To some this may sound obvious but I can’t count how many times I have asked someone performing an exercise what muscle they are working and they have no idea. You can’t properly target a certain muscle if you are not mentally focused on it.
Much has been written about this connection, the mind/muscle connection, over the years but many people do not practice it. The human body is extremely adaptable and tries to perform tasks as easily as possible. Often this results in other muscles kicking in to aid in an exercise. Since we are trying to focus one muscle group at a time, we cannot let this occur.
You have to make the movement as hard as possible rather than as easy as possible in order to force your body to adapt and look the way you want it to. You have to mentally focus on the muscle working at all times and try to shut down any other muscle trying to help out. The more you can focus and feel the muscle work the more you will work that muscle and the faster your results will come.
This will take some time to get the hang of but you will develop this connection. Make sure you feel the muscle stretch as you lower a weight and feel it contract as you raise a weight. Focus on the full range of motion (ROM) with every repetition and you will start to isolate muscles more effectively.