Question

If I hold a weight activating only my biceps it takes 500N of force. My elbow is at 120°. My biceps inserts 5 cm away from the elbow and has a line of action that is directed at 100°. If I hold the weight activating only my brachialis it takes 700N of force. How large is the moment arm of the brachialis?

Answer 1

Angle AOB is 120 degrees and angle OBF is 100 degrees,OB = 5cm. Both Brachialis and Biceps act at the same line of force.

To hold the weight while activating either of the muscles, we can say that the moment generated by Biceps as well as Brachialis should be same. The moment is given by,

Hence the magnitude of moment is,

Here is the angle between r and F vectors, the angle is OBA as per the figure.

This moment should be equal to the one generated by Brachialis. Let the moment arm be at a distance x, then we can write,

Therefore the moment arm is at a distance of 3.6cm from the elbow.