CHAPTER 5
DISCUSSION
Assumptions, Limitations, and Future Work
Joint Model Selection
If the current model cannot adequately reproduce future experimental motions, the
chosen joint models may be modified. For example, the flexion-extension of the knee is
not truly represented by a fixed pin joint (Churchill et al., 1998). When comparing the
fitness of the optimum knee joint model to multi-cycle experimental marker data, the
agreement was quite good for all knee flexion angles with the exception of those
approaching full extension. By eliminating knee flexion angles less than 200, which
comprised 18% of the flexion-extension data, the mean marker distance error was
reduced to 0.48 + 0.23 cm (11.89% decrease) using the optimum model parameters from
the full data set. A pin joint knee may be sufficiently accurate for many modeling
applications. A 2 DOF knee model (Hollister et al., 1993) may account for the
screw-home motion of the knee joint occurring between 00 and 200 (Blankevoort et al.,
1988). If greater fidelity to actual bone motion is necessary, a 6 DOF knee joint may be
implemented with kinematics determined from fluoroscopy (Rahman et al., 2003).
Design Variable Constraints
Certain joint parameters must be constrained to zero with the purpose of preventing
the unnecessary optimization of redundant parameters. Case in point, the medial-lateral
translational model parameter placing the knee joint center in the femur segment must be
constrained to zero. On the other hand, this model parameter may be used as a design