Causes of Muscle Soreness Explored

So, I guess this is sort of question and a discussion of what I've heard to be the causes of muscle soreness. Because after reading more and more stuff, I really have no idea anymore =C

So what exactly causes muscle soreness?

1. lactic acid build-up
cure: drain your legs with a cool down and stretch
2. Microscopic tears in your muscles, esp for endurance sports
cure: not anything you can do
3. Muscle cell exhaustion
cure: not anything you can do - just keep muscles nourished with CHO and protein for recovery
4. Lack of electrolytes so that your muscles are contracted - constantly working, hence soreness
cure: drink a sports drink, and i will personally recommend gatorade =D

at least that's what i've heard. Wondering what you've heard or what you believe to be the cause of muscle soreness, so that we can find a cure to muscle soreness. together.