Reliability and Concurrent Validity of a Clinometer+ Bubble Smartphone Application to Assess Hamstring Muscle Length

Muscle flexibility is a component of physical fitness. Using traditional tools in muscle length evaluation tests creates challenges. Therefore, the use of smartphones and health-related software as an alternative method has become widespread. This study aimed to investigate smartphones' intra- and inter-rater reliability and validity for measuring hamstring muscle length. In a blinded study design, two researchers measured hamstring flexibility through four types of tests on each of the 22 asymptomatic participants with a total of 44 lower limbs. The measurements were compared between the traditional goniometer method and the practical smartphone application method. The intraclass correlation coefficient (ICC) was used to evaluate the reliability of each smartphone measurement, and Bland-Altman analysis was used to check the measurement errors. The validity of the two methods was also investigated. Intra- and inter-rater reliability (ICC≥0.8) were good to almost perfect. In intra-rater reliability, PSLR angle showed consistent imprecision; other tests were free of systematic error and measurement error. The inter-rater reliability revealed a constant error in the right leg's PKE angle. A good to excellent correlation (r = 0.817–0.699) was observed in all the measured values, indicating the two methods' validity. These findings support from intra- and inter-rater reliability and validity of both instruments when measuring hamstring muscle length.