Overview
Although the exercises differ, a common factor of each is a 'curling' motion, where a weight (attached to, or used in conjunction with, an item of equipment listed above) is moved through an arc, primarily using the strength of the biceps.
The fullest range of motion is when the elbows begin in full extension, in a supine grip. The biceps contract to lift the weight upward through an arc, to a point where further movement is not possible. Some lifters also do partial reps (such as in "21s") focusing on the top of bottom position.
Some think it is important that the elbow remain next to the body during this motion to keep stress on the biceps. Others will either bring the elbows forward (to fully shorten the biceps) or bring the elbows back (a "drag curl", to avoid over-active insufficiency and keep parallel forearms) to vary the tensions placed on the biceps and other elbow flexors.
The second part of the motion has the elbow joint extending, this is called the 'eccentric' portion. The weight is lowered back to the start position.
This contraction and extension together constitute a single repetition. As with most weight training exercises, results from biceps exercises can be maximized with a proper understanding of flexion.
The barbell curl specifically targets the biceps brachii which is located on the front part of your arm between the shoulder and the elbow. This muscle's primary function is elbow flexion and forearm supination. The biceps also has two heads known as the long head and the short head.
Read more about this topic: Biceps Curl