Fitness buffs who want to include aerobic exercises and weight lifting in the same workout are wondering whether they should lift weights before or after an aerobic exercise.
According to a Dr. Edward R. Laskowski, there is no scientific evidence which shows that one way is better than another.
Weight lifting first
Lifting weights eats up stored glycogen ("quick burst" energy). This means that if you lift weights first, you may burn less glycogen and more stored fat during your aerobic workout. On the other hand, lifting weights first, you may be too tired to finish your aerobic workout which could jeopardize your weight loss or fitness goals.
An aerobic workout may serve as a good warm-up for weight-lifting. However, doing it first may make you too tired to lift weights with proper form; increasing your risk of injury.
If you want to include both weightlifting and aerobic exercise in the same workout, experiment to find what works best for you.
Source: Mayo Clinic