Studies have reported negative effects of walking on mobile human-computer interaction when compared to standing or sitting. However, the quantitative relationship between walking speed and user performance is unknown. In the study described here, we varied walking speed on a treadmill and measured effects on discrete aiming movements on a touchscreen interface. Their relationship was found to be non-linear with a local optimum: when walking at 40-80% of one's preferred walking speed (PWS),

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... et acquisition performance plateaus, indicating optimal trade-off between speed and interaction. Accelerometer data showed that, despite increasing hand oscillation, users were able to maintain stable interaction performance at 74% of PWS. Interestingly, this speed coincides with the speed users spontaneously walk when interacting with a mobile device.