The lord of Ogasawara had at one time requested Matabei to duel Musashi. Matabei was compelled to fight even though he had not wanted to, as to obey his lord's command. Matabei chose to use a bamboo lance, while Musashi chose an ordinary sword made of wood.
Matabei was compelled to fight Musashi due to his lord's command, despite not wanting to.  Both fighters chose their weapons based on what they felt would be most effective in the duel.