Want to improve the development of your biceps? If so, it’s necessary to understand a little applied anatomy. The biceps brachii is a two-headed muscle, comprised of the short head (which sits on the inner portion of the upper arm) and the long head (which sits on the outer portion of the upper arm). The long head of the muscle crosses the glenohumeral joint at the shoulder while the short attaches at the coracoid process on the front aspect of the scapula (shoulder blade). Why is this significant? Well, during performance of exercises where the upper arm is held behind the body, the long head is placed in a greater position of stretch compared to the short head and therefore is able to generate maximal force. Conversely, during performance of movements where the upper arm is held out in front of the body, the long head is slackened to a greater degree (a phenomenon called active insufficiency) and therefore force production is compromised.
Given that the long head of the biceps also has a secondary role in abduction of the shoulder, it also will become actively insufficient when performing curling movements where the the arms are raised out to the sides (i.e. as if performing a biceps pose in bodybuilding). Thus, the short head will be worked to a greater extent in these exercises.
What’s more, evidence shows that aspects of the biceps are “partitioned”, with both the long and short heads comprised of individual architectural compartments that are innervated by private branches of the primary nerves. EMG studies of the long head of the biceps brachii show that muscle fibers in the lateral portion of the muscle are recruited for elbow flexion, fibers in the medial aspect are recruited for supination, and centrally located fibers are recruited for non-linear combinations of flexion and supination. Furthermore, the short head appears to be more active in the latter part of an arm curl (i.e. greater elbow flexion) while the long head is more active in the early phase.
So what does all this mean in terms of muscle development? It means that you have the ability to target either the short head or the long head by varying arm angle! Exercises such as incline curls and drag curls (where the upper arm is positioned behind the torso) place more emphasis on the long head of the biceps. Alternatively, exercises such as preacher curls and concentration curls (where the upper arm is positioned in front of the torso) work more in the short head, as do exercises such as cross cable curls on a high pulley apparatus. “Traditional” barbell and dumbbell curls (where the upper arm rests at the sides) will place a fairly equal amount of stress on both heads of the biceps. Performing supinated curls as well as hammer curls will ensure complete activation of the fibers in the long head.
This now begs question as to how you should apply this information. Unless there is a visible discrepancy between the two heads of the muscle (which is unlikely and difficult to assess), your best bet is to vary your choice of exercises. When performing a split routine, a good approach is to choose one movement from each group and then perform two to three sets per exercise each session. In this way, you ensure that all available muscle fibers are optimally worked over time, thereby promoting complete and symmetrical development of the muscle. Keep total sets to no more than about nine or so per workout to ensure you don’t overwork the biceps–they are a small muscle and are involved as synergists in many upper body exercises. Also, make sure to train through a full range of motion so that both heads are worked to their fullest extent.
Here are 3 really good reference materials for biceps specific training:
Arm Size and Strength: The Ultimate Guide
Encyclopedia of Muscle and Strength
Strength Training Anatomy