Sforno on Genesis 49:14/ספורנו על בראשית מ״ט:י״ד

1יששכר חמור, not ready to fight a war. This corresponds to the saying of the sages in Avodah Zarah 17 אי ספרא לא סייפא, “if your vocation is books, it cannot be skill with a sword.”

2גרם. A donkey with strong bones. And, due to his strength, רובץ בין המשפמים when he lies down to rest, he does so between two שפיתות, open type of containers able to contain the loads that he will carry. These are placed on either side of him. [the word seems to describe a recessed setting such as on a stove which has holes into which the pots are lowered to be surrounded by the fire underneath. Ed.] The unique nature of this kind of stove is that the pots are not capable of being moved or removed from it until the cooking or boiling is complete and the fire has gone out. Yaakov describes a simile in which Issachar, i.e. the donkey can hold out until he has completed the job assigned to him. The whole verse is a parable symbolising how Issachar accepts the yoke of the Torah and studies Torah with all the energy at his command. He does this in addition to shouldering the burden of derech eretz, i.e. doing justice to the more mundane demands made upon everybody living on this planet.