When Israel came out of Egypt, there was not one feeble person among their tribes:

Psalms 105:23 Israel also came into Egypt; and Jacob sojourned in the land of Ham.

  26 He sent Moses his servant; and Aaron whom he had chosen.

  27 They shewed his signs among them, and wonders in the land of Ham.

  36 He smote also all the firstborn in their land, the chief of all their strength.

  37 He brought them forth also with silver and gold: and there was not one feeble person among their tribes.

So, by the time that Amalek came against them:

Exodus 17:8 Then came Amalek, and fought with Israel in Rephidim.

What had happened, during that the interval, from when they left Egypt, until Amalek came, that there were, by that time, feeble among them?

Deuteronomy 25:17 Remember what Amalek did unto thee by the way, when ye were come forth out of Egypt;

  18 How he met thee by the way, and smote the hindmost of thee, even all that were feeble behind thee, when thou wast faint and weary; and he feared not God.

The people had all ready began to murmur and complain by the time that Amalek came, could that possibly have anything to do with it?

.