ERRORS IN ALIGNMENT OF CENTER OF PRESSURE AND FOOT COORDINATES AFFECTPREDICTED LOWER-EXTREMITY TORQUES

The purpose of the study was to quantify the effect of errors in spati al alignment between the center of pressure recorded from a force plat form and the coordinates of the foot recorded from film on resultant j oint torques in the lower extremity during the stance phase of gait. T wo-dimensional kinematic and kinetic data from eight subjects performi ng walking and running were analyzed using inverse dynamics with the o btained center of pressure values and with +/-0.5 and +/-1.0 cm shifts in the anteroposterior location of the center of pressure under the s upport foot. Shifting the center of pressure posteriorly increased the flexor (dorsiflexor) torques at the hip and ankle and decreased the e xtensor (plantarflexor) torques at these joints. This shift also cause d an increase in the extensor and a decrease in the flexor torques at the knee. Shifting the center of pressure anteriorly caused the opposi te effects at each joint. The +/-0.5 and +/-1.0 cm shifts in the locat ion of the center of pressure caused, on average, 7 and 14% changes, r espectively, in maximum joint torque and angular impulse values. Relat ive transition times between flexor and extensor torques were either i ncreased or decreased, on average, by 7 and 13%, respectively for the two conditions over all trials. Based on these results, it is conclude d that due to potential errors in the spatial alignment of kinetic and kinematic data, joint torques in the literature on gait should be con sidered as approximations of the true values.