KINETICS: FORCES AND MOMENTS OF FORCE - Biomechanics and Motor Control of Human Movement - page 134

Figure 5.20 Free-body diagram of foot segment showing the actual and effective lines
of pull of four of the plantarflexor muscles. Also shown are the reaction forces at the
ankle and the ground as they act on the foot segment.

where θ s (t ) is the angle of the foot segment in the spatial coordinate system.
In Figure 5.20, the free-body diagram of the foot is presented, showing the
internal anatomy to demonstrate the problems of defining the effective inser-
tion point of the muscles and the effective line of pull of each muscle. Four
muscle forces are shown here: soleus F s , gastrocnemius F g , flexor hallucis
longus F h , and peronei F p . The ankle joint's center is defined by a marker on
the lateral malleolus. The insertion of the Achilles tendon is at a distance R
with an angle θ m from the foot segment (defined by a line joining the ankle
to the fifth-metatarsal - phalangeal joint). The foot angular position is θ s in
the plane of movement. The angle of pull for the soleus and gastrocnemius
muscles from this insertion is rather straightforward. However, for F h and
F p the situation is quite different. The effective angle of pull is the direction
of the muscle force as it leaves the foot segment. The flexor hallucis longus
tendon curves under the talus hone and inserts on the distal phalanx of the
big toe. As this tendon leaves the foot, it is rounding the pulleylike groove
in the talus. Thus, its effective direction of pull is F h . Similarly, the peronei
tendon curves around the distal end of the lateral malleolus. However, its
effective direction of pull is F p .
The moment arm length d ei for any muscle required by Equation (5.12)
can now he calculated:

= R i sin β i

d ei (5.14)