A lever is a stiff rod that can rotate about a fulcrum or pivot point. This rotation transmits force.

When the lever is at equilibrium the tendency for it to rotate **clockwise** is balanced by the tendency
to rotate **counterclockwise**.

**What factors determine the equilibrium point of a lever?**

- Add 25 gm weights to the lever and balance it.
- Note the distance from the weights to the fulcrum. These distances are the lengths of the two lever arms (L
_{1 }and L_{2}). - Calculate the product of the weight (force = F) and length (L) of the lever arm on each side of the fulcrum.

**What is the relationship of these two products?** Check yourself

**What happens if one of the weights moves closer to the fulcrum?** Check
yourself

- Replace one of the 25gm weights with a 50gm weight.
- Balance the lever and compare L
_{1}and L_{2} - How long should the lever arm with the 50 gm weight be? Check yourself

In the muscle/skeletal system a contracting muscle functions as a weight. When it contracts it creates an **in-force**
that is translated into an **out-force** by the lever system.

- Write the equation for this relationship when the system is at equilibrium. Check yourself.
- Solve the equation for
**F**. Check yourself_{out}

Changes in the length of skeletal elements can result in more efficient transmission of an in-force to an out-force. Based on the equation there are several ways to increase the force efficiency of a lever:

- increasing the length of the in-lever arm
- decreasing the length of the out-lever
- or doing both of the above

Make sure you are comfortable with the above ideas before you explore the different types of levers. Return to beginning