(Motion Analysis Corp., Santa Rosa, CA), including six HSC-180 cameras, EVa 5.11

software, and two AMTI force plates (Advanced Management Technology, Inc.,

Arlington, VA). Marker data were collected at 180 Hz during 3 seconds for static trials

and 6 seconds for individual joint trials. The raw data were filtered using a fourth-order,

zero phase-shift, low pass Butterworth Filter with a cutoff frequency set at 6 Hz.

Hip Joint

 There are 6 translational model parameters that must be adjusted to establish a

functional hip joint center for a particular patient (Figure 3-4, Table 3-2). Markers placed

over the left anterior superior iliac spine (ASIS), right ASIS, and superior sacrum define

the pelvis segment coordinate system. From percentages of the inter-ASIS distance, a

predicted (or nominal) hip joint center location within the pelvis segment is 19.3%

posterior (pi), 30.4% inferior (p2), and 35.9% medial-lateral (p3) (Bell et al., 1990). This

nominal hip joint center is the origin of the femur coordinate system, which is

subsequently defined by markers placed over the medial and lateral femoral epicondyles.

An additional 3 translational model parameters (p4, p5, and p6), described in the femur

coordinate system, complete the structure of the nominal hip joint center.

 Given the physical hip joint center is located within the pelvic region lateral to the

midsagittal plane, a cube with side lengths equal to 75% of the inter-ASIS distance and

its anterior-superior-medial vertex positioned at the midpoint of the inter-ASIS line

provides the geometric constraints for the optimization of each model parameter (Figure

3-5, Table A-i, Table B-l).

Knee Joint

 There are 9 model parameters (5 translational and 4 rotational) that must be tailored

to identify a patient-specific functional knee joint axis (Figure 3-6, Table 3-3). The