Isaiah 40:30

וְיִעֲפוּ נְעָרִים וְיִגָעוּ וּבַחוּרִים כָּשׁוֹל יִכָּשֵׁלוּ׃

30 Vigorous youths will become faint and weary and young men will fall exhausted.

וְיִעֲפוּ נְעָרִים וְיִגָעוּ Vigorous youths will become faint and weary. The words יעף (be faint) and יגע (be weary) are getting a workout in these verses. The forms are simply waw imperfects; not waw-consecutive imperfects. The distinction here is that parallel ideas are presented, but they don't have a direct relationship to each other, they simply will both take place at times just as the fall of nations will take place.

וּבַחוּרִים כָּשׁוֹל יִכָּשֵׁלוּ and young men will fall exhausted. A בָּחוּר is a young man in the prime of health and physical strength, although Holladay suggests that it also implies that he is still unmarried. Our slang term "young stud" might apply.

The Mp note for וּבַחוּרִים tells us that this word occurs twice -- once written fully (plene -- here in Isaiah 40:30) and once defectively (Jer. 31:13).

The phrase כָּשׁוֹל יִכָּשֵׁלוּ is a good example of an infinitive absolute (כָּשׁוֹל is in the qal) emphasizing the force of the following verb of the same root (יִכָּשֵׁלוּ is a nifal 3rd plural, notice the classic nifal vowel pattern). The effect here is that the "stumbling" of the young studs will be a complete collapse in exhaustion (cf GK §113n).