Board Thread:General Discussion/@comment-28732696-20150506011117/@comment-6331988-20150926001236

The supersonic punch didn't even knock out Girder, so I can see it not breaking Grodd's arm. After all, he is genetically engineered, and could have extremely enhanced durability. If that explanation doesn't work for you, Grodd often has telekinesis in the comics, which could be used to slow the Flash down or weaken the impact of his punch.