Abstract Park K-N, Cynn H-S, Kwon O-Y, Lee W-H, Ha S-M, Kim S-J, Weon J-H. Effects of the abdominal drawing-in maneuver on muscle activity, pelvic motions, and knee flexion during active prone knee flexion in patients with lumbar extension rotation syndrome. Objective To investigate the effects of performing an abdominal drawing-in maneuver (ADIM) during active prone knee flexion on the hamstrings and erector spinae muscle activity, the amounts of pelvic motion and knee flexion, and onset of pelvic movements. Design Comparative, repeated-measures study. Setting University research laboratory. Participants Men patients (N=18) with lumbar extension rotation syndrome. Intervention Subjects performed prone knee flexion in 2 conditions. Main Outcome Measures To measure muscle activity, surface electromyogram (EMG) of both erector spinae and the medial and lateral hamstrings was performed. Kinematic data on the pelvic motion and knee flexion were measured using a 3-dimensional motion analysis system. Repeated 1-way analysis of variance was used for the statistical analysis. Results Significantly decreased electromyographic activity in the right and left erector spinae and significantly increased electromyographic activity in the medial and lateral hamstrings activity were shown during prone knee flexion in ADIM condition using the pressure biofeedback unit. In addition, the amounts of anterior pelvic tilt, pelvic rotation, knee flexion, and perceived pain decreased significantly during prone knee flexion in the ADIM condition compared with the same maneuver in the non-ADIM condition. The onset of anterior pelvic tilt and pelvic rotation occurred significantly earlier in the non-ADIM condition, compared with the ADIM condition. Conclusions ADIM effectively increased activation of knee flexors, decreased activation of back extensors, and reduced the pelvic motions and low back pain during prone knee flexion in patients with lumbar extension rotation syndrome.