I have my own idea while looking at the Torah in the original Hebrew...in particular the first book Beresheet or Genesis....the answer lies in mathematics. Look up "PHI" or the "GOLDEN MEAN" if you're not familiar with them. Then look at the total number of verses in Genesis and the verse numbers that have this hand and thigh mentioned. I found that they are the golden mean from each end of Genesis... or Beginning and End.
When I found this out... I went to the wall of my home and had my height marked on the wall...then I marked where the end of my fingers touched the wall in a normal standing posture. I then measured from tip of my head to tip of my finger and found that it is exactly the golden mean of my body. So, you see... this line runs from fingertip to fingertip through the bottom of the scrotum across the body...and has an important mathematical significance...that the ancients were very aware of.
You can verify what I've said here with this nifty database:
Just type in the word "thigh" under the "search by words" and do the math yourself...
(1-sqrt5)/2 * total verses in Genesis. Then you'll get an idea why the weird symbolism of Jacob wrestling all night and hurting his thigh.
I also found reference to 7 upper chakras and 7 lower chakras..and the mid thigh deals with anger or revenge where the bottom of the foot deals with murder or malice... the 7 upper chakras being good and the 7 lower being bad...according to this site... so there is a chakra just at or above the navel that is the golden mean of the body from the soles of the feet to the tip of the head. I personally had experience with that and know that the spirit attaches to the body in that location. It seems the lower golden mean mark of the body is more to do with animal type instincts where the upper are more spiritual / godly.
This is my ideas. I pray they are not misleading in any way. They are just my present level of understanding..and that changes quite a bit quickly over time..so take it for what it's worth please.