This appears to be linked to sudden contraction of the muscles in the area. (dogster.com)
Did you feel so much pain that you felt yourself passing out, eyes closing, going under, only to wake up screaming without any control with the next sudden contraction? (ecochildsplay.com)
Practical examples can be seen from all over the country that emphasise that a significant and sudden contraction in the public sector can have severe consequences for the private sector and, in particular small and medium enterprises. (politics.co.uk)