An end effector is capable of being used to stimulate a portion of skin at a stimulation frequency. The end effector includes a base portion that is couplable to a motor and an end portion having a plurality of contact points at which the end effector is configured to contact the portion of skin. The plurality of contact points are located at a target distance from each other that is based on an inverse of the stimulation frequency. The end effector is configured such that, when the base portion is coupled to the motor and the motor is operating, the end effector has a resonant frequency based on the stimulation frequency. When the motor is operating and a force is applied to bias the end effector toward the portion of skin, a cyclical stimulus is produced within the portion of skin at about the stimulation frequency.