Sensor should be inserted in :

1-a place where there is a sufficient exposure  to ISF

2-a way from anything that hitting of it may cause pain ,bleeding or erratic readings

First of all, lets examine the body anatomy-:

Peripheral nervous system: (PNS)

General body anatomy 

Dermatome vs Myotome : 

https://pediaa.com/difference-between-dermatome-and-myotome/