It depends on your goal. If the goal is fat loss then I suggest you perform your cardio last.
Here’s why. ATP (Adenosine Triphosphate) is your body’s energy currency or “pocket change”. The money in your pocket is readily available for you to spend whenever you need it. However, once you run out of money you’ll need to replenish it for some other source. It’s the same with the energy systems in the body. Whenever you move, your body uses ATP first. Depending on the duration and intensify of the exercise will determine the source as to where the body will replenish it’s ATP or “pocket change.” This has a lot to do with the amount of oxygen required to perform the exercise and how much glucose or glycogen is stored in your muscles and liver. If the exercise is short and intense, lasting under 30 seconds, the body will replenish it’s “pocket change” from the ATP-CP system. If the exercise lasts from 30 seconds up to about 2 minutes, the Glycolytic system will kick in. The glycolytic system is the burning of glycogen or stored glucose (sugar) from your muscles. Oxygen is not required for both the ATP-CP or Glycolytic systems to work
Weightlifting is considered an anaerobic exercise (oxygen is not required). A normal weight-lifting set usually lasts less than 60 seconds, which means you’ll predominately rely on the ATP-CP or Glycolytic system to replenish your ATP. Remember, you have only so much readily available ATP or change in your pocket. Cardiovascular exercise such as walking, running, cycling, swimming or rowing will rely on the Aerobic-oxidative system because the duration of these activities is longer, therefore, less intense. These types of exercise are continuous and last longer than two minutes, usually lasting 20-30 minutes or more at a time. This aerobic-oxidative system relies primarily on fat to replenish the ATP used.
When you start a fire you’ll always light the kindling before the log. Your body is smart. Think of glycogen or muscle sugar as “kindling” and fat as a “log.” Weightlifting will burn the kindling first because it’s too intense or brief to burn the log. By lifting weights first which burns the “kindling”(sugar) and then performing cardio last, you will be able to burn more of the “log”(fat), which will result in a greater loss of that unwanted fat. In contrast, studies show that performing cardio exercise first before lifting weights, will result in a reduction of quality of the workout and reduction of strength for the muscles.
So if you want to get rid of your “log” perform your cardio last. If you’re an endurance athlete then perform your cardio on one day and your strength training on another. Hopefully, this will help achieve more quality from your workout.