Of course,

you could choose to practice a back kick to the knee or
shin — with either distance. The point is, you need to cover with a
rear line of defense.
Get someone to approach from behind with a focus mit. Practice
kicking the mit -- your partner holds the target in the appropriate postions
for kicking.
If your client is behind you, get comfortable kicking around your client
without offending or injuring your protectee.
Pause For Reflection
Do you see what we’re doing? You are teaching yourself to move professionally
through crowds. You practice moving forward smoothly.
Then with just a shift of focus, you can almost guard your sides. You
just have to preview what will soon be passing on the sides. Anything else,
you are trusting to awareness ... for now (I have another e-report in mind
as a future project).
And in this last exercise, you covered the rear line of attack. You are
now, covering most lines of possible attack.
Note: Peripherally, we see below us easily. Still check the surface
as you move. On the other hand, most humans are very lazy about
looking up when entering a new situation. Keep both in mind.