Although flexibility field tests are commonly used in research, sport, and school settings, there is no conclusive evidence about what they actually assess. The first aim of this study was to assess the contributions of the main joints involved in the back-saver sit-and-reach test using angular kinematic analysis. The second aim was to measure the inter-method agreement between the back-saver sit-and-reach test and the sit-and-reach test. A total of 138 adolescents (57 females, 81 males) aged 14.5 +/- 1.7 years performed the back-saver sit-and-reach test and the sit-and-reach test. Hip, lumbar, and thoracic angles were assessed by angular kinematic analysis while the participants were performing the back-saver sit-and-reach test. Stepwise linear regression models and the Bland-Altman method were used. The hip angle independently explained 42% (P < 0.001) of the variance in the back-saver sit-and-reach test, the lumbar angle explained an additional 30% (P < 0.001) of the variance, and the thoracic angle an additional 4% (P < 0.001). The inter-method mean difference between back-saver sit-and-reach (BSSR) and sit-and-reach (SR) measures (BSSR - SR) was 0.41 cm (P = 0.21). The results suggest that hip flexibility is the main determinant of the back-saver sit-and-reach test score in adolescents, followed by lumbar flexibility. The back-saver sit-and-reach test can therefore be considered an appropriate and valid test for assessing hip and low-back flexibility in this age group. The back-saver sit-and-reach and sit-and-reach tests provide comparable values.