How Much Force Must the Biceps Generate in an Isometric Exercise?

[mwoods18]

Homework Statement

During an isometric exercise for strengthening the biceps, an external force of 200 N is applied perpendicular to the forearm 0.3 meters from the center of rotation of the elbow joint. The elbow is flexed to 90 degrees and the forearm is supinated. The force produced by the biceps muscle, which for our purposes is the only active muscle, has a moment arm of 0.04 meters and a line of application parallel to the long axis of the humerus. How large is the force (B) must the biceps muscle generate to counteract this torque? The weight of the forearm and hand is 13.7 Newtons and the length of the forearm and hand is 0.4 cm. The center of mass is 68% of the length of the forearm and hand from the elbow to the wrist.

Homework Equations

Sum of Torque= 0
F*FA=R*RA

The Attempt at a Solution

(0.028m)(200N)=(0.272m)(Fmuscle)
Fmuscle=20.6N

 

[Doc Al]

There are three forces that produce torques in this problem:
(1) weight of the forearm and hand (where does it act?)
(2) external force of 200 N (where does it act?)
(3) biceps force (where does it act?)

The torque due to the biceps force must balance the torques due to the other forces.