Question About Pre-Fatiguing

[quote]The Mighty Stu wrote:
^Not the explanation most people I’ve known would use.

As multiple muscles work in conjunction on a compound lift, the one that is naturally stronger (larger?) would require more stress (reps? weight? TUT?) to stimulate a response before the smaller ones. [/quote]

? If I am training chest, THAT is my goal. Why would I care about the triceps?

[quote]

So in the case of chest pressing, an isolation movement such as DB flyes would target specifically the pecs, creating an already weakened starting point when they are called into play during the subsequent pressing motion. Essentially lowering the threshold of the strongest muscle (pecs) to a more level playing field with the others (delts and triceps).

S[/quote]

Why would I want to weaken my chest WHEN TRAINING MY CHEST IN PRIORITY??