The lateral pteygoid's attachment is not near the surface and is on the medial side of the ramus/condyle

Lateral pteygoid attaches "behind" the condyle

The lateral pterygoid is not near the surface.  The lateral ligament is another barrier. It is covered by the masseter

Medial pterygoid attaches close to surface on the mandibular border
All of the pictured muscles of masstication (except the LP) have their attachments on the border of the mandible, close to the surface, thereby are accessible to TENS stimulation. 

The lateral pterygoid's attachment is deep and behind the condyle.  The ability of TENS stimulation has not been demonstrated.

Digastric attaches close to surface

The nerve to the Lateral Pteygoid (red arrow, left) branches off the Buccal nerve, which is a division of the mandibular nerve.
Surface TENS electrode is placed over the coronoid notch, with the hypothesis being that the electrical stimulation reaches deep through the notch to the nerve to the lateral pterygoid.