A bow long bent grows weak
(phr.) people who are overworked cannot be expected to continue working to their full potential
References to bows, arrows, archers and archery lie at the root of dozens of English words and phrases, among them such familiar fare as bolt upright (a reference to the straightness of the bolt of a crossbow), another string to your bow (first used in the fifteenth century), upshot (originally the final shot of an archery competition, hence a byword for a result or conclusion), and point blank (describing an arrow fired into the white, or blanc, marker on an archery target, at such a distance that it does not drop from its original trajectory).
At the less familiar end of the scale are the likes of like wood, like arrow (a seventeenth century reminder that children often take after their parents); he that always shoots right, forfeits his arrows (constant success can prove draining over time); and to unstring the bow will not heal the wound (a proverbial warning that it is pointless taking precautions after harm has been done).
A bow long bent grows weak is another of these less familiar expressions—though the sentiment it advises could arguably afford to be much better known.
Alluding to the weakening of overused archers’ bows, it implies that workers who are stressed or overburdened cannot be expected to work to their full potential, and will steadily lose their keenness and consistency over time.
Seemingly, this is a longstanding problem: in the form “a bow long bent at last waxeth weak,” this particular bit of workplace wisdom was first recorded in the sixteenth century, in the English playwright John Heywood’s collection of contemporary Proverbs (1546). But the sentiment behind this expression is apparently considerably older than that—as it is likely that these lines from the Ancient Greek historian Herodotus, written some 2,500 years ago, could have inspired it:
They … who have a bow, bend it only at the time they want it; when not in use, they suffer it to be relaxed; it would otherwise break, and not be of service when … required. It is precisely the same with a man; if, without some intervals of amusement, he applied himself constantly to serious pursuits, he would imperceptibly lose his vigour both of mind and body.
Herodotus, Histories (II.173), (c. 440BCE)