How can we design a training program that will maximize the growth of the biceps? To answer that question, we need to know which factors we need to take into consideration, and how each of these factors affects the content of the program.
What information do we need to identify the best approach to training a muscle group?
The basic functions of muscles can be identified from their anatomy. The most fundamental anatomical features of a muscle are its origin and insertion. These are the points that the muscle attaches to the skeleton. The origin is the point more proximal, while the insertion is the point more distal. By locating the origins and insertions of a muscle, we can make basic predictions about its functions. In addition, when there are multiple muscles within a group, if they have slightly different origins and insertions, we can make predictions about how small differences in exercise performance might affect their relative contributions to a movement or exercise.
Many muscles have origins and insertions that mean that the muscle crosses only one joint, while others have origins and insertions that mean that the muscle crosses two joints. Crossing one joint means that the muscle can only carry out one joint action, while crossing two joints means that it can carry out two joint actions. When a muscle crosses two joints, it can display unpredictable behaviors if a movement or exercise involves moving both of these joints at the same time.
#2. Internal moment arm lengths
Muscles exert turning forces on joints that are the product of their linear tension and their internal moment arm length. Therefore, longer internal moment arm lengths lead to greater turning forces. More importantly, when multiple muscles act on a joint, those with the longest internal moment arm lengths contribute to the greatest extent. Also, which muscles contribute most to the joint turning force alters according to the joint angle.
By studying the way in which the internal moment arm lengths of all the muscles acting on a joint change over the joint angular range of motion, we can identify the joint angle that involves the greatest contribution from each muscle. Having done this, we can then select exercises with strength curves that involve peak forces being required at the joint angles that involve the greatest contribution from the target muscle that we want to develop. This is particularly important when training muscles at joints controlled by multiple different muscles (such as the hip, which is extended by the gluteus maximus, hamstrings, and adductor magnus), but is also relevant when a muscle has multiple heads, as is the case for the triceps brachii or the quadriceps.
#3. Working sarcomere lengths
Muscles grow after their fibers are exposed to a high level of mechanical tension. The amount of tension that muscle fibers experience can be increased by stretching their passive structures (mainly titin). Consequently, using a full range of motion for a strength training exercise can increase hypertrophy, but only if this leads to titin being stretched.
However, the titin molecules inside muscle fibers are only stretched when the sarcomeres of the muscle fibers in a muscle are lengthened as far as the descending limb. Lengthening a muscle to a stretched position using a full range of motion does not always cause the sarcomeres of its muscle fibers to reach this point. Some muscles contain muscle fibers that have sarcomeres that operate entirely on either the ascending limb or the plateau region of the length-tension relationship. As a result, they do not experience any increase in passive tension when the muscle itself is stretched.
Therefore, to identify the best range of motion for a muscle, we need to look at the range of working sarcomere lengths for that muscle, to see whether they can reach the descending limb. When muscles contain muscle fibers with sarcomeres that reach the descending limb, we may decide to use full ranges of motion in most exercises, since this should stimulate stretch-mediated hypertrophy. In contrast, when muscles contain muscle fibers with sarcomeres that remain mainly on the ascending limb or plateau regions, we may decide to use some exercises with partial ranges of motion, either for variety or to reduce the amount of muscle damage that is caused.
#4. Susceptibility to muscle damage
Susceptibility to muscle damage after a workout helps us to identify the right training volume, loading types, and frequency, since these factors are largely a function of how easy it is to damage a muscle during any given workout.
Susceptibility to muscle damage after a workout can be directly assessed by recording strength recovery. Where this information is not available, it can be inferred based on several factors. Factors that affect the susceptibility of a muscle to muscle damage after a workout include: (1) the prevailing fiber type of the muscle, (2) the level of voluntary activation that can be attained by the muscle, (3) the size of the muscle, and (4) the working sarcomere lengths of the muscle fibers inside the muscle.
- Prevailing fiber type of the muscle — the prevailing fiber type of the muscle affects the amount of muscle damage that occurs after a strength training workout because fast twitch muscle fibers are more easily damaged than slow twitch muscle fibers (perhaps due to their less oxidative capacity or to the stiffer titin molecules that they contain). Therefore, muscles that contain a proportion of fast twitch muscle fibers will experience greater damage after a workout.
- Level of voluntary activation — the level of voluntary activation that can be attained by the muscle during a muscular contraction affects the amount of muscle damage that occurs after a strength training workout since higher levels of motor unit recruitment allow the activation of more fast twitch muscle fibers, which are more easily damaged. Therefore, muscles that can achieve greater voluntary activation will experience greater damage after a workout.
- Size of a muscle — the size of a muscle affects the amount of muscle damage that occurs after a strength training workout because larger muscles tend to achieve lower levels of voluntary activation during maximal contractions, and display greater central nervous system fatigue during fatiguing exercise. Both lower levels of voluntary activation and greater central nervous system fatigue are protective against muscle damage occurring. Therefore, it can be expected that smaller muscles will experience greater muscle damage than larger ones.
- Working sarcomere lengths — the working sarcomere lengths of the muscle fibers within a muscle affect the amount of muscle damage that occurs after a strength training workout because longer sarcomere lengths lead to greater passive tension, and therefore produce greater damage to the internal structures within each muscle fiber.
#5. Regional muscle variation
Although we often think about muscles as being single entities, they are actually made up of multiple functional units, which are sometimes called muscles within muscles. These functional units contribute differently to force production depending on (1) the direction of movement at the joint, and (2) the point in the joint range of motion at which force is greatest.
Regional variation can be identified by anatomical study (to identify the presence of any subdivisions inside the muscle, such as may be caused by intramuscular tendons), by analysis of the innervation of the muscle (to assess whether there are multiple nerves leading to different parts), and by assessment of regional muscle activation during a range of different exercises (to see which regions are most active). Where such regions exist, multiple exercises may be necessary to achieve maximal hypertrophy, because different exercises will involve the preferential recruitment of motor units within the different regions.
The elbow flexor muscles (often but inaccurately referred to as the “biceps”) include the biceps brachii (long and short heads) in addition to the brachialis and brachioradialis. In terms of muscle volume, there are very varying reports regarding the contributions of each of these muscles to elbow flexor muscle size. Averaging these data, the biceps brachii contribute 48%, the brachialis 35%, and the brachioradialis 17%.
Generally, the biceps brachii have two heads, but a minority of people may have three heads. There does not seem to be any clear pattern to the anatomy of this third head, and a smaller minority of people may even have four heads. Whether this has any meaningful impact in practice is unclear.
The biceps brachii long head and short head are generally termed “two-joint muscles” that act as elbow flexors and shoulder flexors, while the brachialis and brachioradialis muscles are usually called “single-joint muscles” that act only as elbow flexors. However, both biceps brachii heads are also forearm supinators, and the brachioradialis can act as a forearm pronator or supinator, depending on the orientation of the forearm. This is worth pointing out, given that the functions of these muscles at the wrist have meaningful implications. The brachialis is usually considered to be the only “pure” elbow flexor that does not perform any other function, such as shoulder flexion or forearm supination.
The origins and insertions of the main elbow flexors are as follows:
- Biceps brachii long head — the long head of the biceps brachii originates from the supraglenoid tubercle of the scapula, which is located above the glenoid cavity but at the base of the coracoid process. The tendon of the long head of the biceps brachii runs from the supraglenoid tubercle and over the head of the humerus before attaching to the muscle. The tendon emerging from the long head of the biceps brachii inserts on the ulnar side of the radius, on the proximal half of the bicipital tuberosity.
- Biceps brachii short head — the short head of the biceps brachii originates on the coracoid process of the scapula, which is a curved protuberance on the most lateral, most superior, and anterior part of the scapula, just above the glenoid cavity. The tendon emerging from the short head of the biceps brachii inserts on the ulnar side of the radius, on the distal half of the bicipital tuberosity. Also linked to the proximal part of the tendon emerging from the short head of the biceps brachii is a broad fascial structure called the lacertus fibrosus or bicipital aponeurosis, which inserts into the antebrachial fascia that covers the forearm flexor muscles that originate from the humeral medial epicondyle.
- Brachialis — the brachialis originates on the distal, anterior humerus and inserts on the coronoid process of the ulna, which is a small projection from its proximal, anterior surface.
- Brachioradialis —the brachioradialis originates on the distal, lateral humerus and inserts near to the styloid process of the ulna, which is a small projection from its distal, lateral surface. The muscle runs the length of the whole forearm, on its lateral side when standing in the anatomical position. Owing to its position on the lateral sides of both the humerus and the radius, the brachioradialis is a forearm supinator when the forearm is pronated, but a pronator when the forearm is supinated.
In summary, when we talk about “biceps” training, we are really talking about elbow flexor training, and the biceps brachii comprise 50% of elbow flexor volume. The elbow flexors include the biceps brachii (which are shoulder flexors, elbow flexors, and forearm supinators), the brachialis (which is an elbow flexor), and the brachioradialis (which is an elbow flexor and a forearm pronator or supinator, depending on forearm position).
Internal moment arm lengths
#1. Effect of elbow angle
All of the elbow flexors display a trend to increase their elbow flexion internal moment arm lengths with increasing elbow flexion angle, and they therefore have the least leverage when the arm is straight (the elbow is fully extended).
Yet, the elbow flexion internal moment arm length of the brachioradialis increases to a greater extent with increasing elbow flexion joint angle than the other elbow flexors. Therefore, it contributes proportionally more than the other muscles when the arm is bent compared to when the arm is straight. Also, the biceps brachii display a greater reduction in elbow flexion internal moment arm length at higher degrees of elbow flexion than the other elbow flexors (most markedly in comparison with the brachioradialis). Therefore, the biceps brachii contribute proportionally less than the other muscles when the arm is bent compared to when the arm is straight.
Consequently, exercises with a strength curve involving peak forces exerted with a bent elbow will develop the brachioradialis to a greater extent than the biceps brachii. The easiest way to achieve this type of strength curve is to use a form of accommodating resistance, such as by attaching elastic resistance bands to a barbell.
#2. Effect of forearm action and position
The biceps brachii are forearm supinators, and their forearm supination internal moment arm length increases with increasing pronation angle. In contrast, the brachioradialis is a forearm pronator only when the forearm is supinated, and becomes a supinator once the forearm is pronated.
We can therefore influence the relative involvement of each of the elbow flexors in elbow flexion exercises either by (1) introducing a simultaneous forearm action, or (2) altering the position of the forearm.
When a task involves combined elbow flexion and forearm supination actions, the biceps brachii will be preferentially more activated, while the activation of the brachioradialis will be reduced. When a task involves combined elbow flexion and forearm pronation actions, brachioradialis activation will be increased, and biceps brachii activation will be reduced. In the former case, this effect is mediated by differences in internal moment arm lengths. In the latter case, it is mediated by reflexive inhibition of the biceps brachii due to afferent feedback from the brachioradialis when the forearm is pronated.
Also,biceps brachii elbow flexion internal moment arm lengths increase with increasing supination and decrease with increasing pronation regardless of elbow angle. Assuming that the brachioradialis is not similarly affected, then doing elbow flexion exercises with a supinated forearm will increase biceps brachii involvement and decrease brachioradialis involvement, while doing elbow flexion exercises with a pronated forearm will decrease biceps brachii involvement and increase brachioradialis involvement.
Consequently, exercises that combine elbow flexion and forearm pronation actions or positions may well develop the brachioradialis to a greater extent relative to the biceps brachii. In contrast, exercises that combine elbow flexion and forearm supination actions or positions should develop the biceps brachii to a greater extent than the brachioradialis.
#3. Effect of shoulder action and position
Although the biceps brachii are technically shoulder flexors based on their anatomy, the shoulder flexion net joint moment produced by the electrically-stimulated muscle force of the biceps brachii after deducting the contribution of passive tissues is very small. Moreover, the net joint moment decreases dramatically from 0–60 degrees of shoulder elevation, after which it is zero. This suggests that the biceps brachii do not function as shoulder flexors once the arm moves far beyond the anatomical position.
Many multi-joint pulling exercises such as pull-ups, pull-downs, and rows involve simultaneous shoulder extension and elbow flexion, starting with the shoulders elevated overhead and ending with the shoulders at approximately 60 degrees. If the biceps brachii were effective as shoulder flexors within this joint angle range of motion, they would act as antagonists in such exercises. It therefore makes good sense that the biceps brachii are not shoulder flexors in this range of motion, and can therefore contribute effectively.
Modeling exercises also indicate that shoulder angle has minimal impact on the elbow flexion net joint moment, and that any effect is likely mediated by a change in muscle length, because the elbow flexors function extensively on the ascending limb of the length-tension relationship, and an increase in shoulder elevation angle causes a reduction in muscle length, which therefore reduces the force that can be exerted. Consequently, it seems that elbow flexion exercises are best performed with the shoulder in the anatomical position, albeit due to the length-tension relationship, and not because of the internal moment arms.
Working sarcomere lengths
When measured between 20–120 degrees of elbow flexion (0 degrees = full elbow extension), the working sarcomere lengths of the biceps brachii (long and short heads), brachialis, and brachioradialis remain on the ascending limb and the plateau of the length-tension relationship. This suggests that most of their muscle fibers are unlikely to experience any stretch-mediated hypertrophy during normal strength training or indeed as a result of inter-set stretching. Even so, given the tendency for non-uniform strains in this muscle, it is possible that the muscles fibers of some regions might experience stretch on the descending limb even while others do not.
In practice, this means we can freely make good use of partial range of motion exercise variations when training the biceps brachii, as it will likely respond similarly regardless of the range of motion used (or longest muscle length achieved) in training. Using partial ranges of motion is particularly useful when attempting to increase training frequency, since this type of exercise tends to produce less muscle damage. Even so, care should be taken to avoid training extensively on the ascending limb, such as by working in high degrees of shoulder flexion, as this will reduce the mechanical tension experienced by the working muscle fibers.
Susceptibility to muscle damage
Research that has studied the effects of a standard workout on a number of muscle groups has found that the biceps brachii takes longer to recover from a standardized workout than many other muscles.
This might be expected, given that the biceps brachii muscle group is much more fast twitch than many other muscles, and often reaches quite high levels (94–99%) of voluntary activation, which is predictable given that the muscle group is actually very small. Even so, the biceps may not be as easily damaged by quite as much as we might anticipate, because it operates mostly on the ascending limb and plateau region of the length-tension relationship.
In practice, this means that we should avoid training the biceps brachii in ways that increase muscle damage, such as by using high repetition sets, high volumes, and larger ranges of motion. Also, accommodating resistance in the form of elastic resistance may be a better option than free weights. Despite using these methods, we may still need to train the muscle less frequently.
It has been known for some time that certain motor units within the biceps brachii seem to be recruited in order to carry out certain functions, such as elbow flexion, while other motor units are recruited to perform other functions, such as forearm supination. Such regional activation of muscle fibers is mirrored to some extent by regional innervation, and perhaps also by regional anatomy. Whether such differences in regional motor control occur because different regions have different internal moment arm lengths, or for other reasons, is currently unclear.
Additionally, there is evidence that certain regions of the biceps brachii are recruited at different points during a fatiguing contraction, which suggests that there are regions in which high-threshold motor units are located, and other regions in which low-threshold motor units can be found. Moreover, some research indicates that the fiber type proportion of the biceps brachii varies between regions, which supports the idea that high-threshold motor units are located in certain, specific parts of the muscle.
Finally, modeling work has identified a tendency for non-uniform strains in this muscle, and this could cause some regions to grow to a greater extent than others, after a period of repeated muscular contractions. Indeed, regional hypertrophy of the elbow flexors has been recorded after long-term, conventional strength training, although it seems more likely that the smaller distal region hypertrophy measured in this study merely reflects an overall lack of longitudinal fiber growth, due to few fibers being stimulated on the descending limb of the length-tension relationship.
Consequently, there is good reason to believe that there are individual regions of the biceps brachii that function separately from one another and may respond better to different exercises. However, it is still unclear how these regions might be targeted.
What is the takeaway?
The elbow flexors include the biceps brachii (long and short heads) as well as the brachialis and brachioradialis muscles. For maximum hypertrophy of this muscle group, it is optimal to use exercises and training methods that develop each muscle to its greatest extent. Also, since there is evidence of regional variation within the biceps brachii, a range of exercises may be necessary for targeting each of the functional compartments.
Owing to differences in their internal moment arm lengths, the elbow flexors can be targeted with exercises involving different strength curves and forearm positions. Barbell or dumbbell curls with free weights are ideal for developing the biceps brachii (and the brachialis) muscles, and a more supinated grip can be used to emphasize the biceps brachii, and reduce the contribution of the brachioradialis. In contrast, pronated grip curls against accommodating resistance (elastic bands) can be used to target the brachioradialis.
The elbow flexors are a small, fast twitch muscle group for which we can achieve extremely high levels of voluntary activation. Although they all work on the ascending limb and plateau regions of the length-tension relationship, they are therefore easily damaged by training. This indicates that we will want to use training approaches that reduce muscle damage (such as exercises with descending strength curves, partial ranges of motion, or accommodating resistance). We may also want to reduce volume (if we keep training frequency the same as other muscle groups) or train the muscle less frequently (if we keep training volume the same as other muscle groups).
Since the elbow flexors work mainly on the ascending limb and plateau regions of the length-tension relationship, attempting to achieve stretch-mediated hypertrophy by selecting full range of motion exercises is unlikely to be more effective than partial range of motion equivalents. Given that full ranges of motion cause more muscle damage, and it is important to avoid muscle damage for this muscle group, partial range of motion variations may be the best option.