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A prospective cohort study evaluating the quantitative lumbar flexion-relaxation phenomenon (QLFRP), measured with surface electromyographic (SEMG) signals from the erector spinae during trunk flexion pre- and postrehabilitation, in patients with chronic disabling occupational lumbar disorders (CDOLD).To assess the responsiveness of the QLFRP in documenting change in functional performance during a functional restoration program for CDOLD patients.A recent theoretical construct suggests that QLFRP is responsive to change in lumbar range of motion (ROM) during rehabilitation, with high sensitivity and specificity for abnormal QLFRP predicting ROM.A cohort of normal subjects was tested for QLFRP correlated to inclinometric lumbar ROM measures. The cutoff score was applied to a group of CDOLD patients entering a functional restoration program (N = 135), and to program completers (N = 104). Pain and functional self-report scores were compared with SEMG and ROM measures.The CDOLD group averaged 23.7 months off work. Surgical treatment was provided prerehabilitation to 51% of patients, with 29% receiving lumbar fusions. From pre- to post-treatment, achievement of QLFRP rose from 31% to 74% of patients, while normal ROM rose from 8% to 63% of patients. Compared to the 16% of patients still demonstrating both abnormal QLFRP and ROM, the other groups showed significantly greater improvement in self-reported pain and function, with the best improvements occurring in patients showing normal ROM and QLFRP. The QLFRP showed high sensitivity, but only modest predictive validity and specificity for predicting ROM postrehabilitation. Improvement in sensitivity and predictive validity occur when surgical cases were excluded from the analysis.A majority of patients in an interdisciplinary functional restoration program failed to demonstrate either the QLFRP or normal ROM on admission to the program. A majority of program completers, however, achieved both normal ROM and QLFRP and another 30% demonstrated either normal QLFRP or normal ROM. Both QLFRP and ROM measures were responsive to relevant self-report scales.