Download Levers and Movement

Levers and Movement
We can consider the mechanisms by which muscles act on bones using
descriptions based on levers. A lever is a rigid structure, in this case a
bone that moves on a fixed point called the fulcrum, in this case an
articulation. A lever moves when an applied force or effort is sufficient
to overcome any load or resistance that would otherwise oppose or
prevent such movement. In the body, each bone is a lever and each joint
is a fulcrum, and muscles supply the applied forces. Movement of the
skeleton occurs at joints, so there has to be sufficient muscle power to
move all the bones at these joints.
Levers can change the direction and effective strength of a force as well
as the distance and speed of movement produced by the force. Imagine a
seesaw in a playground. You and a friend could sit on opposite ends of it.
You would then take turns pushing off the ground with your legs as you
each went up in the air. A seesaw is an example of a first-class lever, and
there are three classes of levers. The fulcrum (F) lies at the midpoint of
the seesaw, between the applied force (AF) and the load (L). What this
means is that the seesaw balances you and your friend, as you both
provide the applied force with the push of your legs and the load with
the weight of your bodies.
The body has only a few first-class levers. One is involved in head
flexion (pulling the head down toward the chest) and head extension
(pulling the head back up into normal position). The fulcrum is where
the head moves in the sagittal plane on the first cervical vertebrae. The
load is the weight of the head, and the applied force comes from the
muscles. Extension occurs when the muscles pull the head back up.
Moving your head forward and backward mirrors the action of a seesaw.
In a second-class lever, the load is located between the applied force
and the fulcrum. When you move a load in a wheelbarrow, you lift
upward on the handle and the wheel acts as the fulcrum. The force is
further from the fulcrum than the load is, so you can move a larger
weight with less effort. In the body, you achieve the same effect by
standing on your toes. The fulcrum is on the ball of the foot, the load is
your body weight, and the effort comes from the muscles in the back of
the leg.
In third-class levers the force is applied between the load and the
fulcrum. We don’t often see these types of levers in artificial machines,
but they are the most common levers in the body. Speed and distance
traveled are increased at the expense of force. For the biceps brachii in
the arm, the load will be located in or around the hand. The biceps
muscle applies the force, while the fulcrum is the elbow. For instance,
when you pick up a full bag of groceries, you can lift it quickly using the
third-class lever of the biceps brachii and the forearm. However, the
location of fulcrum prevents great enhancements of load carrying.