It can be caused by a number of factors, including a rapid increase in the frequency of training, sudden increase in the intensity of training, transition from one training method to another, genetic abnormalities of the knee joint and poor base strength of the quadriceps.

BBC: Hargreaves faces career concerns

Obviously part of the role of the Joint Chiefs in the process was to talk to the President about the health and the strength of the force, understanding that a lot has been asked of it over the past eight years.

WHITEHOUSE: Press Briefing