Methods: Forty-three pain-free controls and 43 patients with chronic NSLBP were enrolled. Using non-invasive techniques, synchronized lumbar spine motion and erector spinae (ES) EMG activity were recorded during standardized trunk flexion-extension cycles. Results: Several variables differed significantly between the two groups. Logistic regression identified two variables with significant odds ratios for the presence of chronic NSLBP: time spent with the spine flexed beyond 90% of its maximum range (odds ratio, 0.92; 95% confidence interval, 0.86–0.99) and the ES relaxation ratio (odds ratio, 1.08; 95% confidence interval, 1.04–1.13). Conclusions Individuals with and without chronic NSLBP exhibit distinct spinal motion and ES activity patterns during trunk flexionextension. These differences—specifically the time of maximum lumbar flexion and the relaxation ratio—can be objectively and easily measured. Their assessment may offer valuable clinical utility for the diagnosis and follow-up of NSLBP patients.