A method, a device, and a non-transitory storage medium having instructions to detect, via an ultrasound transducer, an ultrasound event that includes an ultrasound signal that is transdermally propagated on the body of a user and generated by an on-body touch in an area in which the ultrasound signal has propagated; detect a state of a limb of the user on which the device is worn, wherein the state is a contracted state or a relaxed state; and select an input based on the receipt of the ultrasound signal and the state of the limb. An accelerometer may be used to detect muscle tremor and determine whether the state of the limb.