...But let's take it from the start. Some basic anatomy:
What we usually refer to as a "shoulder" is two bones, the collarbone and the scapulae. At the far end away from your throat they connect, basically making a frame for muscles to attach to (such as delts, traps and upper pecs). The upper-arm bone ends in a dome, which slides around on a shallow cavity of the scapulae, thus enabling us to move our arms in pretty much any way that we wish. And one of the biceps-joints (the long head) starts just above that shallow cavity. So far, so good. Now, there's a little problem that occurs when lifting your arms, and that is that the space between the upper-arm dome and the connected bones of the shoulder girdle is pretty tight. Why don't we do dumbbell lateral raises above 90 degrees? Because you start lifting your shoulders! And why is that? Simply because the bone-dome and the far end of the connected bones starts pressing against each other, thereby forcing the entire structure to elevate!
An easy test: Put your finger on top of your shoulder. Hold your arm out slightly and flex the delts, and you'll find the spot with no muscle covering it. Now you're holding your finger on acromion, the outermost point of the scapulae. Keeping your finger on it while raising your arm, you'll notice that no matter how hard you try, that sucker starts going upwards at a certain point no matter what you do! That's not it wandering by itself, it's being PUSHED upwards by underlying dome. With this in mind, you probably quickly realize that it's not a good idea to have a vulnerable joint being stuck in between ... Especially not if holding a heavy dumbbell increasing the pressure tenfold!
At this angle though, sitting straight up, you're squeezing the joint of M.Supraspinatus, a member of the rotatorcuff. The bicep joint is located slightly further down and anterior, thus saving it from most stress at this angle ... But when you're leaning back on a bench and start doing some heavy flies, boy, are you hitting the spot or what!
Just like you were squeezing and tearing at the rotator cuff when sitting straight, you're doing the same to the biceps when you're angled backwards. It sucks, but it's one of the main reasons for shoulder-ache among serious trainers! "What am I talking about," you ask, "the shoulder? I thought we're dealing with the biceps here!" Yes, we are, and the biceps are so much more than the musclebelly you see in the mirror at the locker-room. The joint keeps going under the delts and all the way up and back above the upper-arm bone. Delt pain? Not quite - it's under the delts! If you experience weird, stubborn ache in your shoulders once in a while, give it a try and replace those flies with cablecrosses instead (which are safer because of their angle and range of motion).
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