OBJECTIVE: To investigate if there is training reserve in the maximal lumbar extension. DESIGN: Three-year longitudinal study. BACKGROUND: Among adults there is variation in the normal range of sagittal motion of the lumbar spine, but reduced spinal flexibility does not predict future occupational back pain. In various sports and in ballet, maximal extension of lumbar spine is a common manoeuvre, and low-back pain is also common. It is not known whether training increases maximal extension of healthy back. Forceful training of maximal extension may injure the anatomical structures limiting the extension range. METHODS: We compared lumbar sagittal flexibility to hip flexor and hamstring flexibility in a 3-year longitudinal study on female ballet dancers (n = 18), athletes (n = 31), and controls (n = 17) before and after their adolescent growth spurt. RESULTS: Ballet dancers had more flexible hamstrings and hip flexors than controls, but there were no group differences regarding the maximal lumbar flexion or extension. These results persisted throughout follow-up. CONCLUSIONS: The maximal physiological extension of the lumbar spine cannot be increased by training in healthy adolescents. RELEVANCE: An attempt to exceed the physiological maximum extension may only cause overly hard strain on specific anatomical structures of the lumbar spine. This knowledge should be considered when the rules of sports and choreography of dance performances are considered.