Joe Weider introduced the principle of isolation into bodybuilding – laying the foundation for split system training. Isolation exercises don’t truly exist. Regardless of the type of exercise one way or another body momentum can be generated in muscles of the core as well.
For example, the bench press is a well known compound movement that works the chest, shoulders, and triceps. However, almost any Powerlifter will tell you how important lat involvement is, as well as leg drive. And even an isolation movement like front lateral raise uses the muscles of the forearm to grip the weight and core muscles for stabilization.
In fact, some bodybuilders such as Mike Mentzer claimed they didn’t even need to train certain body parts such as the calves, forearms, and abdominal muscles because of all the secondary stress placed on these muscles from other exercises.
However, in order to continue to push the muscles harder – as bodybuilders advance from beginner to intermediate and then to advanced – workouts need to be split allowing for isolation.
Basic beginner and intermediate routines, such as upper/lower body and push/pull workouts, rely heavily on large compound muscles as the objective is to work multiple muscles group in one workout.
As the routine transitions from intermediate to advanced, the program becomes split into smaller segments allowing more isolated work to be done in each area.
Extreme examples are when bodybuilders isolate each individual muscle group into a separate workout and train twice a day, such as Arnold Schwarzenegger when preparing for Mr. Olympia. This type of training is really advanced and involves training each muscle group with as many as 30 sets.
It is at this point the principle of isolation is most evident. Any overlaps in the muscles being worked would quickly result in overtraining.
So when choosing isolation exercises remember to pay attention to the secondary stress placed on other muscles and the arrangement of exercises within your training split.