Biomedical Engineering Reference
In-Depth Information
done by a constant torque
T
is the product of the torque and the angle through
which it turns. The muscle power (i.e., the rate of doing work) is defined as the
scalar product of the joint torque and the segment's angular velocity.
5.2.3 Leverage
Forces arising due to various activities of the body are transferred to other anatomi-
cal structures. The amount of force transferred or required to perform an activity is
a function of the framework of the anatomical structure and the location of joints.
The concept levers are used to understand how body forces act. A lever is a simple
mechanical device used to produce a turning motion about an axis with an appli-
cation of the torque. The bones act as the levers about which the muscular system
generates the movements. Lever components include:
The fulcrum, which is the point of support, or axis, about which a lever may
be made to rotate;
The force arm, which is the distance from the fulcrum to the point of applica-
tion of the effort;
The resistance arm, which is the distance from the fulcrum to the weight on
which force is acting.
The ratio of the force arm to the resistance arm is called a mechanical advan-
tage. If the mechanical advantage is greater than one, more load can be lifted than
the applied effort. If mechanical advantage is less than one, less load can be lifted
than the applied effort. Levers are classified into three types [Figure 5.4(a)] based
on the location of the fulcrum, effort, and resistance:
First-class lever
First-class lever
Second-class lever
Second-class lever
Third-class lever
Third-class lever
F
m
F
m
Rotating
component
Rotating
component
Angle of
Angle of
attachment
<90
attachment
<90
°
°
E
E
R
R
R
R
E
E
R
R
E
E
F
F
F
F
F
F
dislocating component
dislocating component
F
m
F
m
Angle of
Angle of
E
E
attachment
=90°
attachment
=90°
E
R
F
F
R
F
m
F
m
F
Angle of
Angle of
R
attachment
>90°
attachment
>90°
stabilizing
component
stabilizing
component
(a)
(b)
Figure 5.4
Leverage in musculoskeletal system. (a) Different classes of lever showing the location where
E
is
the effort,
R
is the resistance, and
F
is the fulcrum. Also shown are examples from different parts of the body.
(b) Resolving forces in bone-muscle actions.
