In an ideal world, strength and aerobic (cardio) training would be done on different days. There are some studies suggesting the two components play against each other and can inhibit improvements in either realm. But, there is only so much time in the day for you to get your workout done. You may not even be able to get to the gym more than 2 times a week so you want to do both cardio and strength each time.
The thing is our bodies go through several energy systems. Roughly, the first 10 seconds of exercise uses up our fastest energy source. This limited amount of energy is readily available in our muscles and helps us perform explosive exercises such as sprinting or jumping. The next 1-2 minutes is our second energy source which allows for exercises such as longer sprints. Because it works so rapidly, it can only last so long. Once our inventory of quick energy is used up we go into our third and slowest way of supplying energy. At this point you will start to feel your body slow down. This happens because our body has to go through a longer process to generate more usable energy. The first two stockpiles will not be fully restored unless you take several (>10) minutes for passive recovery.
If we do aerobic exercise first we will then deplete our body of the energy stores that are most useful for strength training. We will still be able to perform the exercises but they may not be as fast or explosive. You might even be able to lift more weight (pounds) if you do strength training before your aerobic exercise.
So the next time you go in the gym to do both strength and aerobic training, consider doing a short warm up (example below) to get your blood flowing. Hit the weights before you hit the treadmill. Give what energy and focus you can to those specific exercises. All that energy you have left can go to your cardio which requires quite a bit less concentration.
Example Warm Up:
- Active stretches for muscle groups with limited range of motion (i.e active leg raise for hamstrings, thread the needle for upper back, laying windmill for chest and shoulders)
- Body weight squats x10-20
- Modified push ups (on an incline) x10
- Jumping jacks, run in place, or jump rope x15-30 seconds