recent research from the University of California shows that concurrent training can improve endurance and strength (Journal of strength and conditioning research 22(5), p1487-1502, 2008)
Some studies have proved that training in one area will automatically interfere with the development of another. I can remember this being the 'gospel' according to McKardle, Katch and Katch! The flaw is we believe this to be true for whatever type of fitness there is. The fact is a lot of studies used untrained subjects and did not provide adequate recovery for their subjects. However, research on trained subjects with adequate recovery tells a different story (again published in JSCR: 2001, 15, p172-177; 2003, 17, p393-401; 2006, 20, p541-546).
The findings from the latest research, 2008, suggests that for 'well conditioned athletes concurrent training can develop both the endurance and strength of the trained subject, in particular integrated training, where the sequencing of training has a major impact on the results, is more effective than serial training'.
In a nutshell, if you are well conditioned, weight training and aerobic training can be done in the same training cycle and even in the same session. Alternating sets of resistance exercise with brief intense aerobic work is more beneficial for developing both strength and endurance at the same time than starting with a block of resistance and ending with a block of cardio. The only drawback for the 2008 study that i can see is that the subjects trained for 1hr and 50mins so adequate rest is a must or fear overtraining!
Is there a case for circuit training?