Or HaChaim on Deuteronomy 25:2/אור החיים על דברים כ״ה:ב׳

1 אם בן הכות הרשע, "if the person found guilty (the wicked one) is convicted he is to receive 39 lashes, etc." The reason the Torah wrote the word בן with the vowel chirik instead of the customary vowel segol is that he who administers the lashes must evaluate if the victim is physically able to absorb these lashes without injury. In the event that the health of the individual in question cannot absorb the normal number of lashes, i.e. that he is not ben hakkot, he receives only as many lashes as are compatible with maintaining his level of health (compare Makkot 22).