Purpose and Background. Strengthening the lumbar extensor musculature is a common recommendation for CLBP. Although reported as effective, variability in response in CLBP populations is not well investigated. This study investigated variability in responsiveness to isolated lumbar extension (ILEX) resistance training in CLBP participants by retrospective analysis of 3 RCTS. Methods and Results. Data from 77 intervention participants was available (males = 43, females = 34) 37 control participants (males = 20, females = 17). Intervention participants all underwent 12wks of ILEX resistance training and changes in ILEX strength, pain (VAS) and disability (ODI) measured. True inter-individual response variability was examined through calculation of difference in the standard deviation of change scores for both control and intervention. Intervention participants were classified into using k-means cluster analysis for strength changes and using MCIC cut-offs for VAS and ODI. Analysis suggested true inter-individual responses to the intervention existed. Participants were classified for strength changes as low (n = 31), medium (n = 36), and high responders (n = 10). Participants were classified for VAS changes as negative (n = 3), non-responders (n = 34), responders (n = 15), and high responders (n = 19). Participants were classified for ODI changes as negative (n = 2), non-responders (n = 21), responders (n = 29), and high responders (n = 25). Conclusion. Considerable variation exists in response to ILEX resistance training in CLBP. Future work should identify factors prognostic of successful outcomes. Conflicts of interest: None. Funding obtained: None