Understanding how to combine both cardio and strength work into your program can be confusing.
You may ask yourself “Should I do my cardio before? After? Or another day?”
Contrary to popular belief that you should perform cardio after a strength session, the current research offers mixed results.
Schumann et al. (http://www.ncbi.nlm.nih.gov/pubmed/20101919) Compared the effects of 3 different 8-wk training programs endurance training, strength training and concurrent training (strength & endurance mixed)
Total fat mass significantly decreased in the endurance and concurrent groups but not in the strength group, whereas fat free mass significantly increased in the strength and concurrent groups.
The above findings is what I would expect, as performing the programs with cardio endurance work included is going to increase overall energy expenditure hence the results showing a drop in total fat mass in the 2 groups which included cardio.
A point to consider is that without knowing the nutritional intake of these people during the study, we can't be sure if the program alone was the reason for these results.
The increase in fat free mass in the groups that included strength training again is to be expected as these are the benefits of including strength training within your program. If someone wanted to build muscle they wouldn’t perform cardio alone.
A possible drawback to this study is that it was performed on untrained subjects, who will respond to pretty much any consistent method training.
In another study Chiara et al. (http://www.ncbi.nlm.nih.gov/pubmed/18545210) compared performing endurance & resistance work before or after a Circuit training program and a circuit program alone. The resistance-type circuit training targeted strength endurance (weeks 1-6) and explosive strength and power (weeks 7-12).
They found that circuit training alone induced strength and power improvements that were significantly greater than when resistance and endurance training were combined.
This study brings us to the conclusion that if your primary goal is to gain strength, you may get better strength gains from performing your cardio work separately.
Specificity is a very important point when structuring your program, as the findings above show. The type of response you get is dependent on the type of training you perform (S.A.I.D principle).
So should you do cardio before or after weights to get strong?
From a practical point of view, tiring yourself out with some high intensity intervals, then trying to going and lift heavy weights isn’t going to be very productive. 20 mins walking on the treadmill on the other hand, isn’t going to affect the strength session much.
Be sensible about the choices you make and consider the time you have available to commit to your program, if you are limited then focus your efforts on what's most important to you & going to give you the best results.