These systems involve the use of isolation exercises to preferentially fatigue a muscle whilst eliminating synergistic muscle weaknesses. Consider the following example:
In the bench press, the agonist is the pectoralis major, with the main synergist being the triceps brachii. In the majority of clients the triceps will fail before the stronger pectoral muscles. By employing either a pre or post exhaust isolation exercise it is possible to “bypass” the weak synergist and permit greater fatigue of the target muscle – in this example pectoralis major.
In pre-exhaust the pectoral isolation exercise is performed prior to the compound e.g. cable crossovers performed before bench press.
In post-exhaust the pectoral isolation exercise is performed after the compound e.g. bench press performed before cable crossovers.
Active IQ, Level 3 certificate in personal training, programming personal training with clients (2014)