mysql repair threads not spawned

In my my.cnf I specify

myisam_repair_threads=4

When I add an index to large table in the show processlist I see that mysql outputs:

 Repair with 2 t开发者_Python百科hreads |
 ALTER TABLE arman.files
 ADD INDEX (md5sum)

Why mysqld does not use more than 2 cores are there more tweaks in configfile to use more cores?

thanks Arman.

PS I am using mysql 5.5.14