When I was playing/While I was...

[Vladv1]

While I was playing football, I hurt my knee/ When I was playing football I hurt my knee.
What is the difference here? To me they sound and mean the same.

 

[emsr2d2]

1. While I was playing football, I hurt my knee.
2. When I was playing football, I hurt my knee.

What is the difference here? To me they sound and mean the same.

Note my corrections above. When you give us more than one sentence to consider, please number them. You remembered to use a comma after "football" in the first but not the second.
There is no real difference in meaning. By that, I mean that you'd almost certainly hear both from native speakers with the same intended meaning. One could argue that using "while" makes it very clear that you hurt your knee during the act of playing football (you were on the pitch taking part in a game or practice) whereas using "when" could mean that you hurt your knee during a time of your life in which you regularly played football but didn't necessarily do it on the pitch. However, it is unlikely that most readers/listeners would immediately think it meant the latter.