KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 110

muscles. Thus, unless the muscle is silent, it is impossible to determine
the contribution of these passive structures.
5.0.3 Joint Reaction Forces and Bone-on-Bone Forces ∗
The three forces described in the preceding sections constitute all the forces
acting on the total body system itself. However, our analysis examines the
segments one at a time and, therefore, must calculate the reaction between
segments. A free-body diagram of each segment is required, as shown in
Figure 5.3. Here the original link-segment model is broken into its segmental
parts. For convenience, we make the break at the joints and the forces that
act across each joint must be shown on the resultant free-body diagram.
This procedure now permits us to look at each segment and calculate all
unknown joint reaction forces. In accordance with Newton's third law, there
is an equal and opposite force acting at each hinge joint in our model. For
example, when a leg is held off the ground in a static condition, the foot is
exerting a downward force on the tendons and ligaments crossing the ankle
Figure 5.3 Relationship between the free-body diagram and the link-segment model.
Each segment is “broken” at the joints, and the reaction forces and moments of force
acting at each joint are indicated.
∗ Representative paper: Paul, 1966.