During medical interventions, direct interaction with medical image data is a cumbersome task for physicians due to the sterile environment. Even though touchless input via hand, foot or voice is possible, these modalities are not available for these tasks all the time. Therefore, we investigated touchless input methods as alternatives to each other with focus on two common interaction tasks in sterile settings: activation of a system to avoid unintentional input and manipulation of continuous values. We created a system where activation could be achieved via voice, hand or foot gestures and continuous manipulation via hand and foot gestures. We conducted a comparative user study and found that foot interaction performed best in terms of task completion times and scored highest in the subjectively assessed measures usability and usefulness. Usability and usefulness scores for hand and voice were only slightly worse and all participants were able to perform all tasks in a sufficient short amount of time. This work contributes by proposing methods to interact with computers in sterile, dynamic environments and by providing evaluation results for direct comparison of alternative modalities for common interaction tasks.