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Human Body Leverage?

  1. Aug 10, 2011 #1
    Hi guys/gals,

    I had a couple random questions on leverage and the human body. I am an active individual that enjoys lifting weights and working out. At the gym today, I got to thinking about the "mechanical advantages" that certain body types have doing particular lifts.

    First question is curling a weight, by utilizing the bicep. So my question is this:

    Is there a formula/animation/drawing to point to the advantages of a shorter arm (lever) when doing a curl. Remember, the weight is on the hand and you bend at the elbow. So therefore, it appears quite obvious to me that a shorter arm (bicep + forearm combo) can lift more, due to the decreased angle.

    With that question, can anyone show how a 1" further placement up the hand can create a harder lift? In other words, how much more "force" is applied when 50 lbs is moved up 1"?

    Second question is pressing (bench pressing) a barbell weight from the chest, moving upwards. Obviously (like above), a short armed individual will have less trouble moving the same weight as one with longer arms. Is there any math to back this up?

    I hope this made sense and is in the right sub-forum.

    Thanks for any assistance.
     
  2. jcsd
  3. Aug 10, 2011 #2

    russ_watters

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    Staff: Mentor

    Mechanical advantage is the concept you are looking for and the formula for the advantage of a shorter arm is just the ratio of the lengths.

    For the 1" difference, I think it would be best seen in terms of percent, so for example if the normal distance is 12", a 1" increase in the distance raises the torque required by 8%.

    The math works the same for a bench press.
     
  4. Aug 10, 2011 #3

    Drakkith

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    Staff: Mentor

    Just to chime in, while it is easy to calculate things like leverage, when it comes to the human body things are far from easy. There are so many different variables that it becomes nearly impossible to figure out anything meaningful unless you spend a lot of time tracking them all down. Just because someone has a shorter arm doesn't necessarily mean they have an advantage. A taller person with a longer arm could have much more muscle overall even if the bulk isn't noticeably increased.
     
  5. Aug 11, 2011 #4

    A.T.

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    Google for "arm biomechanics". You need the the lever arm of the biceps, to compute how much force it must generate to lift a certain weight, at a certain distance:

    Sm_elbowfbd.jpg

    Ignoring the forearms own weight it is about 6x times the load that you hold in your hand.
     
  6. Aug 11, 2011 #5
    Thanks for that link; it seems to me that the length of the 'lever arm' between the elbow joint and where the biceps tendon attaches to the forearm is going to be proportionally shorter in a short-armed individual as well.

    I think this is also significant.

    It seems that most of the people I hear espousing the idea that "weightlifting is easier for short people" are tall guys who can't lift as much. Maybe the short guys just train harder :smile:
     
  7. Jan 18, 2012 #6
    Ha. I am looking for a new way to lift weights (to prevent plateauing) and stumbled into this forum. I am totally lost... ) At first, I thought I was onto something, but I still don't get it. I wouldn't expect anyone to explain this in a more layman's terms...if so, great. If not, I understand. You all are speaking in your own language. This might make for a good article in Men's Health though...
     
  8. Jan 18, 2012 #7

    sophiecentaur

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    The tall guys are probably makin' out with the girls instead. :biggrin:
     
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