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