Rashi: It is like a hand coming out of his helmet opposite his nose; it reached to his legs. ¹

Radak: It is a plate hanging on his legs to cover them. This is a 'borrowed' expression. Mitzchah primarily refer to a plate of gold or silver that girls wear on their Metzach (forehead).

Rashi citing Targum Yonasan: It is like a Chanis (spear) coming out of his helmet. It extends to his shoulders, to guard his neck from a sword.

Radak: David said (verse 45) "you come against me with a sword, Chanis and Kidon." It seems that Kidon is a spear to strike, not for protection, like "Netei ba'Kidon" (Yehoshua 8:18). Galyus held the Chanis in his right hand and another weapon that is not mentioned in his left hand, and the Kidon was between his shoulders, to take it after he throws the Chanis at his opponent.