Biomedical Engineering Reference
In-Depth Information
SYNTHESIS OF HUMAN
MOVEMENT — FORWARD
SOLUTIONS
8.0
INTRODUCTION
The vast majority of kinetic analyses of human movement have been inverse
dynamics. As detailed in Chapters 5 and 7, this type of analysis took the
kinematic measures and combined them with measured external forces (i.e.,
ground reaction forces) to estimate the internal (joint) reaction forces and
moments. We used the outcome measures plus a link-segment model to pre-
dict forces that were the cause of the movement. This, of course, is the inverse
of what really happens. The real sequence of events begins with a varying
neural drive to the muscles, resulting in varying levels of recruitment of ago-
nist and antagonist muscles. The net effect of all muscle forces acting at each
joint is the generation of a time-varying moment, which in turn accelerates (or
decelerates) the adjacent segments and ultimately causes the displacements
that our cameras record. If we model this approach in the computer, we are
doing what is called a
forward solution
.
The constraints of forward solution models are considerable when com-
pared with inverse solutions. For example, if we wish to calculate the ankle
and knee moments on one limb, we do not need data from anything but
the segments concerned (in this case, the foot and the leg). No kinematic or
kinetic data are required from the thigh, the opposite limbs, the trunk, the
arms, or the head. For a forward solution, we must model the entire body
before we start, or in the case where part of the body is fixed in space, all seg-
ments that are capable of moving must be modeled. If the link segment that
we use is not a valid replication of the anatomical situation, there will likely
be major errors in our predictions. The reason why we must model the entire
link system is the interlimb coupling of forces. Our inputs could be net mus-
cle moments at each joint plus the initial conditions of positions and velocity.
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