The physics of a biceps curl

Factual Questions
21

What is the experimental answer?

22

According to the abcbodybuilding site above…
The following conclusions of moment arms for elbow flexion were derived from van Zuylen , van Velzen , van der Gon (1988), Murray , Delp , Buchanan (1995), and Murray , Buchanan , Delp (2000).

Magnitude of Moment arms

  1. The Brachioradialis has the largest.
  2. The Biceps Brachii is in second place.
  3. The Brachialis comes in third.
  4. The Pronator teres has the smallest MA.

As stated, however, moment arms peak at a certain level of elbow flexion (49, 76, 48,55). Here is a breakdown:

Peaks of Moment Arms

  1. The brachioradialis peaks at approximately 90 degrees of elbow flexion (49, 76, 48, 55).
  2. The biceps brachii peaks also at approximately 90 degrees of elbow flexion, and has been shown to vary up to 110 degrees of elbow flexion (49, 76, 48, 55). Recall that the elbow has been shown to flex at an average of 135-150 degrees of flexion (30, 31, 62, 63).
  3. The brachialis peaks at approximately 100 degrees of flexion, and up to 120 in some individuals (49, 76, 48, 55).
  4. The pronator teres peaks at approximately 75-80 degrees (49, 76, 48, 55. )

In conclusion, the Lever arms of the elbow flexors peak at the top range of motion. For the three larger muscles it peaks between 90 and 120 degrees, and including all muscles at 75-120 degrees. Thus, if we only analyze this aspect of torque, the greatest moment is realized at this range of motion. However, we know that force of contraction is also key to producing torque. We must therefore analyze each muscle’s force production capabilities before making absolute conclusions.

23

AskNott already did it, but I’m just driving by to yell: “There’s no such thing as a bicep, goddammit!”

Now, back to the science bit.

24

Even tho’ you guys have pretty much stated everything, I’m bored at work so I’d thought I’d do some real numbers.

Using treis’ illustration:

     |
     |
     |<---elbow
    /|
  /  |<-- theta is the angle between vertical and the forearm
/

W
|
/<–downward force of 10 lbs

So you can see at the hand there’s a downward force. At the elbow, there’s both a vertical force of 10 lbs (ignoring the weight of the forearm) and a moment (the force resisting rotation).

Moment is distance time force and in this case, the distance is the horizontal distance from the hand to the elbow which is sin(theta)*(forearmlength). Let’s assume the forearm is 1 foot and theta is 45 degrees. So the moment is:

M = sin(theta)(forearmlength)(weight) = sin(45)(1’)(10 lbs) = 7.07 ft-lbs

This makes it obvious that when theta is zero (the arm is hanging straight down), sin(0) = 0, and there is no moment to resist, only the downward force.

Similarly, when the arm is at a 90 degree angle, sin(90) = 1, so the moment is 10 ft-lbs and at a maximum. Once you get past 90 degrees, the curl becomes progressively easier as the angle again gets smaller.

Ok, who’s bored now?

25

Crap, meant to hit preview, not post. I guess the leading spaces got removed in the illustration. Here it is again:

…|
…|
…|<—elbow
…/|
…/ |<-- theta is the angle between vertical and the forearm
…/
…W
…|
…/<–downward force of 10 lbs

26

IIRC, the HTML specs state that multiple spaces should be treated as a single space, and vB takes it one step further by eating all leading spaces. Use the code tags to maintain your formatting (quote this post to see how):



         |
         |
         |<---elbow
        /|
      /  |<-- theta is the angle between vertical and the forearm
    /
  W
   |
  \/<--downward force of 10 lbs