Ok guys there is a lot of gym science and missunderstanding floating around on there. This is FALSE. Your muscles are sore because they are injured - if you look at the mucle tissue under a microscope you would see that the cells are damaged. In otherwords, your ripped/tore a muscle and it needs to heal. Reread my post up above for your answer as to why or why not to exercise a sore muscle.
Lactic acid is a byproduct of anaerobic metabolism and is responcible for the fatigue you feel when doing rigerous exercise. It doesn't cause sore muscles. It is infact converted to a chemical called piruvate and burned off as an energy substrate within a few hours after your workout...this is why there is such a metabolic effect of anaerobic exercise which can last up to two days in some cases.
Your biceps muscles flex your elbo and very weakly flex your shoulder due to its attachments. If you repeatedly flex your elbo against resistance WITHOUT proper joint stability at the shoulder and without adequate healing time and without proper progression and without exercising the muscle THROUGH A FULL RANGE OF MOTION, then you get some unstable joints, damaged muscle tissue, and short and tight biceps. Your body doesn't want to lengthen it out by extending your elbow all the way out for these very reasons - mostly because you are lifting too heavy of a weight and shortening and tightening that **** thing so much!!!
On a side note about training through a full range of motion. I "work" my abs by doing crunches on a stabliity ball with a full range of motion and a 50 lb dumbell on my chest no more than twice per week for about 4 sets total for the week. That means I do about 40 heavly loaded crunches per WEEK and I have what would most would consider the model midsection (ie. six pack)!! Just some food for thought.
The undertrained athlete always outperforms the overtrained athlete. My advice: work hard, then rest hard. If you cannot best your last workout by at least 1-3% improvement, then you need to rest. LISTEN TO YOUR BODY - NOT TO YOUR BUDDIES IN THE GYM OR THESE GOOFBALLS THAT WRITE THESE RIDICULUS ARTICLES IN MAGAZINES!!