthopedic surgeon and fitness expert Vonda Wright says she can't think of a single athlete who has a thigh gap because "Skinny does not mean fit or muscular."
And apparently, being obese doesn't necessarily mean you are unhealthy, either.
Researchers recently discovered that the answer seems to be in how different people build or expand fat cells, according to a small study of 27 sets of twins in Finland.
In healthy obese people, their bodies create more fat cells with weight gain, but in unhealthy populations fat cells expand to their teeny tiny breaking points. When the number of cells increase, so does the number of mitochondria in each cell, which is what helps the body harvest energy from food.
An Australian study earlier this year found that nearly a third of all obese adults manage to maintain good health, but described that state as "transient."
For example, that could mean a newly-obese young person remains healthy early in life because they are relatively healthy to begin with, but their low blood pressure can rise to match their sky high BMI with time.
Of course, most people don't spend much time worrying about their weight on a cellular level, even if that's what could matter most in the long run.