To test the sensitivity of knee flexion-extension moment patterns to variation in estimates of the knee center location (KCL), inverse dynamics analysis was performed using three different KCLs for data collected from 18 healthy adult subjects who walked at five different speeds (25%, 50%, 75%, 100% and 125% of 0.785 statures.s< ?? opnbop ?? ?-1). The KCL, which was determined using data from static subject calibration trials, was varied in software by plus or minus 10 mm in the anteroposterior direction. The effect of this KCL variation on knee moments was similar across subjects and at all five walking speeds. The relative knee center variation effect, expressed as a percentage of the mean knee moment, was progressively greater at slower walking speeds, up to 123% of the first extensor peak at the slowest speed. The implication of these results is that, while KCL variation does not affect the general shape of the moment pattern at more natural walking speeds, it can change the sign of the moment-and thus the interpretation-when the moment magnitude is small (e.g. at the very slow speeds used by some patients). When knee moments are of magnitudes less than or equal to the KCL variation effect, one cannot confidently interpret them as representing either a net flexor or extensor knee control strategy. Copyright 1998 Elsevier Science B.V.