In Iyengar, the bandhas (energy locks in the body) are considered an advanced practice only to be used as part of advanced pranayamas. Here's an interesting article over at AshtangaNews.com by Ruth Jones, an Ashtanga Vinyasa practitioner (in which Mula Bandha, the root lock, is used consistently throughout the practice) and PhD candidate who is conducting a study on contraction of the pelvic floor muscles.

Tags: Ashtanga Vinyasa