SYNTHESIS OF HUMAN MOVEMENT—FORWARD SOLUTIONS - Biomechanics and Motor Control of Human Movement - page 201

A given joint moment acts on the two adjacent segments, and they, in turn,
create reaction forces on segments further away from the original cause. For
example, in gait the pushoff muscles at the ankle (plantarflexors) create a
large plantarflexor moment, which results in a rapid ankle plantarflexion. The
horizontal and vertical reaction forces at the knee are drastically altered by
this plantarflexor moment, and they, in turn, act on the thigh to alter its accel-
eration. In turn, the reaction forces at the hip, contralateral hip, and trunk are
also altered. Thus, the acceleration of all body segments is affected by the
ankle muscles at this time. If we have errors in any part of our anatomical
model, we will have errors in our prediction. If any segment has the wrong
mass or a joint has a missing or unrealistic constraint, the entire link system
will start to generate displacement errors, and these errors will accumulate
with time. Thus, a poor anatomical model, even with valid time histories of
moments, will start to accumulate trajectory errors very quickly.
8.0.1 Assumptions and Constraints of Forward Solution Models
1. The link-segment model has the same assumptions as those presented
in Section 5.0.1 for the inverse solution.
2. There must be no kinematic constraints whatsoever; the model must be
permitted to fall over, jump, or collapse as dictated by the motor inputs.
3. The initial conditions must include the position and velocity of every
segment.
4. The only inputs to the model are externally applied forces and internally
generated muscle forces or moments.
5. The model must incorporate all important degrees of freedom and con-
straints. For example, the hip and shoulder joints must have three axes
of rotation, but with limitations on the range of movements (because of
the passive internal structures such as ligaments), modeled as passive
internal forces and moments.
6. External reaction forces must be calculated. For example, the ground
reaction forces would be equal to the algebraic summation of the
mass — acceleration products of all segments when the feet are on the
ground. Partitioning of the reaction forces when two or more points of
the body are in contact with external objects is a separate and possibly
major problem.
8.0.2 Potential of Forward Solution Simulations
The research and practical potential of simulations are tremendous, but
because of the severe constraints described in the previous section, the
potential unfortunately has not been realized. The kinds of questions that
can be posed are prefaced with “What would happen if ... ?” For example,