BACKGROUND CONTEXT: Imbalanced patterns of erector spinae activity and reduced trunk extension strength have been observed among patients with low back pain (LBP). The association between LBP and neuromuscular imbalance still remains unclear. PURPOSE: To examine the relationship between LBP, neuromuscular imbalance, clinical symptoms, and trunk extension strength on two independent occasions, after dynamic neuromuscular changes through a back exercise program. STUDY DESIGN/SETTING: Experimental longitudinal study of the lower back in a clinical setting. PATIENT SAMPLE: Eighty-two elite amateur tennis players with and without LBP. OUTCOME MEASURES: Clinical testing of spinal mobility and muscular flexibility of the lower back. Isometric voluntary maximum trunk extension strength. Surface electromyography (EMG) of lumbar erector spinae. METHODS: Athletes underwent a standardized clinical examination with common tests for spinal mobility and muscular flexibility, followed by an isometric trunk extension tests in a specially built apparatus with simultaneous surface EMG recording from right and left lumbar erector spinae. Imbalance quotients were calculated using integrated EMG (IEMG) measures. The relationships between LBP, neuromuscular imbalance, trunk extension strength, and clinical findings were investigated before and after a back exercise program using univariate and multivariate logistic regression models. RESULTS: A significant association between neuromuscular imbalance of erector spinae and the occurrence of LBP was observed, whereas no significant imbalances were found in subjects without LBP. The number of subjects with LBP decreased proportionally with the occurrence of neuromuscular imbalance in the lumbar region after the completion of a back exercise program. However, neuromuscular imbalances were still present during retesting among subjects whose LBP did not resolve; the relationship between neuromuscular imbalance of lumbar erector spinae and LBP was significant again. On the contrary, no significant association between LBP and maximum isometric trunk extension strength or neuromuscular imbalance and maximum isometric trunk extension strength was found before or after the exercise program. No clear relationship between LBP and clinical testing of the lumbar spine or neuromuscular imbalance and clinical testing was found on the two independent testing occasions. CONCLUSION: A direct relationship between LBP and neuromuscular imbalance was documented in athletes with LBP. Maximum isometric trunk extension strength had no relationship to the presence of LBP or the occurrence of neuromuscular imbalance of erector spinae. Common clinical testing of spinal mobility and muscular flexibility had only limited correlation to LBP and neuromuscular imbalance.