buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 231
In the case of dynamic analysis, during the gait, the force and the moment
in the ankle joint can be determined, through inverse analysis, using the method
of bodies isolation, by writing the motion equations either for the shank, or for
the foot, thus being removed one of the segments of the biomechanical system
shank-ankle-foot. For inverse dynamic analysis, the linear and angular
accelerations of the foot must be known, these being usually experimentally
determined. At the same time, the contact force between foot and ground must
be known, in order to solve the system of dynamics equations.
A
M
3
L
M
L L
B
M
2 L
C
M
Fig. 1 – Structural biomechanical model shank-foot.
2. Forces in the Ankle Joint
Determining the reaction forces and the rotation moment in the ankle
joint, requires to know all internal forces acting on muscles, tendons and
ligaments and between bones surfaces in contact, the insertion points of the
muscles, as well as the distances between the insertion points and the rotation
center of the ankle. Taken together, all these conditions are, practically,
impossible to be realized, because the static or dynamic biomechanical model
would be very complex. To get rid of this drawback, the method of force
reducing can be used. Through this method, the resultant force and moment in
the articulation is determined. The method of force reducing, applied to the
ankle, implies to cover the following steps (Fig. 3):
i) there are highlighted, on the structural model, all internal and external
forces acting on the system ankle-foot;
ii) a force denoted with F, is considered to be the resultant of all forces
exerted by muscles, tendons, ligaments and bones in contact;
1