I've read and heard from several bodybuilders (Arnold, big Youtube guys) that the pectorals will only grow in proportion to the upper lats. Both must be trained equally to see growth.
What is the scientific explanation for this? How do the muscle fibers in the pecs know how the muscle fibers in the back are doing?
Does this rule apply to all pairs of opposing muscles?
Granted, the pecs and upper lats are not mechanically opposing, but are physically placed opposite of each other. Examples of other opposing muscles include bicep/tricep, quads/hamstrings, tibialis anterior/gastrocneumus. Arnold said that the tricep should be two-thirds of the upper arm to be aesthetically pleasing, so the proportion requirement could not always be 1:1 for all muscle pairs. He said that was an aesthetic requirement, not a physical requirement, but this doesn't rule out that a minimum physical requirement of some other proportion exists.