One strategy for a human joint experiencing an excessive load is to hyper extend the joint or 'lock' it.  This should not be done habitually as it takes the strain off your muscles and loads up your ligaments, tendons and cartilaginous tissue, and is dangerous normally as it risks damage to those structures if the joint is further over loaded.  Some people's joints hyper extend naturally and strengthening exercises can help control the joint with muscle.  This is probably a combination of nature and habit leading to the saddest knees on the planet.