For Women
Insert 2 fingers into your vagina and contract.
For Men
Insert one finger into the rectum and tighten your muscles around it.
When you find your inner squeeze, it will feel like the opposite of bearing down to make a bowel movement. It is a drawing in and up around your inserted fingers. You can aim to bring your tailbone and your pubic bone together, and while you may not notice this actually happening, using the image might get your pelvic floor muscles into a working contraction.
While you are doing this test, continue breathing -- holding your breath alters the way in which muscles are being used, and defeats the purpose of the test.
Take heart, the contraction you feel may be small, but once you locate a contraction you are ready to strengthen your pelvic floor muscles with exercise.

