"Bodybuilding is about training muscles, not lifting weights". I think this was said by Arnold. It is a concept that is very misunderstood when you look at the way the average gym goer trains. I don't spend all that often in public gyms but when I do I always walk away scratching my head with some of the things I see. The technique is horrible and people are increasing their risk of injury instead of improving performance and making gains. When you consider most of the people training there just want to look good, or are want to be bodybuilders, then I think they should be focusing on creating as much tension as possible in the working muscles than they currently do.
When doing isolation work, instead of using so much momentum and bringing many muscle groups into play to lift a given weight, you should be focusing on contracting the muscle as hard as possible. You want to make the lift harder, not easier. You can overload a muscle using the same weight each workout as long as you created more tension and contracted it harder from workout 1 to workout 2 for example. You want the target muscle to be doing all the work hence the term "isolation". If you use body motion and momentum to move the weight then you aren't isolating and muscle group. Sure certain muscle groups will fire to support the working muscle but you want to minimise this as much as possible.
You must also focusing on contracting the target muscle group first. What ever you contract first gets the most stimulus. So if doing curls, you must focus on contracting the biceps first and not the shoulders as is commonly seen.
Use weights that aren't too heavy to feel the target muscle. This is one of if not the biggest reasons people are unable to feel certain muscle groups when training them. If the weight is too heavy for the muscle you are targeting then other muscle groups are going to kick in to help you lift the weight. As an example, when I do face pulls or a seated row variation, there is a point in which the weight is heavy enough for me to really feel the contraction of my scapula retractors but if i go just slightly over that point I lose the feeling of contraction in those muscles, even though my technique still looks good and I can lift the weight quite comfortably.
When training isolation exercises or trying to bring up a weak point, tension is the key. If you learn to contract those muscles more efficiently then they will be able to support you during your bigger lifts. It isn't just about moving the weight from point A to point B when bringing up a weak muscle group. You have to focus hard on the target muscle and try to get it to do the majority of the work. Focus on keeping constant tension in that muscle group for the whole set and you will feel a very big difference. You are basically flexing against a resistance. To do this you will have to lift more slowly as too much acceleration will make you lose tension in the muscle. Maybe use a 3131 type tempo when doing a regular biceps curl for example. The transition point is also where a lot of people lose tension so that is why I think it is important to pause at each point the first few sessions you do.
So it doesn't really matter what your goal is. If you just want to look good and are doing some isolation work or if you are an athlete trying to bring up a weak point, you have to focus on creating as much tension, and contracting the target muscle as hard as possible.