Biomechanics - Principles of Biomedical Engineering - page 167

1. A fi rst-class lever is used to gain either force or distance, depending on
the relative length of force and resistance arm. It is frequently observed in
the body for maintaining posture and balance. Examples include the
atlanto-occipital joint, where the head is balanced by the neck extensor
muscle force and intervertebral joints, where the trunk is balanced by
extensor spinae muscle forces.
2. A second-class lever is the applied force and resistance on the same side of
axis, with the resistance closer to the axis. It provides a force advantage
such that large weights can be supported or moved by small forces. An
example is a person standing on his or her toe.
3. A third-class lever is the force and resistance on the same side of the axis,
but with the applied force closer to the axis. This is more common in the
body, and most muscle-bone lever systems are of the third class. Based on
the application that a musculoskeletal system is performing, the lever
classifi cation is subjected to change.
The angle at which a muscle pulls on a bone affects the mechanical effective-
ness of the muscle bone lever system [Figure 5.4(b)]. The force of muscular tension
is resolved into two force components: one perpendicular to the attached bone and
one parallel to the bone. Only the component of the muscle force acting perpen-
dicular to the bone actually causes the bone to rotate about the joint center. The
angle of maximum mechanical advantage for any muscle is the angle at which the
most rotary force can be produced. The angle at the elbow at which the maximum
flexion torque is produced is approximately 80°. The greater the perpendicular
distance between the line of action of the muscle and the joint center (force arm
distance), the greater the torque produced by the muscle at the joint.
The way that muscles are arranged in the body plays an important role in how
they function. Flexion muscles are arranged to decrease the angle between articu-
lating bones, whereas extensors are arranged to increase the angle between articu-
lating joints. The velocity of contraction of muscles is influenced by the intrinsic
speed of the shortening (the rate of conformational changes in the cross-bridge)
number of sarcomeres in series. Factors such as the length of muscle influence
the force of contraction. At lengths greater than their resting length, they develop
tension or force. This force is passive, since it exists whether or not the muscle is
active. The passive force acts in a direction from the muscle's points of attachment
toward its center.
5.2.4 Impulse-MomentumRelation
The tolerance of various tissues to forces depends on the magnitude as well as the
duration of forces acting on them. The sum of all forces acting during a certain
period is termed the impulse. If the force is constant, then the product of force and
time is the impulse. When the force is not uniform, a normal practice is to plot
the changes in force as a function of time, and impulse is the area under the curve.
Consider a person jumping. If the force generated against the floor is plotted over
the jump time (Figure 5.5), then the shaded area represents the impulse generated.
The impulse of a force is a vector quantity. To determine net impulse acting on the