A new Back Strain Monitor (BSM) device has been developed in order to measure, record and analyze movements of the lower back. The purpose of this study was to examine the inter-tester and the intra-tester reliability of the movement measurements given by the BSM accelerometers, and compare it with the reliability of two other conventional measurement methods: the Double Inclinometer method (DI) and the Modified-Modified Schober (MMS) method. The clinical studies included 23 participants (16 males, 7 females) with no recent history of lower back pain, who wore the device during a combination of different anatomical movements (flexion, extension, left lateral flexion and right lateral flexion of the lumber spine). The tests were conducted by three therapists (testers). The reliability results for the BSM accelerometers clearly outperform the results obtained for the DI and the MMS methods. The inter-tester reliability gives the Intra-Class Correlation (ICC) value of 0.95 for the BSM flexion, 0.89 for the DI flexion and 0.74 for the MMS. The intra-tester reliability gives the ICC value of 0.99 for BSM flexion, 0.94 for DI flexion and 0.77 for the MMS. The BSM accelerometers were highly reliable in assessing back movements, measuring these movements with less error than the DI and MMS methods.