Leg kicks in mixed martial arts have long been one of the attacks that bring out a universal reaction of "oh man, I wouldn't want to be that guy." Nate Marquardt's leg was decimated by Tarec Saffiedine in this past weekend's Strikeforce welterweight title fight with five rounds of heavy leg kicks.
Over at Fightland, Dr. Michael Kelly explains that there can be significant long term damage from leg kicks like those taken by Marquardt:
The biggest problem with something like this, where you have a repetitive injury in the same area and it’s that extensive, is the development of Myositis Ossficans Traumatica. What that is, is you get heterotropic bone formation in the muscle as a result of repetitive strikes or repetitive blood trauma to the same area. As your body heals, it starts to reabsorb some of that blood, but it leaves deposits of calcium in the muscle, and the body actually starts to form bone in the muscle, which affects the function of the muscle. And looking at this bruise, you can see varied levels of Ecchymosis, of bruising. That tells me he’s more likely to develop big calcium rocks in the muscle.
The article then goes on to explain that this could cause loss of function of the muscle, loss of strength, deformity or other complications.
There's also a chance that muscle tissue could be broken down due to the injury.