Let me ask you a question: What is the main difference between a dumbbell fly on a flat bench and a dumbbell press on a flat bench?
Now, there are a multitude of directions you could go with your answer, so for some semblance of direction we will say this question is to be looked at from the perspective of the joints involved in creating the respective motions.
Okay, that doesn't exactly narrow it down, either, because it could be correctly argued that every joint in the body is involved in both motions, but some joints have motion occurring at the joint while at other joints there isn't any motion occurring. SO, for sake of this post, I will once again narrow down the focus of my original question to say, "What is the main difference between a dumbbell fly on a flat bench and a dumbbell press on a flat bench from the prospective of the joints involved, with limiting the discussion to joints where there is motion at said joint?"
Longer than I had originally intended, but alas, hopefully my idea has been conveyed appropriately. So how would you answer this?
At first glance, almost everybody is going to cite the difference in elbow flexion and extension between the motions, and I would agree that is the main difference. Now, what would you say the second biggest difference is?
Here's the reason I bring this up: When looking at movements, don't just take into consideration the muscles that are allegedly working during said movement; look at the stress placed on the joints, as well. When you perform a flye on a flat bench, there is a tremendous about of stress placed on the gleno-humeral (GH) joint of the shoulder. The reason for the increased stress on this joint is because the moment arm of the applied force is significantly longer when the humerus is at 90 degrees of abduction and the elbow is in a position of 0 degrees of flexion (as is the case during a flye on a flat bench) than when the humerus is horizontally abducted and the elbow is flexed at approximately 90 degrees (as is the case during a dumbbell press on a flat bench).**
**Side note: An appropriate precursor to this post probably would have been along the lines of defining a moment arm and defining what abduction is in relation to the humerus. If you are confused, Google it, or drop a comment below and I'll make sure to post on these subjects in the future.
So, how much greater is the stress on the GH in this position? Well, this is going to vary completely on the individual. BUT, for the sake of this post, let's say the three divisions of pec major attach 1" away from the axis (the GH, in this case). We'll say the person's humerus is 10" long (the distance between GH and elbow) and the distance between the elbow and where the weight (length of radius and ulna plus some--RU+) is in their hand is 10". That means that at a position of 90 degrees of humeral abduction and 0 degrees of elbow flexion (flye) the length of that lever is 20", whereas at a position of 90 degrees of humeral abduction and 90 degrees elbow flexion (press) the length of that level is 10". There is also a component of external rotation, among other things, in both of these scenarios.
Let me stop here and say that this is an INCREDIBLY simplified model, with many, many assumptions, not the least of which is that the person in question can both achieve enough elbow extension to allow for a full ten inches of distance between the weight in their hand and the elbow joint as well as actually be in this much elbow extension at the bottom of their flye rep. Also, I would like to say that for this example we will not be taking into consideration the stress upon the joint at all of the infinite number of positions throughout the flye and press motions, nor will we be considering what is actually happening during the motions. I will simply (or not) be explaining the scenario from the two positions described--90 degrees of humeral abduction with 1) 0 degrees of elbow flexion and 2) 90 degrees of elbow flexion.
Okay, back to the numbers. First, let's look at the press.
At a position of 90 degrees of humeral abduction and 90 degrees of elbow flexion, there is a moment of resistance that is 10" long. The moment of effort is, as stated before, 1". Assuming the person is holding a 5-pound dumbbell, the fibers that horizontally adduct the humerus will have to generate 50 inch-pounds of force to horizontally adduct the humerus from that position. The 45-pound difference between the weight that is being held and the force that has to be generated to move it is what is placed on the GH.
Now, looking at the flye.
At a position of 90 degrees of humeral abduction and 0 degrees of elbow flexion, there is a moment of resistance that is 20" long. We still have the same 1" moment of effort and are using the same 5-pound dumbbell, BUT there will have to be 100 inch-pounds of force generated to horizontally adduct the humerus from this position. This will result in an excess of 95 pounds of force being placed upon the GH, over twice as much as with the press.
One could argue that you use less weight with a flye than with a press, so that might make the stress on the GH equal. True, but once we talk about actually moving the weight there are too many other variables to take into consideration because now we are talking about a moving resistance instead of a static resistance so inertia and other variables will come into play.
What is my point of writing this? It is not to say flyes are bad, or even that presses are superior or something along those lines. Quite frankly, the hierarchy of good and bad regarding different exercises is 100% based on the limitations of the individual performing them and his or her goals, so I could not logically talk about this in a blog post. My reason for writing this is to get you to think in a different light about the movements you are performing and the toll that they may or may not be taking on your body.
- An Educated Approach To Training
If we all can begin to take a more educated approach to our training, one in which we actually understand what we are doing, we can begin to truly manipulate these variables to our advantage to create positive adaptations in a safe and effective manner instead of randomly flinging our bodies and weights in different directions and assuming the intended outcome will be the actual outcome.
Also, I am by no means an expert on the terminology I used in this post today, and this way of viewing movement is very new to my thought process. If you are and notice I have messed something up in my verbiage or explanation, please let me know via the comment section or e-mail at charlie@selfmadefitness.com. Many thanks!
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Charlie Cates, CSCS
Self Made®, Owner and Founder
Charlie Cates is a strength and conditioning specialist and the owner and founder of Self Made® (http://selfmadefitness.com/) in Chicago, IL. He has worked with competitive and everyday athletes of all ages and ability levels, from 9-year-old kids to NFL MVP’s. He can be reached via e-mail at charlie@selfmadefitness.com.
This article may be reproduced with biographical information intact.
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