Another thing, aiming at the chest joint is only good if:

a) your kick isnt moving vertically too fast

and

b) if a is happening, you're hitting the upper half of the chest joint.

If you aim too low on that joint with a vertical kick you can get your foot breaking the pecs but for some odd reason the chest can take a massive point beating but refuse to break. Higher up the chest joint increases the chance of a chest/neck break.