The biceps curl is the iconic move that all weight lifters point to when looking at their progress in building muscle. The biceps has two muscle heads and the triceps has three muscle heads. Often while men point to their biceps, the more important muscle to build is the often neglected triceps on the rear part of the upper arm.
To build muscle you need to stretch the muscle fibers apart causing a local inflammation. The muscle fibers are single cells with many nuclei and look like spaghetti, but they contain two proteins that interlock called myosin and actin. The actin is like a little engine that motivates the movement of the myosin and acting together causing the contraction of muscle that gives muscle its force. The contraction of muscle fibers is a passive event that is triggered quickly by the entry of calcium into the muscle cell. The energy for the contraction is provided by mitochondria which are little energy factories that convert sugars and fats to energy producing carbon dioxide and water. They use oxygen to help with this energy conversion which is why it is important to breathe correctly while lifting weights or even during aerobic exercise.
So much for the conduct of exercise but how do you build muscle? You have to stretch the muscle fibers apart so that the actin and myosin are separated during an ECCENTRIC contraction. As displayed on the video this is the opposite of the biceps curl upward movement which is a CONCENTRIC contraction. By moving down slowly, you stretch your muscle fibers and cause an inflammation. This inflammation brings in what are called satellite cells that fuse with the existing muscle cell to make it bigger. What ultimately helps the muscle get bigger is adequate protein consumed within 30 minutes to one hour after the exercise. The proteins are broken down into amino acids and then within the muscle cell re-assembled into more muscle protein.
The optimum amount of protein for this to occur is about 20 grams of protein in young healthy volunteers. Whatever is not used to build muscle may be used by other organs or converted to carbohydrates, but nothing is wasted as some websites imply. You need to take a high quality protein with enough carbohydrate or fat to make it tasty.