We studied the effect of walking at a self-selected and at a slower speed on the angular movements of the pelvis and lumbar spine. We also studied how interpretation of speed effects on lumbar spine movements was influenced by frame of reference, either relative to the pelvis or relative to a global reference frame. Twenty-seven subjects without pathology walked on a treadmill at either self-selected or 60% of self-selected speed. The movements of the pelvis and lumbar spine, as represented by surface markers, were recorded by videocameras and the three-dimensional angles computed by the PEAK motion measurement system. Results indicated that the amplitudes of pelvic list (P<0.05) and pelvic axial rotation (P<0. 05) were decreased at slow walking speed. Relative to the pelvis, the amplitude of lumbar lateral flexion was decreased with slower walking (P<0.01). In contrast, when lumbar spine movements were measured relative to a global reference frame, no differences were detected due to decreased walking speed. This suggests, firstly, that the effect of walking speed when evaluating the significance of decreased movements of the pelvis and of the lumbar spine (relative to the pelvis) of subjects walking at slower than self-selected speeds should be considered and secondly, that movement of the lumbar spine should be interpreted with respect to a frame of reference.