Spatial User Interfaces, such as wearable fitness trackers are widely used to monitor and improve athletic performance. However, most fitness tracker interfaces require bimanual interactions, which significantly impacts the user's gait and pace. This paper evaluated a one-handed thumb-to-ring gesture interface to quickly access information without interfering with physical activity, such as running. By a pilot study, the most minimal gesture set was selected, particularly those that could be executed reflexively to minimize distraction and cognitive load. The evaluation revealed that among the selected gestures, the tap, swipe-down, and swipe-left were the most 'easy to use'. Interestingly, motion does not have a significant effect on the ease of use or on the execution time. However, interacting in motion was subjectively rated as more demanding. Finally, the gesture set was evaluated in real-world applications, while the user performed a running exercise and simultaneously controlled a lap timer, a distance counter, and a music player.