Research has shown that compound movements (i.e., exercises involving more than one joint) activate muscles in a different manner compared with isolation exercises. Thus, to adequately train a muscle you should add isolation exercises.
Untrained subjects were divided into two groups: one performing compound exercises only (C), and the other performing compound exercises plus isolation exercises (C+I). The compound-exercise group performed only the bench press and lat pulldown. The C+I group performed the bench press, lat pulldown, triceps extension, and curl.
There were no differences in gains between the two groups. Although the C+I group trained biceps and triceps in addition to the compound exercise, arm size and strength changed an equal amount for both groups.
The addition of isolation exercises will not significantly increase your gains.
The foundation of your training should be based on compound exercises such as the squat or leg press, chest presses at multiple angles, and pulls and rows. Now, it should be noted that the subjects in this study were untrained, meaning that they are going to be much more responsive to training than a seasoned lifter. For more advanced lifters, the addition of isolation exercises is appropriate not because the underlying principle is different for experienced lifters, but because experienced lifters require more volume to get the same response. - FLEX