What is the significance of the knees and the calves?
Targum Yonasan: ' ... on the knees' -because you knelt in order to sin, and 'on the calves' - because you ran to the Avodah Zarah'.
Why does the Torah insert "asher Lo Suchal Leherafei" in the middle of the Pausk?
Targum Yonasan (according to Na'ar Yonasan): Even though boils on the legs and on the calves are generally curable, you will not be able to cure them if you do not do Teshuvah (because they are heaven-sent). 1
What's more, the boils will spread from the soles of your feet to your skulls. Perhaps only illnesess that come on a person via Mal'achim or that person brings on himself are curable. See also Na'ar Yonasan, who elaborates.
