Isolation exercises are mostly a waste of time. If you're grossly misproportioned (like me
) and are want to say build your delts and arms more in comparison to your chest and back for instance, the addition of an isolation exercise for that area should help it grow. Just don't go crazy, more than ~2-3 sets will be counter productive for most people, if you're doing a couple of heavy compound exercises that hit it indirectly.
EMGs, while interesting, are really pretty useless - your form will deviate because your body structure is different and the exercise has a different groove for you. Your best bet is to go by what feels the best.