Then let mine arm - The strong language which Job uses here, shows his consciousness of innocence, and his detestation of the offences to which he here refers, Job 31:16-22. The word rendered "arm" here (כתף kâthêph) means properly the shoulder. Isaiah 46:7; Isaiah 49:22; Numbers 7:9; compare the notes at Isaiah 11:14. There is no instance, it is believed, unless this is one, in which it means arm, and the meaning here is, that he wished, if he had been guilty, his shoulder might separate from the blade. So Herder, Rosenmuller, Umbreit, and Noyes render it; and so the Vulgate and the Septuagint.
From my shoulder-blade - The scapula - the flat bone to which the upper arm is attached. The wish of Job is, that the shoulder might separate from that, and of course the arm would be useless. Such a strong imprecation implies a firm consciousness of innocence.
And mine arm - The word arm here denotes the forearm - the arm from the elbow to the fingers.
From the bone - Margin, "the chanelbone." Literally, "from the reed" - מקנה miqâneh. Umbreit renders it, Schneller als ein Rohr - quicker than a reed. The word קנה qâneh means properly a reed, cane, calamus (see the notes at Isaiah 43:24), and is here applied to the upper arm, or arm above the elbow, from its resemblance to a reed or cane. It is applied, also, to the arm or branch of a chandelier, or candlestick, Exodus 25:31, and to the rod or beam of a balance, Isaiah. xlvi. 6. The meaning here is, that he wished that his arm should be broken at the elbow, or the forearm be separated from the upper arm, if he were guilty of the sins which he had specified. There is allusion, probably, and there is great force and propriety in the allusion, to what he had said in Job 31:2 l: "If his arm had been lifted up against an orphan, he prayed that it might fall powerless."

Let mine arm fall - Mr. Good, as a medical man, is at home in the translation of this verse: -
"May my shoulder-bone be shivered at the blade,
And mine arm be broken off at the socket."
Let judgment fall particularly on those parts which have either done wrong, or refused to do right when in their power.

[Then] let mine (p) arm fall from my shoulder blade, and mine arm be broken from the bone.
(p) Let me rot in pieces.

Then let mine arm fall from my shoulder blade,.... With which the upper part of it is connected; let it be disjointed from it, or rot and drop off from it; a dreadful calamity this, to lose an arm and the use of it, to have it full off immediately, as a judgment from God, and in just retaliation for lifting up an hand or arm against the fatherless; as Jeroboam's arm withered when he put it forth from the altar, and ordered hands to be laid upon the prophet for crying against the altar, 1-Kings 13:4; and mine arm be broken from the bone; from the channel bone, as the margin of our Bibles, or rather from the elbow, the lower part of the arm and so may be rendered, "or mine arm", &c. Eliphaz had brought a charge against Job, that the arms of the fatherless had been broken, and suggests that they had been broken by him, or by his orders, Job 22:9; and Job here wishes, that if that was the case, that his own arm was broken: such imprecations are not to be made in common, or frequently, and only when a man's innocence cannot be vindicated but by an appeal to the omniscient God; an instance somewhat like this, see in Psalm 137:5.

Apodosis to Job 31:13, Job 31:16-17, Job 31:19-21. If I had done those crimes, I should have made a bad use of my influence ("my arm," figuratively, Job 31:21): therefore, if I have done them let my arm (literally) suffer. Job alludes to Eliphaz' charge (Job 22:9). The first "arm" is rather the shoulder. The second "arm" is the forearm.
from the bone--literally, "a reed"; hence the upper arm, above the elbow.