Final answer:
The flexor hallucis brevis and adductor hallucis muscles insert onto the proximal phalanx of the hallux, enabling the flexion and adduction of the big toe respectively.
Step-by-step explanation:
The muscles that insert onto the proximal phalanx of the hallux (big toe) include the flexor hallucis brevis and the adductor hallucis. These muscles are responsible for the flexion of the big toe. The flexor hallucis longus, while it does act on the hallux, actually inserts onto the distal phalanx, not the proximal phalanx.
The flexor hallucis brevis muscle has two heads: the medial head and the lateral head, both of which insert onto the base of the proximal phalanx of the big toe.
Similarly, the adductor hallucis has two heads: the oblique head and the transverse head, which also insert onto the proximal phalanx of the hallux, assisting in the toes' adduction and flexing movements.