"The splits" vs "a split"

I've always heard it as "the splits," but I'm not a gymnast. A bit of research shows that gymnasts use the singular "split." (They use the plural "splits" when referring to two or more types of splits.)

http://www.gymnasticsrevolution.com/Parents14.htm

http://gymnastics.about.com/od/trainingadvice/ss/frontsplit.htm

You can do "a split" or "the splits". They are interchangeable. But you would never say "a front the splits". You would say "a front split" and walk away smiling, even if you pulled a groin muscle.

I’ve never heard of singular split with this meaning, and nor has the OED, which provides a citation from 1861 for the first use of the splits. The use of a plural form for what looks as if it should be singular is not unprecedented. First there are the ‘plurale tantum’ words like trousers, scissors and clothes. But then, too, we have plurals like the runs (diarrhœa), the heebie-jeebies (feeling of discomfort) and the mockers (bad luck). The splits seems to belong morphologically with these.

I have been a touring musician for years, and discovered this problem along the way. Somewhere in the midwest was the first time I heard of "The Splits". They thought I was an idiot for calling it "A Split". The explanation: The olympic event is called "The Splits"...as in, "She did well on the parallel bars, now it's time for the splits"....

I thought that was an inadequate explanation for why whole populations of people, more or less by region, shared this terminology.

What's needed for futher clarification is a map like this one, for the usage in question:

http://www.facebook.com/photo.php?fbid=2752093039136&set=a.2190936530574.2113805.1161952054&type=1&theater