The argument for doing isolation exercises when you have been training less than a year is not a very good one and most coaches will agree that if you want to gain muscle you need to stick to compound exercises. The difference is that compound exercises will directly affect your core strength which will make you stronger.
Sports science has now proven conclusively that the only way to get your muscles to change shape is by getting stronger and the only way to get stronger is by doing compound exercises. A compound exercise or movement is when you are using more than one joint to complete it.
It is compound movements like squats, bench-press and deadlift that have been the basics which allows all future bodybuilders, weight trainers and those who want to add muscle to get started and build the foundation of strength which is needed before doing isolation exercises.
There is no doubt that isolation exercises can help to re-shape your muscles and some top professional bodybuilders have proven this to be true. A good example of this is how Lee Haney changed the shape of his biceps completely to help him win 8 professional Mr Olympia titles in a row.
The problem is that you are wasting your time in doing isolated exercises if you do not have the strength and the basic foundation of muscle which is needed to make the doing of these isolated exercises more beneficial. The stronger you are the more muscle you carry and you need to concentrate on those basics by doing compound exercises to increase your core strength.
One always needs to be very aware of your body’s response to the hard workouts that you are doing and that requires monitoring on a regular basis. Over training is something that holds a lot of beginners back as they can spend the first year making big mistakes on thinking that if 4 sets per body-part is good then 10 sets must be better.
It is this type of training that can slowly lead to you stopping to gain muscle because you are over-training, not getting the required amount of sleep and/or the required nutrition will quickly lead you into a state of over-training which happens sometimes very slowly.