Definition: "An ergogenic aid is any substance or phenomenon that enhances performance "
Muscle groups work harder during a set if you focus on them
In the gym there's no exercise that trains only the muscle group you want to stimulate. Other groups are always involved. That's why it's not so easy to achieve directed growth of particular parts of the body. If you have strong shoulders and weak chest muscles, it's pretty difficult to develop good pecs. Or is it not that difficult after all? Sports scientists at the University of South Carolina Upstate discovered a very simple method. Focus.
The researchers then repeated the activity. First they got the women to perform one set. Then a trainer told the women that they should concentrate on getting the power for the movement out of their latissimus dorsi muscles. That was why they were doing lat pull-downs: to develop the broad upper back muscle.
The trainer explained to the women exactly where the lats are located, and touched the muscles in the women. Then the women performed another set. The figure below shows the electrical activity in the muscles.
Focussing your attention on a muscle group while performing a set increases the intensity with which your muscle takes part in the movement. Isolating muscles is a matter of good ol' fashioned focus & concentration.