Background/Objectives: Ultrasound-guided flexor pollicis longus (FPL) injection is limited by the muscle’s deep location and its proximity to the cephalic vein (CV) and superficial branch of the radial nerve (SBRN). We aimed to identify the wrist and forearm posture producing the largest measured corridor width and target exposure. Methods: This prospective single-center study evaluated 51 forearms from 27 healthy adult volunteers with full range of motion, using transverse ultrasound of the distal volar forearm in six standardized positions. Two corridors were measured: CV-to-SBRN (L₁, Approach A) and CV-to-radial-cortex apex (L₂, Approach B), with FPL exposure height (F). Results: L₂ was present in all limbs and was wider with forearm supination than in neutral (supination [SUP], +2.36 mm; flexion plus supination [F+SUP], +2.05 mm; extension plus supination [E+SUP], +2.66 mm; each p < 0.001), an ~40% increase; the three supination postures did not differ. F increased across the same postures and was greatest in E+SUP (+2.32 mm), which did not differ from SUP. L₁ was absent in 12–28% of limbs and, where present, was narrower with supination (both p < 0.001). Conclusions: In healthy adults, forearm supination widened the safety corridor and increased target exposure. Applicability to spasticity requires confirmation.