Gait variables, pelvic rotations in the frontal and sagittal plane and RA-EMG (rectified and averaged EMG) of the three columns of the intrinsic lumbar back muscles (= ILBM) were recorded simultaneously during 48 succeeding strides of 11 subjects on a treadmill. Bilateral activity is found in all parts of the ILBM during the double support phase. After right heel strike the right ILBM (and after left heel strike the left) show in most cases more activity than the contralateral ILBM. This is especially so in the intermediate and lateral columns, which consist mainly of the longissimus thoracis and the iliocostalis lumborum muscle and less so in the medial column, made up mainly by the multifidus and spinalis muscle. This difference is probably due to the difference in moment arm for the two directions. Pelvic rotations are described, but no evident relationships between pelvic rotations in the different planes and ILBM-activity could be seen, probably because the changes in the position of the torso are of more importance.