Bend Over Backwards

Draws an obvious comparison between a physically demanding backbend and going to demanding lengths for someone else, but sources disagree sharply on when it actually caught on, with dates ranging from around 1920 in America to claims of related gymnastic imagery being used as far back as the 10th century. The most popular theory points to acrobats and contortionists, who bent themselves into extraordinary positions to impress an audience and keep working, mirroring the extreme effort the idiom now describes figuratively.