There are many unknown factors that affect loading at the hip joint during functional activities. One of the major clinical concerns with total hip replacements (THRs) is long term loosening. Torsional loading has been identified as one factor related to loosening of THRs.

This dissertation related the muscle forces at the hip joint to the torsional loading of a THR. A parametric approach was used to solve the indeterminate problem of determining the necessary muscle forces during functional activities. The kinematics and kinetics of the lower extremity joints during gait and stairclimbing were measured with an optoelectronic system and a force plate. An analytical computer model of the musculoskeletal system was then used to parametrically determine the potential range of muscle forces during these activities. The effect of muscle force distributions on the loading of the hip joint was explored along with the additional effect of functional adaptations in subjects with THRs. The relationship between the functional adaptations and possible losses in muscle strength due to surgical trauma were explored. Similarities between preoperative and postoperative functional adaptations were also examined to evaluate the possibility that the postoperative adaptations were learned responses from the preoperative condition.

The muscle forces were the predominate factor in determining the stem rotatory torque at several instances during gait. The choice of muscle force distribution and level of antagonistic muscle activity had a large impact on the stem rotatory torque. Therefore, even though the kinematics and kinetics of two subjects with THRs may be similar it is possible that the resulting implant stability from the rotatory torques may be very different based on the individual's muscle activation patterns.

The largest rotatory torques during gait occurred later in stance during the peak hip extension moment. Increased rotatory torques were associated with increased iliacus and psoas muscle activity. The rotatory torque could be reduced by increasing rectus femoris and tensor fasciae latae muscle activity in preference to that of the iliacus and psoas. The most common gait adaptation in subjects with THRs was a decreased hip extension moment which partially resulted from a decreased hip range of motion. This occurred when the rotatory torques were greatest. The decreased hip extension moment found in the THR group lessened the overall demand on the hip flexors. This dramatically decreased the rotatory torque as compared to that of the normal group by lowering the overall demand on the iliacus and psoas muscles.

The rotatory torques in this study were of the same order of magnitude as those calculated by other investigators who either used THRs with transducers or determined the forces analytically using optimization methods. In some cases the range of rotatory torques exceeded those that caused micromotion in in vitro cadaveric testing. This emphasizes the importance of designing THRs to resist torsional loads.