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Background: The B-B Score is a straightforward kinematic shoulder function score including only two movements (hand to the Back + lift hand as to change a Bulb) that demonstrated sound measurement properties for patients for various shoulder pathologies. However, the B-B Score results using a smartphone or a reference system have not yet been compared. Provided that the measurement properties are comparable, the use of a smartphone would offer substantial practical advantages. This study investigated the concurrent validity of a smartphone and a reference inertial system for the measurement of the kinematic shoulder function B-B Score. Methods: Sixty-five patients with shoulder conditions (with rotator cuff conditions, adhesive capsulitis and proximal humerus fracture) and 20 healthy participants were evaluated using a smartphone and a reference inertial system. Measurements were performed twice, alternating between two evaluators. The B-B Score differences between groups, differences between devices, relationship between devices, intra- and inter-evaluator reproducibility were analysed. Results: The smartphone mean scores (SD) were 94.1 (11.1) for controls and 54.1 (18.3) for patients (P < 0.01). The difference between devices was non-significant for the control (P = 0.16) and the patient group (P = 0.81). The analysis of the relationship between devices showed 0.97 ICC, −0.6 bias and −13.2 to 12.0 limits of agreement (LOA). The smartphone intra-evaluator ICC was 0.92, the bias 1.5 and the LOA −17.4 to 20.3. The smartphone inter-evaluator ICC was 0.92, the bias 1.5 and the LOA −16.9 to 20.0. Conclusions: The B-B Score results measured with a smartphone were comparable to those of an inertial system. While single measurements diverged in some cases, the intra- and inter-evaluator reproducibility was excellent and was equivalent between devices. The B-B score measured with a smartphone is straightforward and as efficient as a reference inertial system measurement. [ABSTRACT FROM AUTHOR]