Measurement of the quality of trunk stabilization is of great interest to identify its role in first occurrence, recurrence or persistence of low-back pain (LBP). Our research group has developed and validated a method to quantify intrinsic and reflex contributions to trunk stabilization from the frequency response function (FRF) of thorax movement and trunk extensor EMG to perturbations applied by a linear actuator. However, the reliability of this method is still unknown. Therefore, the purpose of this study was to investigate the between-day reliability of trunk FRFs in healthy subjects and LBP patients. The test-retest ICC's in patients were substantial for both admittance and reflex gains (ICC3,1 > 0.73 and 0.67). In healthy subjects, the reliability of admittance gain was also substantial (ICC3,1 0.66), but the reliability of the reflexive gain was only moderate (ICC3,1 0.44). Although sample sizes were limited (13 healthy subjects and 18 LBP patients), these results show that trunk stabilization can be measured reliably, and represent a promising step towards using this method in further research in LBP patients. (C) 2015 Elsevier Ltd. All rights reserved.