In the absence of a cool-down, the muscles will tighten up very quickly. This leads to stiffness, inflexibility, and soreness. When you cool down gradually, the muscles don't tense up and contract. Even though there are a number of ways to warm-up and cool down, the most efficient for runners is to incorporate brisk walking at the beginning and at the end of their run. I also recommend ending every run with some stretching.
Calcium carbonate is the most popular, probably because it's also the cheapest.
Calcium carbonate needs to be taken with meals to ensure absorption. Calcium carbonate often causes bloating and is difficult for some people to take. A new study shows that calcium citrate has a superior absorption rate, independent of whether it's taken with food. However, when calcium citrate is taken with food, its absorption rate is even higher - 2.5 times the rate for calcium carbonate. Finish it, but make sure you take it with meals.