What causes muscle soreness?

Here is a very controversial question with an answer. In the past it was believed that muscle soreness occurs because of "deposits" of lactic acid (a substance resulting from the metabolism of carbohydrates to release energy for rebuilding ATP) which is supposed to accumulate in muscle after intense exercise. It turned out it was a wrong assumption, lactic acid is removed from the muscles much faster, 98% excreted in the muscles trained in 1 hour and 30 minutes (with 50% in the first 15 minutes), so too quick to generate fever muscle for several days.Lactic acid causes muscle fatigue actually not muscle soreness, as some claim today "experts" on our land. In reality no one knows the exact cause of muscle soreness, but the most plausible explanation is given by training musulare cell destruction and oxidative stress caused by free radicals produced during exercise.
The muscle fiber rupture occurs because tensions during exercise with weights. This combination of factors affecting muscle and connective tissue, immediately signaled by pain nerve receptors are restored Are two important aspects:
1. Contractions eccentric (lowering phase the weight, so the elongation of the muscle under tension) produce a large amount of tension and friction intracellular (more than concentric contractions when the muscle shortens). So negative reps or greater during this portion of the next few days miscatii generates more intense muscle soreness.
2. Since muscle soreness is generated by muscle abrasions (both mechanical and chemical) that need time to heal, it is logical that a hard workout too frequently lead to trauma and older. Muscial simply will not have time to recover between two session.