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BackgroundThe ability to maintain joint stability and motion control of the extremities is important in injury prevention and recovery. Quantitative measurement of proprioception, especially the upper extremity, is difficult and requires expensive equipment.ObjectiveWe investigated the reliability of a custom iPhone app to compile accelerometer data and calculate a path length of movement during 20 second trials.DesignThis study used a prospective test-retest design. Subjects completed three trials on each upper extremity (RA and LA) 48 hours apart (MWF). A convenience sample of subjects was used.SettingThe study was conducted at a small liberal arts undergraduate institution.Patients (or Participants)Subjects were students associated with exercise science or athletic training programs. The activity level ranged from recreational athlete to NCAA Division II athletes. Twenty-three subjects started with 15 completing all upper extremity trials. Exclusion criterion was an upper extremity injury in the last month.Interventions (or Assessment of Risk Factors)An iPhone 6 app was developed to utilize accelerometer data in a Cartesian grid. Data was obtained at 20 Hz to calculate a path length of motion for each trial. Each subject stood, holding the iPhone in the palm of the hand directly in front of the shoulder with palm up, attempting to minimize motion for 20 seconds.Main Outcome MeasurementsReliability measures were calculated between trials using the path length of motion in each trial.ResultsThe mean path length for RA was 2669 with a standard error of 372, with LA being 2774 and 424. The intraclass correlation coefficient was 0.86 for the right arm and 0.74 for the left across three trials.ConclusionsThe accelerometer in an iPhone 6 is a moderately reliable instrument for assessing motion control stability in the upper extremity. We plan to add additional data filtering to the app.