This study is testing whether a non-invasive nerve monitoring device (TetraGraph) can accurately detect early signs of ICU-acquired weakness by comparing readings from the device with standard nerve conduction/EMG tests done by a specialist to see if they match. They will focus on muscle response signals in the hand and leg over about 10 days in three groups: healthy volunteers, ICU patients who are not on ventilators, and ICU patients who are on ventilators.