How can we design a strength training program that will maximize the growth of the trapezius (commonly called “traps”)? What factors do we need to take into consideration, and how do each of these factors affect the different variables within the training program?
Unfortunately, as we will see, of all the muscles that are commonly trained in the gym, the trapezius is the most difficult to analyze.
What information do we need to identify the best approach to training a muscle group?
To understand how to train the trapezius, we need to work through the research literature to find several important pieces of information.
- Anatomy — if we know the origins and insertions of a muscle, we can make basic predictions about which exercises will be effective for training them. Where the origins and insertions differ between the muscles of a group (or between regions of a muscle), we can predict which exercises will be most effective for which muscle (or region).
- Regional anatomy — sometimes a muscle has multiple internal regions, and each of these regions has different characteristics. When this happens, multiple exercises may be necessary to train the muscle, to target each region in ways that suit its individual characteristics.
- Internal moment arm lengths — the internal moment arm lengths of a muscle determine its contribution to a joint moment, relative to other muscles. Where these change over the joint range of motion, they determine which muscles within a group will be trained most effectively by an exercise, depending on its strength curve.
- Working sarcomere lengths — the working sarcomere lengths of the muscle fibers of a muscle throughout its physiological range of motion determine whether it can experience stretch-mediated hypertrophy. Only when the sarcomeres of the muscle fibers of a muscle are stretched onto the descending limb of the length-tension relationship can mechanical tension be increased during full range of motion exercises.
- Susceptibility to muscle damage — the amount of muscle damage that a muscle experiences after a workout is the main determinant of the frequency we can use for training it. This is because we want to avoid doing workouts when central nervous system (CNS) fatigue is present, and the amount of muscle damage is the main factor that determines how long CNS fatigue lasts after a workout.
The trapezius muscle is a large back muscle that originates on the spine and attaches to the clavicle and the scapula. The muscle is innervated by the spinal accessory nerve (the eleventh cranial nerve), after this nerve has descended through the sternocleidomastoid.
The trapezius muscle is subdivided into three main regions: upper, middle, and lower. Each region has a different origin and a different insertion, making them highly independent of one another and causing them to carry out very different joint actions. Indeed, with biofeedback training, individuals can learn how to activate each region independently. Additionally, the upper, middle, and lower regions have different internal moment arm lengths for shoulder abduction.
Unfortunately, the trapezius has been described extremely inconsistently in both anatomy textbooks and also in research, making accurate information difficult to obtain. One very serious difficulty is the consistent reference by research groups to the “upper trapezius” when assessing the behavior of muscle fibers that originate from the C7 vertebra and insert on the scapula (this is common when using electromyography to record muscle activation). Anatomically, these are fibers that actually belong to the middle region, while the upper region refers to all fibers above this level.
Alongside the complicated nature of the attachments of this muscle, and its role in stabilizing the scapula in a wide range of shoulder movements, these disagreements in terminology between researchers makes it very difficult to infer the biomechanical roles of the muscle regions to various joint actions, especially shoulder (rather than neck or scapula) movements. The following analysis therefore focuses mainly on the roles of the trapezius in neck and scapular movements.
- Upper trapezius — the upper trapezius originates at the base of the skull, on the posterior neck ligaments, and on the cervical vertebrae down to C6. All of the fibers originating on these points insert on the posterior, lateral side of the clavicle. The fibers of this region are directed horizontally, suggesting that they may not elevate the shoulder and cause shoulder elevation (shrugging) or head extension, although they are often described as having these functions. Indeed, strength training with shrugs has no effect on neck muscle size (the upper trapezius occupies one third of neck size by volume). Rather, analysis of the origins and insertions suggests that the main function of at least the clavicular portion of the upper trapezius is to pull the clavicle backwards and medially, which most likely contributes to scapula adduction (retraction).
- Middle trapezius — the middle trapezius originates on the spinous processes of two vertebrae (C7 and T1). The section arising from C7 inserts on the medial side of the acromion of the scapula, while the section arising from T1 inserts on the scapula spine. Given that its muscle fibers are directed largely horizontally, this region is well placed to carry out scapula adduction (retraction). Yet, the fibers also point slightly upwards from the scapula to the spinal column, meaning they can cause shoulder elevation (shrugging). The insertions of the middle trapezius are close to the center of rotation of the scapula, making it hard for the region to cause scapula upward (inward) rotation from the anatomical position.
- Lower trapezius — the lower trapezius originates on the spinous processes of the thoracic vertebrae (T2 to T12). Importantly, all of its fascicles insert on the deltoid tubercle of the scapula spine, which is on the medial aspect of the scapula. By pulling the medial aspect of the scapula spine towards the spine, it causes scapula adduction (retraction). Its varying origins along the length of the spine make its contribution to scapula upward (inward) rotation limited, although it is often described as having this function in textbooks. Similarly, whether it can cause scapula depression (the opposite of shrugging) is unclear, because some of its origins are vertically higher than its insertion, others are horizontally level with it, and others are vertically lower down. Notwithstanding this verifiable point, some researchers still assert that this region does depress either the scapula or the shoulder joint.
In summary, from the anatomical data, it seems that the main role of all three regions of the trapezius is to carry out scapula retraction. In addition, the middle region can carry out the shrugging action that we closely associate with this muscle. Its other roles are unclear.
Anatomical studies have found that the lower trapezius is the largest region, occupying approximately 48% of the total volume of the muscle. The middle and upper trapezius are of similar size, with the upper trapezius being slightly smaller and occupying 23% of the total volume of the muscle, while the middle trapezius occupies 28%. As we can see from the above anatomical analysis, shrugs may only train 28% of the trapezius effectively, leaving a larger portion of the muscle untrained.
In practice, it is difficult to infer the contributions of the trapezius to key shoulder movements (such as shoulder abduction and shoulder horizontal abduction) based on an analysis of its anatomy. It seems likely that only the middle trapezius contributes to shrugging, but all three regions of the trapezius are scapular adductors (retractors). The contributions of each of the trapezius regions to scapular upward (inward) rotation and scapular depression are unclear. Given evidence that the regions function independently of one another, to achieve maximum growth of the trapezius, all three regions should be trained.
Multiple lines of evidence (internal moment arm lengths, muscle activation, and differences in the responses to muscle-damaging workouts) suggest that each of the three regions of the trapezius muscle can actually be subdivided into smaller subregions.
Indeed, each of the three regions of the trapezius muscle can be subdivided based on their internal moment arm lengths for shoulder abduction and also for neck movements. Also, some evidence suggests that there are differences in muscle activation within areas of what is termed the upper region (which is probably actually the middle region, given that the line of action runs from the C7 vertebrae in these studies) of the trapezius muscle. Finally, after a workout involving eccentric contractions, it has been shown that the regions of the trapezius muscle are affected to differing extents, perhaps due to differences in either their muscle architecture, fiber type, or activation.
Some of the differences in internal moment arm lengths are quite important, and suggest that some of the subregions actually have very different functions from other subregions. For example, the middle and upper regions of the trapezius have subregions that have shoulder *adduction* internal moment arm lengths instead of shoulder *abduction* internal moment arm lengths, once the arm is elevated above 100 degrees.
In practice, there are probably subregions within each of the main three regions of the trapezius muscle. Some of these subregions may respond better to training with certain joint movements than others, due to differences in internal moment arm lengths. This underscores the importance of using a variety of suitable exercises for training the trapezius muscle.
Internal moment arm lengths
The most famous role of the trapezius muscle is to perform shoulder shrugs. Even so, researchers have also identified that the muscle has internal moment arm lengths for neck movements in various directions, in addition to internal moment arm lengths for shoulder abduction, and an analysis of its anatomy suggests that the muscle must also have meaningful internal moment arm lengths for scapular retraction.
Importantly, although the trapezius muscle is also very active during many shoulder joint movements, it may not actually have an internal moment arm length for causing such movements, since its main role of scapular retraction may be called upon to stabilize the shoulder joint during exercises in various directions.
#1. Shoulder abduction
Some research has examined the internal moment arm lengths of the trapezius during shoulder abduction. For most of the normal shoulder abduction range of motion, the middle region has the longest moment arm length, followed by the upper region, and finally the lower region. Indeed, exercises involving shoulder abduction typically involve the greatest levels of middle region muscle activation (often described as he upper region, despite measuring fascicles that originate at C7 and insert on the scapula). However, the moment arm lengths change substantially with joint angle.
Shoulder abduction internal moment arm lengths of the middle and upper regions follow a very similar pattern with increasing shoulder abduction in the frontal plane. They are relatively constant between 0 and 90 degrees (from the anatomical position to the arms out to the sides) and then decrease dramatically to nothing at approximately 140 degrees. In contrast, the shoulder abduction internal moment arm length of the lower region is very small initially but increases gradually from 0 to 120 degrees.
Thus, we can expect the middle region to play the greatest role in shoulder abduction between 0 and 90 degrees, followed by the upper region. However, the lower region may be more involved in shoulder abduction when the arms are elevated substantially above 120 degrees. Some (but not all) research investigating muscle activation during shoulder abduction partially confirms this, reporting increased lower region muscle activation contributions with increasing arm elevation at least up to 90 degrees.
#2. Other shoulder movements
The internal moment arm lengths of the trapezius for most shoulder joint actions are unclear. Moreover, it seems likely that the trapezius acts mainly as a scapular stabilizer while other shoulder muscles are producing a turning force at the joint. In other words, the trapezius may have small moment arm lengths for many shoulder joint actions, yet may be strongly (isometrically) activated in many shoulder movements. Its contribution may depend on its ability to stabilize the scapular in opposition to the turning forces that are generated at the shoulder.
Indeed, greater activation of the middle and lower trapezius regions has been noted during free weight rowing exercises than in machine rowing exercises, possibly due to the requirement for increased scapular stabilization. Similarly, higher levels of middle and lower trapezius muscle activation have been observed during unstable push ups than during stable push ups.
Notwithstanding the above analysis, many researchers have analyzed the role of the trapezius during shoulder flexion and extension and during shoulder horizontal extension, most often in an attempt to identify those joint angles or ranges of motion that lead to a greater contribution from the lower trapezius , and a smaller contribution from the upper (in reality middle) trapezius.
#3. Neck movements
Although basic anatomical analysis suggests that the trapezius muscle does not contribute meaningfully to neck movements, research has shown that the upper trapezius has substantial internal moment arm lengths such that it can perform head extension, lateral bending, and axial rotation. Therefore, direct neck training, such as with a neck harness, or using isometric contractions, could be useful for training this region.
#4. Scapular retraction
An analysis of trapezius anatomy indicates that all three regions have a strong ability to carry out scapular retraction. Moreover, when scapular retraction is carried out as an isolated joint action with the arm at shoulder height, muscle activation is high in all three regions of the trapezius, with little apparent difference between regions. Interestingly, the activation of the various regions of the trapezius differs during scapular retraction at different joint angles.
- Shoulder flexion angle — some research indicates that the contribution of the regions of the trapezius to scapular retraction differs with shoulder flexion angle. At 90 degrees of shoulder elevation, the lower region is a greater contributor. Yet, at 130 degrees, the upper region contributes to a slightly greater extent than the lower region. In contrast, research that has investigated scapular retraction in pulling movements that involve shoulder extension in the sagittal plane, and which involve starting in a position of shoulder flexion, have found that the middle region seems to be more active than the lower region when the arm is elevated slightly above shoulder height at the start of the exercise.
- Shoulder abduction angle — some research indicates that the contribution of the regions of the trapezius to scapular retraction differs with shoulder abduction angle. Across the upper and middle regions, muscle activation increases as shoulder abduction increases from 0 to 90 degrees. In general, middle region activation is higher than upper region activation. However, the relative contribution of the upper region increases with shoulder abduction. Thus, the position with the arms in the anatomical position (by the sides of the body) is better for isolating the middle trapezius, while the position with the arms out to the sides is better for training the upper trapezius.
#5. Shoulder shrugging
Even so, an analysis of the anatomy of the trapezius indicates that the middle region may well be the best placed for carrying out this joint action, and assessments of muscle activation during shrugs support this idea (noting that researchers usually misclassify the middle region as the upper region in most electromyography studies). Interestingly, the lower parts of the trapezius are much less active during shrugging than during scapular retraction (while the middle region is similarly active during shrugging and scapular retraction, which is exactly what the anatomical analysis would predict.
The activation of the various regions of the trapezius muscle may alter with shoulder abduction angle. Some (but not all) research has found that the activation of the upper and lower regions during shrugging increases with increasing shoulder abduction.
In practice, we have little information regarding the internal moment arms of three regions of the trapezius during neck, scapular, and shoulder movements. The data that are available suggest that all three regions of the trapezius act mainly as scapular retractors, while the middle region is also responsible for the shrugging movement. Even so, the contribution of the middle region to scapular retraction and shrugging is similar. The upper region may also be involved in various neck movements
Working sarcomere lengths
When muscles produce force, the amount of force they produce is primarily determined by the force-velocity and length-tension relationships of the working muscle fibers.
If different muscles within a group (or different regions within a muscle) are contracting at different velocities or from different starting lengths, then they will produce different amounts of force, and therefore also experience different amounts of mechanical loading. Moreover, when a muscle works predominantly on the descending limb of the length-tension relationship, it is more likely to experience an additive effect of passive and active tension during strength training with a large range of motion, because of greater stretch-mediated signaling.
The working sarcomere length ranges of the trapezius have been estimated by researchers. Across all regions (and with little difference between regions), the fibers of the trapezius work almost entirely between 0.5 — 1.0 times normalized fiber length, suggesting that they work entirely on the ascending limb and plateau regions of the length-tension relationship, and do not reach the descending limb. This indicates that they are unlikely to experience any stretch-mediated hypertrophy as a result of passive tension contributing to muscle fiber force.
In practice, the trapezius cannot experience stretch-mediated hypertrophy. Nevertheless, it may suffer from active insufficiency if trained predominantly at short muscle lengths. Therefore, training the trapezius using sustained isometric contractions in the contracted position or pauses (holds) is unlikely to be as effective as working the muscle to a greater extent at longer lengths.
Susceptibility to muscle damage
The ability of the muscle to recover will depend upon its (1) voluntary activation percentage, (2) fiber type, and (3) working sarcomere lengths. The level of voluntary activation that can be achieved for a muscle affects its susceptibility to muscle damage as the muscle fibers of the high-threshold motor units are the most easily damaged. If voluntary activation is higher, then more of those muscle fibers can be activated, and therefore more muscle damage will be caused. The prevailing fiber type of a muscle affects its susceptibility to muscle damage because less oxidative type II muscle fibers are more easily damaged than more oxidative type I muscle fibers. The working sarcomere lengths of a muscle affect its susceptibility to muscle damage because muscle fibers that reach the descending limb experience greater levels of passive tension, which causes damage to the myofibrils.
- Voluntary activation — the level of voluntary activation of the trapezius muscle has been found to be 94–96% in untrained individuals, which is in line with other large muscles of the upper body such as the triceps brachii, but lower than the high levels that are observed in smaller upper body muscles, such as the biceps brachii.
- Muscle fiber type — in males, the fiber type of the trapezius varies between regions (note that nomenclature for each region varies between studies). The lower region comprises a very high proportion of type I muscle fibers (80%), and the remaining type II fibers are predominantly type IIA (18%). Similarly, the middle region also comprises a relatively high proportion of type I fibers (66 — 76%), and the remaining type II fibers are also predominantly type IIA (21 –– 29%). Only the upper (clavicular) region has fewer type I fibers (44 — 51%), a moderate amount of type IIA fibers (29–36%), and a surprisingly high number of type IIA fibers (20%). Similar results have also been observed in females. This suggests that the middle and lower regions of the trapezius will experience much less muscle damage than the upper region after a workout.
- Working sarcomere lengths — the working sarcomere length ranges of the trapezius have been estimated by researchers. Across all regions (and with little difference between regions), the fibers of the trapezius work almost entirely between 0.5–1.0 times normalized fiber length, suggesting that they do not reach the descending limb, and will likely experience less damage than those muscles that do contain fibers that extent further onto the descending limb.
Overall, the trapezius has moderately high levels of voluntary activation and its fibers do not reach the descending limb of the length-tension relationship. Additionally, the fibers of its lower and middle regions are mostly type I. Thus, the muscle is unlikely to be damaged severely by most workouts, and it will recover fairly quickly. Even so, given the predominantly type II nature of the upper region (and its very high proportion of type IIX fibers), the upper region of the trapezius will likely take far longer to recover from a workout than the other two regions.
In practice, the trapezius as a whole is unlikely to be damaged severely by strength training workouts, and will therefore recover quite quickly. Even so, the upper region will likely take far longer to recover from a workout than the other two regions, as a result of its very high proportion of type II (and type IIX) fibers.
What is the takeaway?
The trapezius is a large back muscle that is commonly subdivided into three major regions (upper, middle, and lower). It is hard to identify the main roles of this muscle, because it seems to be activated whenever the scapula needs to be stabilized. Yet, this stabilizing activity may not be sufficient for training the muscle, since it is only playing a synergistic or supporting role. The main role of all three major regions of the trapezius seems to be scapular retraction (which can be trained by horizontal shrugging). The middle region also carries out scapular elevation (which can be trained by vertical shrugging), but the middle region is similarly active during both scapular retraction and shrugging. Thus, normal shrugging may be superfluous.
As a whole, the trapezius is unlikely to be severely damaged by strength training workouts, will recover quickly, and can therefore be trained relatively frequently. The upper region will likely take longer to recover than the other two regions, due to a larger proportion of fast twitch fibers. Yet, if one exercise (scapular retraction) is being used to train all regions, then training frequency will be determined by the recovery of the most easily-damaged region. Thus, it help to train the trapezius slightly less frequently than other muscle groups.
The trapezius cannot experience stretch-mediated hypertrophy and therefore using a large range of motion will not enhance gains in muscle size. Yet, it may suffer from active insufficiency if trained at short muscle lengths, meaning that isometric contractions in a contracted position are unlikely to be ideal. Dynamic movements starting from a longer muscle length will probably have superior effects on muscle size.