Update only Those records which are affected using Update Tirgger

COLUMNS_UPDATED

500 is a bit mask for checking number of columns affected or you want to check ...

IF (COLUMNS_UPDATED() & 500 > 0)
    BEGIN
            INSERT INTO TableName
            select  'update' as [Action], * from inserted 
    END

For more update or info use below link ...

http://technet.microsoft.com/en-us/library/ms186329.aspx