There are advanced electro-stimulation programmes which combine an agonist muscular contraction with a subsequent anagonist contraction. Both contractions are controlled by the electro-stimulator. However, right now we refer to carrying out a voluntary antagonist muscle contraction using EMS.
The posture should be such that that the muscle being worked voluntarily does so isometrically. That is, the aim is to create a contraction without causing joint movement.
This muscular training is more intense than may seem a priori; the muscle being worked will be subjected to an unusual strain, because as the brain sends a signal to cause the contraction, the agonist muscle sends another signal which relaxes the antagonist muscle, which facilitates movement, and in this type of voluntary exercise we are going to take no notice of these relaxation signals and cause a isometric contraction. As we will realise, it is a load with an intensity greater than that which we would assume. This is an interesting type of training because, almost any place, we can develop this type of work with electro-stimulation without needing much more than at electro-stimulator.
One additional note: you must always make sure that the lever on which the muscle in question acts does not move.
In any case, the electrical stimulus intensity which we apply to the agonist muscle must be so that it doesn’t exceed the effort carried out by the antagonist muscle. The isometric training allows to do this training in various angles. It would be very interesting to specifically work the concrete angle which we want to develop to maximum strength, though there is no problem in working and trying various angles.