In short, normally flexed in an intermediate position, against a resisting surface.
If you look at the pad position videos of Globus, limbs are always in an intermediate position. They are better off locked against an object, furniture, bench. See for instance the position in this short pad placement video in my site, by clicking on Video Pad Placement; in it, a world-level beach volleyball athlete, is being prepped for dynamic EMS with a leg press.
We recommend to use completely extended limbs only if needed by particular cases. EMS specialist Giampaolo Boschetti, who has extensively consulted for both Globus and Compex, explains that training should target the positions in which most power is transmitted by the muscle. Therefore, if for example, in a sport event your quads do their peak work between 45 and 90 degrees of flexion, you probably are better off immobilizing your foot against the wall at a 60-70 degree angle.