I was feeling beaten up on the roads so switched to grass about 5 years ago and felt great until 2 months ago when developed what felt like a tibial plateau stress fracture (pain below the inner side of my knee). I took 5 weeks off and was pain free but within 2 minutes of easy jog on the grass I developed pretty severe pain in the same location with every stride so I shut it down. Walking back to my car, I had no pain so I decided to get on the nearby track (asphalt) out of curiosity to see how it would respond. No pain at all for 20 minute jog. Sitting here now about 3 hours later and it feels fine.