Albert Barnes Commentary Psalms 129:3

"The plowers plowed upon my back; They made long their furrows." — Psalms 129:3 (ASV)

The plowers plowed upon my back - The comparison here is undoubtedly taken from the plowing of land. The idea is that the sufferings they had endured would be well represented by a plow passing over a field—tearing up the sod, piercing deep, and producing long rows or furrows. The direct allusion seems to be to stripes inflicted on the back, as if a plow had passed over it.

This means that they had been subjected to sufferings like those of slaves or criminals when the lash cut deep into their flesh. The psalmist likely had in mind the hard bondage of the children of Israel in Egypt, when they were subjected to all the evils of servitude.

They made long their furrows - On my back. The word used here and translated "made long"—ארך 'ârak—means to make long, to prolong, to extend in a straight line. It can be used either in the sense of making long in extent or space, or making long in regard to time, that is, prolonging. The latter seems to be the meaning here, as it is difficult to see how stripes inflicted on the back could be made physically long. They could, however, be continued and repeated; the sufferings might be prolonged as well as deep. It was a work of long-continued oppression and wrong.