As mobile device sizes get smaller, the thumb is becoming a primary mode for mobile technology interaction. This may expose users to new risks of developing thumb-related overuse syndrome. æKey locations of smartphones affect thumb performance and postures; with awkward postures associated with poor performance. æHowever, the muscular effort required to reach different key locations hasnÍt yet been determined. Therefore, the goal of this study was to determine muscle activity during thumb reaching tasks on a smartphone. We measured muscular effort of 6 hand/forearm muscles using electromyography (EMG) on 5 smartphone users. The tasks involved tapping keys located at 3 distances: close, far, and a middle distance. The results indicate that muscular efforts for the flexor digitorum muscle were higher for taps on keys located farther apart (mean of the % maximum voluntary contraction across users and tasks, and +/-S.E. of 0.045+/-0.010) compared to closer (0.037+/-0.005) and mid-range (0.036+/-0.005). This suggests that finger muscles may be more involved in readjusting the device for far reaching tasks. æAlso, EMG was greater for middle distances for both the abductor pollicis (0.106+/-0.012) and dorsal interossei (0.040 +/- 0.006) muscles compared to the far (0.096 +/-0.010 and 0.024+/-0.005 respectively) and close (0.018 +/-0.008 and 0.018+/-0.002 respectively) distances. æThis unexpected result may indicate that the thumb muscles were more actively involved in reaching for the middle distances. These results can be used to guide smartphone software design to determine key locations that minimize muscular effort, a known musculoskeletal injury risk factor.