To stiffen the sinews is to, by one’s own efforts, become resolute and purposeful. The sinews are the fibrous cords that connect bone to muscle – we stiffen them when we prepare for action.

The expression ‘stiffen the sinews’ comes from the celebrated ‘Once more unto the breach’ speech, from Shakespeare’s Henry V, 1598:

Once more unto the breach, dear friends, once more;
Or close the wall up with our English dead.
In peace there’s nothing so becomes a man
As modest stillness and humility:
But when the blast of war blows in our ears,
Then imitate the action of the tiger;
Stiffen the sinews, summon up the blood…