Concurrent training is defined as combining both resistance exercise, such as weight training, with some form of endurance training, popularly known as "aerobics." You don't do both in the same workout (although some do suggest this, as will be discussed) but rather the usual pattern is to first do a weight workout, followed by an aerobic session. That aerobic session can occur either immediately following the weight workout, or several hours later depending on individual goals and time constraints. Some people insist that aerobics training isn't necessary if you maintain a rapid pace during the weight workout. According to this notion, what's important is maintaining a higher heart rate, and how you accomplish that--whether through lifting weights with a short rest period between sets or by doing straight aerobics--isn't important. There is some truth to this. Certain styles of weight training can indeed provide a significant cardiovascular benefit. Specifically, this style of weight training involves circuit training, where you do several exercises in a non-stop fashion with no rest between exercises, and only brief rests between sets. The late muscle entrepreneur, Joe Weider, opted to add this style of exercise to his "Weider Training Principles," and renamed it "Giant sets." While there is little doubt that circuit training or giant sets can indeed lead to an elevated heart rate if done correctly, studies have repeatedly shown that the extent of oxygen intake that occurs with circuit training is still considerably less compared to a standard aerobic session. And it's the increased oxygen intake that sets aerobics apart from weight training and even explains the term "aerobics." For one, fat oxidation occurs more readily with a higher oxygen intake and this is the reason why doing aerobics is more effective than weight-training for fat-burning purposes.
What it comes down to is the well-established principle of Specificity of Exercise. This refers to engaging in exercise for specific purposes best served by doing that type of exercise. For example, if your primary goal is to build added muscle and increase strength, the best form of exercise is progressive resistance exercise, exemplified by weight training. If you want to train your cardiovascular system, and also boost fat oxidation, you should do aerobics because it features the greatest degree of oxygen intake. To increase flexibility, the choice is to do some form of stretching exercise. Of these three forms of physical fitness, the most questionable is stretching. You can get the benefits of stretching while lifting weights simply by employing a degree of pre-stretching during the execution of a full-range repetition in any weight exercise, although some exercises are more amenable to this than others. One added benefit of pre-stretching any muscle is that it lines up the working muscle fibers in a way that increases the strength of the . . .