When Building Muscle, Do Not Perform Cardio Before Weight Training
If muscle building is a primary goal then make sure you are not performing intense cardio sessions prior to lifting. You want to enter your resistance training sessions feeling 100 percent and ready to go for two reasons.
The first reason has to do with energy and performance. You want to be as fresh as possible when you start your weight training session. This will allow you to lift as much weight as possible, and maximize every set. This combination will help you string together quality workouts, and keep your muscle gains coming.
The second reason has to do with exercise form. If you lift while you are fatigued or not physically 100% there is a chance that your exercise form might suffer. If your form isn’t up to par, you increase the risk of injury.
While it is ok to do 5 to 10 minutes of very light cardio to help get warmed up prior to resistance training, you should avoid intense cardio sessions at this time and save them until the heavy lifting is done.
Gymjunkies has a great write-up outlining the pros and cons of doing cardio before or after weights. Definitely check them out!