KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 111

joint. This is seen as a downward force acting on the leg equal to the weight
of the foot. Likewise, the leg is exerting an equal upward force on the foot
through the same connective tissue.
Considerable confusion exists regarding the relationship between joint
reaction forces and joint bone-on-bone forces. The latter forces are the actual
forces seen across the articulating surfaces and include the effect of muscle
activity and the action of ligaments. Actively contracting muscles pull the
articulating surfaces together, creating compressive forces and, sometimes,
shear forces. In the simplest situation, the bone-on-bone force equals the
active compressive force due to muscle plus joint reaction forces. Figure 5.4
illustrates these differences. Case 1 has the lower segment with a weight of
100 N hanging passively from the muscles originating in the upper segment.
The two muscles are not contracting but are, assisted by ligamentous tissue,
pulling upward with an equal and opposite 100 N. The link-segment model
shows these equal and opposite reaction forces. The bone-on-bone force is
zero, indicating that the joint articulating surfaces are under neither tension
nor compression. In case 2, there is an active contraction of the muscles, so
that the total upward force is now 170 N. The bone-on-bone force is 70 N
compression. This means that a force of 70 N exists across the articulating
surfaces. As far as the lower segment is concerned, there is still a net reaction
force of 100 N upward (170 N upward through muscles and 70 N downward
across the articulating surfaces). The lower segment is still acting downward
with a force of 100 N; thus, the free-body diagram remains the same. Gener-
ally the anatomy is not as simple as depicted. More than one muscle is usually
active on each side of the joint, so it is difficult to apportion the forces among
the muscles. Also, the exact angle of pull of each tendon and the geometry of
the articulating surfaces are not always readily available. Thus, a much more
Figure 5.4 Diagrams to illustrate the difference between joint reaction forces and
bone-on-bone forces. In both cases, the reaction force is 100 N acting upward on M 2
and downward on M 1 . With no muscle activity (case 1), the bone-on-bone force is zero;
with muscle activity (case 2) it is 70 N.