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PRÉCIS: The scientific literature has yet to characterize the relationship between handheld technology use and thumb performance. This study finds that thumb movement performance varies with device size, movement orientation, and direction. These results can be used to design new mobile phone 35 devices and keypad interfaces. 1 Objective: The aim of this study was to determine if thumb motor performance metrics varied by movement orientation, direction, and device size during single-handed use of a mobile 40 phone device. Background: With the increased use of mobile phones and the associated increased risk for musculoskeletal disorders, understanding how design factors affect and improve performance can provide better design guidelines. Method: A repeated measures laboratory experiment of 20 right-handed participants 45 measured the thumb tip’s 3-D position relative to a phone during reciprocal tapping tasks acros