What is the length of the moment arm.

[zdickz]

So here's the problem:

Bodybuilder “Hulk” Heise is doing leg curls on a Cybex machine. “Hulk” gets stuck (see figure) with 90° angle at the knee joint. The force exerted by the machines (FA) on his leg is 500 N, directed at an angle of 55° (angle α) relative to the tibia, and the distance from “Hulk’s” knee joint to the point where the force is being applied is 55 cm (distance L). For this problem, assume the weight of the shank and foot is negligible and the muscle force (FM) acts perpendicular to the tibia in this position (5 points each) (Units!).

What is the length of the moment arm from the knee joint to the applied force (FA)? (hint: drawing a line and use trigonometry will help)

I attached a diagram of the problem. My question is this. I'm looking at the picture and thinking that the moment arm IS the tibia, or the distance from the knee to where the force Fa is applied. Is this not correct?

Thank you for any help on this question folks. I appreciate it!

 

[PhanthomJay]

The moment arm is the perpendicular distance from the line of action of the applied force to the knee joint (see hint). What you are calculating is the moment arm of the component of FA (perpendicular to the tibia) to the knee joint. You get the same moment either way, but the problem specifically asks for the moment arm of the applied force FA.