I have a swollen knee after recent impact, will the magnetic mat be helpful?

If you have recently had an acute joint injury, it should not be heated for the first 48 hours after an injury or until the hot and swollen symptoms subside. 

If you have a joint or joints that are chronically hot and swollen, these joints may respond poorly to vigorous heating of any kind, but you may still use Photon (Red Light) and PEMF.