Do you train your core while lying on your side?

Question

When you lie on your side and tense your core (six-pack), does this train it?

Also, do you train flexibility in your legs by just tensing them?

Answer 1

It trains them as much as any other isometric exercise.  That means they will get stronger at that position plus a little carry over into another position.  As for flexibility the answer is no unless you are doing it in a very specific way.  If you were to only tense the hamstrings there would be a small degree of carryover into flexibility but not much.  You would have to do this tensing every fifteen degrees to build up enough strength to gain any real change in flexibility.