Related על ברכות 2:1

One who reads the Shema must have intent in his heart. Rebbi Achai says in the name of Rebbi Yehudah: “If he had intent in his heart in the first paragraph [of Shema], even though he did not have intent in his heart in the last (i.e. second) paragraph [of Shema], he fulfilled his obligation [of saying the Shema].”

A carrier, even though his burden is on his shoulder, he may read [the Shema]. But while he is unloading [the burden] and loading [the burden] he should not read, because he is not paying attention [to what he is saying]. Either way [i.e. whether he is carrying or loading, or unloading], he should not pray [the Shemoneh Esreh] until he [completely] unloads [and can stand in one place without holding his load].