Article which appeared in the American Journal of
Clinical Nutrition, Vol. 79, No. 5, 913S-920S, May 2004:

The evolution of physical activity recommendations: how much is enough?
http://www.ajcn.org/cgi/content/full/79/5/913S

Another article which may be of interest and which appeared in the latest
edition of the Medical Journal of Australia, out today:

Is exercise good for you?
Peter D Brukner and Wendy J Brown.
Med J Aust 2005; 183 (10): 538-541.
http://www.mja.com.au/public/issues/183_10_211105/bru10410_fm.html

well wishes,
Kate