During the trip to Namek Goku used Senzu Beans to use the Zenkais to power him up, so it can be self inflicted through training in high gravity. Goku became more than 10x stronger and able to use Kaioken X10 without strain.
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http://i43.photobucket.com/albums/e373/Gaiash/KumaVsZangief.jpg
LOL!!!! I hope that's a joke....names don't get more n00bish than that