Deuteronomy 8:4

‘ Your clothes did not grow old on you, nor did your foot swell, these forty years.'

They were to remember how, while they wandered in that wilderness, they were fully provided for. They did not live in poverty so that their clothes grew old and ragged. Rather they had had sufficient so that they could afford to renew their clothing. Nor had they suffered from debilitating feet problems. This may have been seen as because they had abundance of oils to soothe them. They had been short of nothing. They had always been able to clothe themselves well and keep their feet in good condition and go forward. Yahweh had watched over both their wardrobe and their health, as a father sees to the needs of his children.

The word translated ‘swell' is rare. LXX has the sense of callused. The point is as much that Yahweh supplied the means to keep their feet healthy as that He worked a constant miracle. His care was over them constantly. They had lacked nothing (compare Deuteronomy 2:7).