I've only recently become interested in the psoas muscle and its effects on running. But is actually very well studied in yoga and other disciplines.
Resources
* http://www.yogajournal.com/practice/169/?page=3
* http://touchoftao.blogspot.co.uk/2010/10/overview-of-psoas-muscle.html
* http://earthbalance-taichi.com/2012/01/psoas-muscle-in-tai-chi/
* http://upload.wikimedia.org/wikipedia/commons/b/b2/Gray430.png
* Relationship to bulging belly
