From my Facebook page....
A muscle can shorten, lengthen, or be "at rest." During all of those, it can also be contracting or relaxed. This is a fact. A muscle that is being lengthened can be contracting and generating tension in the opposite direction of its lengthening. For example, when you lower the bar in the bench press, your pecs are lengthened (stretched) but under tension (contracting). When you reverse that movement, the pectorals and other pressing muscle (triceps, shoulders, etc) contract, shorten, and apply enough force to the bar to move it up. A muscle cannot apply force in the opposite direction of its contraction. Muscles can only PULL. When a muscle goes to apply force, it will contract, i.e. attempt to shorten. This is a fact. So to clear all this confusion up, all we need to understand is this.
1. When one muscle group contracts and pulls the limb it’s attached to, the antagonist stretches. So if you do a bicep curl, the triceps stretch while the biceps are contracting. So when you curl the weight up, the triceps lengthen. So the triceps cannot apply much force to the barbell curl, because the antagonist (the bicep) is being asked to contract, i.e. apply force. If the triceps applies a lot of force, you are working against yourself and making the curl harder.
2. To beat the dead horse just a little bit, once again, a muscle in the antagonist role cannot apply too much force before it becomes counterproductive to the movement.
3. One of the major antagonists for the pectorals is the latissimus dorsi. The lats.
4. When they contract and shorten, they pull the humerus backwards (not forward like in the bench press) and also pull it into the body (the opposite of the “lat flaring” and outward elbow drive on the ascent of the bench). The pectorals of course, stretch and lengthen during this activation phase caused by the lats.
5. During the eccentric portion of the bench press, when you lower the bar, the lats contract to provide stability as the pecs are stretched while also contracting. Then as the bar is pressed, the lats lengthen as the pecs are called to contract, thus they cannot be applying much force to the bar (to make it go up) as that would pull AGAINST the pecs and decrease force into the bar.
6. Because the lats DO provide for internal rotation and attach to the humerus, it is important for them to be tight so as to provide shoulder stability during the bench. Lats do play an important role in providing for stability on the bench and from an overall anatomical point. However they cannot apply too much force to the bar, because as we have shown they are the antagonists to the pecs, and the pecs are contracting to apply force to the bar.
7. Some guys say they get sore in the lats from benching. Well I can see that happening if he is contracting them strongly enough on the eccentric portion for stability on the bench. It is NOT anecdotal evidence that the lats are helping to "press" the bar, however. If you do negative only work on the bench press with say, 110% of your max, your chest will be screaming for days....yet you never pressed the bar.
8. All of this has nothing to do with how you bench or anything else. When guys talk of "loading the lats" it is indeed factual....on the descent, and especially to pull the elbows in closer to the body at the bottom. Once the bar is pressed, the lats have no choice but to mostly relax and lengthen, and when that happens they are not providing force to the bar. I understand the concept of "flaring" your lats to provide internal rotation that "appears" to make the bar move upwards. But “flaring the lats” is really just using your medial delts to pull the bar toward your face. This is preceded by contracting the lats, but the actual “flaring” is a partial relaxation and lengthening of the lats. This puts the bar backwards and shortens the moment arm of the movement, while also putting your pecs in to a better position to exert force.
I hope this helps a little bit and that some people are open minded enough to read it without acting like assholes.