BACKGROUND: The beneficial effects of thermotherapy on analgesia and relaxation are widely known for various diseases. To date, however, thermotherapy in chronic low back pain is not explicitly recommended in international guidelines. The effects of thermotherapy on biomechanical parameters within a multimodal back pain treatment concept are also unknown. METHODS: Within a multimodal treatment concept, 176 patients with chronic low back pain were treated either with or without supplemental heat wrap therapy. The range of movement and strength parameters of the trunk in flexion, extension, lateral flexion and rotation were measured before and after 12 weeks of treatment. FINDINGS: The range of movement as well as strength parameters of the trunk improved on average within the multimodal treatment. Patients receiving additional thermotherapy supplemental to basic multimodal treatment showed a further improvement of strength parameters regarding extension (P = 0.09, 1 - β = 0.41), rotation to the right (P = 0.09, 1 - β = 0.41) and rotation to the left (P = 0.08, 1 - β = 0.42) in comparison to those conducting only the multimodal treatment. No group differences were detected in flexibility. INTERPRETATION: The implementation of thermotherapy for several hours a day (heat wrap therapy) in daily clinical practice additional to an individualized, evidence-based multimodal treatment concept can be recommended to enhance strength parameters. The potential causes of improved strength parameters as well as the meaning for the patients in activity of daily living are discussed.