Section: 1 (muscle contraction)
In order for a muscle to contract, it needs calcium. Without calcium the muscle cannot contract at all. It also needs two proteins called actin and myosin. The calcium goes into the sarcomere which causes the actin and myosin proteins to bind together to make the muscle contract. ATP (adenosine triphosphate) is required for this process to take place. This is a chemical chain that happens very very fast. When there is no more muscle contraction, calcium is no longer needed.
Section: 2 (bone density)
When you exercise over a period of time you build bone density. In order to build new bone your body must first get rid of old bone. In order for this to take place the parathyroid hormone lets the osteoclasts remove old bone and calcium from the bones before new bone can be built. But first, the thyroid gland has to release a hormone called calcitonin. Calcitonin slows down the rate of the osteoclasts so that the osteoblasts can come in and build new bone and restore calcium as well.
Section: 3 (fat burning)
In order for you to burn fat, you must use fat as fuel. Fatty acids are a form of fat that's always moving around in your body looking for a working muscle. They are known as Free Fatty Acids because they are always on the move. When fatty acids are not in use or have no work to do, they go into a fat cell for storage, but not for long. Once a muscle is working, the fatty acids are on the move again and ready to be used. This is how you burn fat. Weight training is the best way to burn fat. Remember that fat is your friend as long as you use it as fuel. When fatty acids are not in use, you get fat, because fatty acids bond together in triplets known as triglycerides.
Section: 4 (weight control)
As to gaining weight or losing weight, people say it's because you have either a slow metabolism or fast metabolism. You have many metabolisms in your body: protein metabolism, carbohydrate metabolism, fat metabolism and the list goes on. When we talk about metabolism we are talking about calorie burning, known as your (BMR) basal metabolic rate. Your BMR is how many calories your body needs just to exist. Think calorie burning, not metabolism.
The sections shown above are brief descriptions, not full details