Most of the motion generated by joints in the human body are examples of levers.

Question

Most of the motion generated by joints in the human body are examples of levers. An example of a third class lever is the forearm; the force which generates the motion is located in between the pivot point and the resistance force. The elbow acts as the pivot point, the biceps muscle pulls up on the forearm to move the mass which is held in the hand. Imagine you are holding an object in your hand with your forearm horizontal and your upper arm vertical so your elbow creates a 90degree angle. If your biceps attach to your forearm 0.0425 m from the elbow and make an angle of 26.0degree with the vertical while a 3.14 kg mass is held in your hand 30.75 cm from the elbow, what force is exerted by the biceps muscle? Ignore the mass of the forearm.

Explanation / Answer

This is done by the equilibrium of torques about the elbow. The formula for torque is FL (as long as we have the perpendicular component.

The torque from the biceps mucsle must balance the torque from the mass

For the biceps muscle = F(cos 26)(.0425)

For the mass, = (2.76)(9.8)(.3075)

Set them equal

F(cos 26)(.0425) = (2.76)(9.8)(.3075)

F = 217.7 N

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