KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 135

where: β i is the angle between the effective direction of pull and the line
joining the insertion point to the joint center.
In Figure 5.20, β for the soleus only is shown. Thus, it is now possible
to calculate S ei (t ) over the time that the plantarflexors act during the stance
phase of a running cycle. Thus, each muscle force F ei (t ) can be estimated
by multiplying S ei (t ) by each PCA i . With all five muscle forces known,
along with R g and R a , it is possible to estimate the total compressive and
shear forces acting at the ankle joint. Figure 5.21 plots these forces for a
middle-distance runner over the stance period of 0.22 s. The compressive
forces reach a peak of more than 5500 N, which is in excess of 11 times this
runner's body weight. It is interesting to note that the ground reaction force
accounts for only 1000 N of the total force, but the muscle forces themselves
account for over 4500 N. The shear forces (at right angles to the long axis
of the tibia) are actually reduced by the direction of the muscle forces. The
Figure 5.21 Compressive and shear forces at the ankle calculated during the stance
phase of a middle-distance runner. The reaction force accounts for less than 20% of the
total compressive force. The direction of pull of the major plantarflexors is such as to
cause an anterior shear force (the talus is shearing anteriorly with respect to the tibia),
which is opposite to that caused by the reaction forces. Thus, the muscle action can be
classified as an antishear mechanism.