Originally posted by KryptoAllez
But... is it not true that the latissimus dorsi, one muscle but it is also actually made up of MANY individual muscle FIBERS which insert at different angles into the fascia? Is that not true? Cause I believe it is, just from what I have read and see in illustrations of muscle anatomy. Therefore, if the muscle fibers are inserted/attached at different angles, would it not be possible to place more STRESS on some fibers (perhaps the lower fibers in this case) than on others? Maybe they ALL contract, to SOME degree, but perhaps because of the angle of movement you are using, you can actually contract some sections MORE than others or to more of a degree. Cause it's not like the sections are all "glued" together or something. It's not one solid block.
You're quite correct. Its not a solid block, and it is composed of many different fibers. However, neurologically, muscles aren't wired that way. When the signal is sent to contract, it contracts.
Angle of pennation (the angle of attachment) isn't really a factor, for a reason I'll elaborate on in a moment.
There actually has been research done on so-called "compartmentalization" of motor units-- this would be what you're referring to.
However, there's very little work done on it, and what work was done involved very light weights or electrostimulation-- while this did show some preferential growth in different parts of a muscle, there's two reasons I discount it.
Firstly, its a genetic thing. The MU compartments don't change with training, so if your muscle is predisposed to grow in one section over another, then that is how it will grow, regardless of the way you train.
Secondly, the tensions used were too small to cause hypertrophy. Once the tension becomes high enough to cause growth, the muscle's mechanism of transmitting force to the insertions will all but eliminate any preferential tensions in individual parts of the muscle, so even the MU compartments are a moot point.
Personally, I think that the people who keep arguing that you can't target or stress certain sections of muscles more than others are looking at muscle anatomy with too simplistic of a view. The muscles are made up of INDIVIDUAL fibers, INDIVIDUAL cells and this alone should show that it is not as simplistic as saying "it's one muscle and either it contracts or it doesn't", it is just NOT that simple! The idea that a muscle would respond in a uniform fashion just seems implausible in light of the fact that there are distinct physiological/anatomical differences within a single muscle! And that's my 2 cents. 
Anatomy's part of my reasoning, yes. However, more in-depth study of the neurological processes at work and mechanical properties of the muscle itself have only reinforced my viewpoint.
Unfortunately, my vocabulary is somewhat limited.
