does anyone know if doing 10-20 minutes (or longer) of moderate cardio BEFORE weight training has any positive fat burning effects on your body?
No positive effect but does have a definite negative effect on your workout.
It's sort of a "yes, but" answer. You will burn off more glycogen prior to lifting which will have an effect on fat burning BUT it will also raise cortisol levels faster, weaken your lifting abilities, and create a higher degree of muscle tear down that a person with normal recovery abilities will not get any benefit from.
If you're a genetic athelete freak then sure, it'll probably be great. But in terms of building or retaining muscle mass for normal people its not so good.
ill warm up on a bike for 5 minutes. but im definitely for resistance training, prior to cardio. ideal to be on an empty stomach anyways so giving yourself another hour or so after you last meal isnt such a bad idea. As mentioned, aerobic exercise can take away strength during lifting.
The only way I can see it as a benefit is if you do your cardio in the morning on an empty stomach, and your weight training in the evening. But not one right after the other. I believe there is a benefit if you do your cardio (15-20) after your weight training. Your heart rate is already elevated, so no warm up is necessary.
there is a good article in the recent Iron Man about cardio and weights and how balance them in a workout. Everything I have ever read on the subject says to lift first then do cardio. 5 - 10 mins warm up on a treadmill before a workout is good but be careful not to use up your glycogen levels.
One thing you could do is drink a gatorade or eat some applesauce during workouts to keep sugars up.