Fundamentals of Biomechanics

142 FUNDAMENTALS OF BIOMECHANICS
Figure 6.7. Typical angles of pull of the biceps brachii muscle in an arm curl. The angular positions of the shoulder
and elbow affect the angle of pull of the muscle, which determines the size of the components of the muscle
force. Muscle forces (F) are usually resolved along the longitudinal axis of the distal segment (F L ) and at right angles
to the distal segment to show the component that causes joint rotation (F R ).
in the extended position, the rotary component
is similar to the stabilizing component.
In the more flexed position illustrated,
the rotary component is larger than the
smaller stabilizing component. In both positions
illustrated, the biceps muscle tends
to flex the elbow, but the ability to do so
(the rotary component) varies widely.
This visual or qualitative understanding
of vectors is quite useful in studying
human movement. When a muscle pulls at
a 45º angle, the two right-angle components
are equal. A smaller angle of pull favors
the longitudinal component, while the
rotary component benefits from larger angles
of pull. Somewhere in the midrange of
the arm curl exercise the biceps has an angle
of pull of 90º, so all the bicep's force can
be used to rotate the elbow and there is no
longitudinal component.
Vectors can also be qualitatively added
together. The rules to remember are that the
forces must be drawn accurately (size and
direction), and they then can be added together
in tip-to-tail fashion. This graphical
method is often called drawing a parallelogram
of force (Figure 6.8). If the vastus lateralis
and vastus medialis muscle forces on
the right patella are added together, we get
the resultant of these two muscle forces.
The resultant force from these two muscles
can be determined by drawing the two
muscle forces from the tip of one to the tail
of the other, being sure to maintain correct
length and direction. Since these diagrams
can look like parallelograms, they are called
a parallelogram of force. Remember that
there are many other muscles, ligaments,
and joint forces not shown that affect knee
function. It has been hypothesized that an
imbalance of greater lateral forces in the
quadriceps may contribute to patellofemoral
pain syndrome (Callaghan &
Oldham, 1996). Does the resultant force (F R )