I've been riding a fixed-gear for almost 2 years now (commuting about 14 miles daily--not including the occasional miscellaneous trips to the store, etc), and I haven't experienced any signs of knee injury, soreness, etc.
I came across this old article about fixed-gears (
http://www.oldskooltrack.com/files/la.times.frame.html)
Because even in freewheeling San Francisco--where the bike-riding culture carries political weight--police, messenger firms and older couriers say track bike Turks cross a serious line of irresponsible behavior. Citing chronic knee damage and accidents, many wonder whether it's time to rein in such risk-takers.
Veterans riders dismiss the bikes as "suicide rockets" and "leg-beaters" for the unhealthy pounding they impose on a rider's knees. Many track bike aficionados display the scars of their stubbornness, bearing incisions from knee surgery to repair everything from ligament pops to cartilage tears.