How should we train the deltoids?

How can we design a strength training program that will maximize the growth of the deltoids? What factors do we need to take into consideration, and how do each of these factors affect the different variables within the training program?

What information do we need?

We can use the research literature to enhance our training programs if we search for information about the gross anatomy, regional anatomy, and internal moment arm lengths of a muscle, in addition to its working sarcomere lengths, and susceptibility to muscle damage. Each of these factors provides information that is useful for different reasons (read more).

• Gross anatomy describes the locations of the attachments of the muscle to the skeleton. Learning the basic anatomy of a muscle helps us figure out suitable exercises, and also helps us see how we might alter them to target different muscles within a group.
• Regional anatomy describes the way in which a muscle divides into several internal regions, and this tells us whether we are going to need multiple exercises to train the muscle.
• The internal moment arm lengths of a muscle determine its leverage on the joint, and therefore its contribution to a joint moment, relative to other agonist muscles. This allows us to see where peak force in an exercise joint range of motion needs to be, to target one muscle within a group (or one region of a muscle). We can alter the point where peak force occurs by our exercise selection and by our choice of external resistance type.
• The working sarcomere lengths describe the lengths of the sarcomeres inside a muscle over its joint angle range of motion. It allows us to see if the muscle can experience (1) active insufficiency (and so will be trained poorly by exercises involving peak forces at very short muscle lengths), and (2) stretch-mediated hypertrophy (and so will be trained more effectively by exercises involving peak forces at very long muscle lengths).
• The susceptibility of a muscle to damage is how easily a muscle is damaged by a workout. It is affected by: (1) muscle fiber type proportion, (2) its level of voluntary activation, and (3) the working sarcomere lengths of its muscle fibers. The amount of muscle damage that a muscle experiences after a workout is the main determinant of the frequency (and volume) we can use for training it, and it also influences our choice of exercises (single-joint vs. multi-joint, single-limb vs. multi-limb, and full vs. partial range of motion).

Anatomy

It is widely known that each deltoid muscle is subdivided into three heads, the anterior, middle, and posterior heads. Each head has a different origin.

The anterior head originates on the lateral part of the clavicle. In contrast, the middle head originates on the lateral part of the acromion of the scapula, and the posterior head originates on the inferior side of the spine of the scapula. All heads insert in the same place on the lateral side of the humerus.

As a result of their origins, all of which are more medial (closer to the mid-line of the body) than the insertion, the three heads carry out shoulder abduction (lifting the arm away from the side of the body in the frontal plane). Also, since the anterior head is positioned in front of the insertion, it flexes the shoulder (lifts the shoulder in the sagittal plane), horizontally adducts the shoulder (brings the arms together in front of the face, in the transverse plane), and internally rotates the shoulder. In contrast, since the posterior head is positioned behind the insertion, it extends the shoulder (brings the arm down towards the side of the body in the sagittal plane), horizontally abducts the shoulder (moves the arms away from the face, in the transverse plane), and externally rotates the shoulder.

Practical implications

In practice, the deltoid is a relatively large muscle that can be subdivided into three functionally-different anatomical regions or heads. All three heads cause shoulder abduction. The anterior head also carries out shoulder flexion and horizontal adduction, while the posterior head also carries out shoulder extension and horizontal abduction.

Regional anatomy

Although the deltoid muscle is often divided into three heads, some researchers have identified that it can be further subdivided into even smaller subregions.

Indeed, one careful analysis that assessed both internal muscle moment arm lengths and muscle activation showed that there were likely seven individual subregions of the deltoid. Visual inspection of the diagram provided in this study reveals that the anterior region was not subdivided, and was designated as subregion one. The middle region was subdivided into subregions two and three, while the posterior region was divided into subregions four, five, six, and seven.

These subregions had clearly different internal muscle moment arm lengths for producing shoulder flexion and extension and for producing shoulder abduction and adduction, when measured at 20 degrees of flexion or abduction, respectively. Unfortunately, the moment arm lengths during horizontal flexion and extension were not measured. The differences between regions were as follows:

• Shoulder flexion — the subregions with internal moment arms for shoulder flexion were those that were part of the anterior and middle deltoids (one, two, and three). The lengths of these internal moment arms differed between subregions, with the longest lengths occurring in subregions one and two. This suggests that, in contrast to our common assumptions about the middle head of the deltoid, it probably contributes somewhat to shoulder flexion, as well as to shoulder abduction.
• Shoulder extension —the subregions with internal moment arms for shoulder extension were those that were part of the posterior deltoid (four, five, six, and seven). The lengths of these internal moment arms differed between subregions, and they increased from four through seven.
• Shoulder abduction (and adduction) — while the whole deltoid is widely considered to be a shoulder abductor, only the first six regions had muscle moment arm lengths for shoulder abduction, while the seventh was actually a shoulder adductor. The lengths of these internal moment arms differed between subregions, and increased from subregion one to a peak at subregion three, before declining to subregion six. Thus, the middle head of the deltoid is a greater contributor to shoulder abduction than the anterior and posterior heads.

Similarly, each subregion was activated most strongly during the joint actions for which it had the longest internal moment arm lengths. During shoulder abduction, the middle subregions (two, three, four, and five) were all strongly activated, while the outer subregions (one, six, and seven) were not. During shoulder flexion, the front subregions (one, two, three, and four) were all strongly activated, while the rear subregions (five, six, and seven) were not. Finally, during shoulder extension, the rear subregions (five, six, and seven) were all strongly activated, while the front subregions (one, two, three, and four) were not. Unfortunately, the regional muscle activation during horizontal flexion and extension were not measured.

Clearly, one other factor that was not assessed in this study was the role of force direction. The internal moment arm lengths and muscle activation of the subregions were only measured during vertical shoulder flexion or extension and vertical shoulder abduction or adduction. As with the pectoralis major, it is quite likely that different force directions (such as by using different inclines during bench pressing) might involve different contributions from each of the subregions.

Practical implications

In practice, the deltoid may each be subdivided into as many as seven subregions. Analysis of these regions shows that the middle head is also involved in shoulder flexion, albeit to a lesser extent than the anterior head, and that the contribution of each of the subregions to shoulder abduction increases towards the center of the muscle, with only one subregion failing to contribute at all (the lowest subregion on the posterior side). It seems likely that performing shoulder exercises at different angles may allow the preferential targeting of each subregion.

Internal moment arm lengths

The most famous role of the deltoid muscle is to perform shoulder abduction. However, various segments of the muscle are also involved in shoulder flexion and shoulder extension.

#1. Shoulder flexion and extension

When moving the shoulder in the sagittal plane, many muscles are involved. The anterior and middle deltoids, pectoralis major, and supraspinatus are the primary shoulder flexors, while the posterior deltoid, latissimus dorsi, teres minor, and teres major are the primary shoulder extensors.

• Anterior deltoid — the anterior deltoid has a relatively large shoulder flexor moment arm throughout the joint range of motion, although its leverage is smallest when the arms are by the sides and greatest when the arms are elevated away from the sides of the body.
• Middle deltoid — although it is not often described as a primary shoulder flexor in textbooks, the middle deltoid does have a small shoulder flexor moment arm throughout the joint range of motion, although its leverage is minimal when the arms are by the sides. Like the anterior deltoid, the leverage of the middle deltoid increases gradually as the arms are elevated away from the sides of the body.
• Posterior deltoid — the posterior deltoid has a meaningful shoulder extension moment arm throughout the joint range of motion. Its leverage decreases with increasing shoulder elevation angle, being greatest when the arms are by the sides.

Alongside the pectoralis major, the anterior and middle deltoids are primary shoulder flexors. The anterior deltoid has a much longer moment arm than the pectoralis major for shoulder flexion, but the moment arm length of the middle deltoid is probably shorter. Thus, the anterior deltoid is the main contributor to most shoulder flexion movements. In contrast to the shoulder flexion moment arms of the pectoralis major, the moment arms of the anterior and middle deltoids increase with increasing shoulder flexion angle. Therefore, shoulder flexion exercises involving peak forces at higher degrees of shoulder flexion (such as overhead presses) will preferentially target the deltoids, while those that involve peak forces in lower degrees of shoulder flexion will involve larger contributions from the pectoralis major.

Alongside the latissimus dorsi, the posterior deltoid is a primary shoulder extensor. All regions of the latissimus dorsi display shoulder extension moment arm lengths that peak between 30 and 60 degrees of shoulder elevation, and decline to minimal levels above 120 degrees. Similarly, the shoulder extension moment arm lengths of the posterior deltoid are also high at low levels of shoulder elevation, and decline with increasing shoulder elevation angle, being greatest at low degrees of shoulder elevation. Even so, the posterior deltoid retains a meaningful shoulder extension moment arm length at high degrees of shoulder elevation, whereas the majority of the latissimus dorsi muscle does not. Therefore, shoulder extension exercises involving peak forces at higher degrees of shoulder flexion (such as machine pullovers focusing on the top half of the exercise) may target the posterior deltoid more than the latissimus dorsi.

Practical implications

In practice, the anterior and middle deltoids work alongside the pectoralis major to carry out shoulder flexion, while the posterior deltoid works alongside the latissimus dorsi to carry out shoulder extension. The anterior and middle deltoids are likely more involved in shoulder flexion when the arms are elevated above the head (such as during overhead presses). Similarly, the posterior deltoid is also likely more involved in shoulder extension when the arms are elevated above the head (such as during machine pullovers focusing on the top half of the exercise).

#2. Shoulder adduction and abduction

When moving the shoulder in the frontal plane, many muscles are involved. The anterior and middle deltoids and two of the rotator cuff muscles (the supraspinatus and infraspinatus) are the primary shoulder abductors, while the pectoralis major, latissimus dorsi, teres major, and subscapularis are the primary shoulder adductors.

• Anterior deltoid — the anterior deltoid has a shoulder abduction moment arm throughout the joint range of motion. Its leverage is minimal when the arms are by the sides and increases gradually as the arms are elevated further away from the sides of the body, being greatest at higher degrees of shoulder elevation.
• Middle deltoid —the middle deltoid has a shoulder abduction moment arm throughout the joint range of motion. Even so, its leverage is lowest when the arms are by the sides and increases very quickly to a plateau as the arms are elevated away from the sides of the body. Below 90 degrees of shoulder elevation, the middle deltoid has a longer shoulder abduction moment arm length than the anterior deltoid. Above 90 degrees, their shoulder abduction moment arm lengths are actually quite similar.

Since there are no other muscles that contribute to shoulder abduction other than the rotator cuff muscles, the anterior and middle deltoids are likely trained very effectively by this movement. Shoulder abduction below shoulder height will emphasize the middle head more than the anterior head, while shoulder abduction above shoulder height will involve similar contributions from both anterior and middle heads.

Practical implications

In practice, shoulder abduction is primarily achieved by the middle and anterior heads of the deltoid. Below shoulder height, the middle deltoid is a much greater contributor. Above shoulder height, the middle and anterior deltoids contribute more equally. Therefore, to focus on the middle deltoid, use lateral raises only up to shoulder height or below. Overhead dumbbell presses with the arms out to the side will involve similar contributions from the middle and anterior deltoids.

#3. Shoulder horizontal flexion

When moving the shoulder at shoulder height in the transverse plane, many muscles are involved. The pectoralis major and anterior deltoid are the main horizontal flexors, and the subscapularis plays a smaller role. The posterior deltoid, infraspinatus, supraspinatus, and teres minor are the main horizontal extensors between the sagittal plane (hands together in front of the body) and the frontal plane (hands outstretched to the sides), although the teres major and latissimus dorsi take over beyond this point (behind the body).

Alongside the pectoralis major, the anterior deltoid is a primary shoulder horizontal flexor. The shoulder horizontal flexor moment arm lengths of the pectoralis major and of the anterior deltoid both decrease from the frontal plane (hands outstretched to the sides) to the sagittal plane (hands together in front of the body). However, the anterior deltoid has better leverage when approaching the sagittal plane, while the pectoralis major has better leverage before this point. Therefore, exercises that involve peak forces at longer muscle lengths (such as dumbbell chest flys) will likely preferentially target the pectoralis major, while exercises that involve peak forces at shorter muscle lengths (such as machine pec deck using accommodating resistance) will preferentially target the anterior deltoid.

The posterior deltoid is a primary shoulder horizontal extensor. In fact, the posterior deltoid is the most dominant shoulder horizontal extensor at all relevant degrees of shoulder horizontal flexion, such that almost any shoulder extension exercise (rear delt fly or machine reverse pec deck) will work fine.

Practical implications

In practice, when using shoulder horizontal flexion exercises to train the anterior deltoid, exercises that involve peak forces close to the point where the hands meet are best (such as pec deck with accommodating resistance). When using shoulder horizontal extension exercises to train the posterior deltoid, almost any shoulder extension exercises are appropriate.

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 each major region of the deltoid have been estimated by researchers. The anterior and middle deltoids both reach extensively onto the descending limb of the length-tension relationship, while the posterior deltoid does not. This means that anterior and middle deltoids will experience stretch-mediated hypertrophy during strength training with larger ranges of motion, while the posterior deltoid will not. Conversely, the middle and posterior deltoids both reach onto the ascending limb of the length-tension relationship, while the anterior deltoid does not. This means that the middle and posterior deltoids might suffer active insufficiency if trained at very short muscle lengths, while the anterior deltoid will not.

Practical implications

In practice, the anterior and middle deltoids (but not the posterior deltoid) can experience stretch-mediated hypertrophy as a result of strength training using long ranges of motion involving peak forces where the muscle is lengthened. The anterior deltoid is unlikely to experience active insufficiency when working at short muscle lengths, but the middle and posterior deltoids can.

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.

There do not seem to be any studies that have measured voluntary activation in the deltoids. Yet, voluntary activation is often quite closely related to the size of a muscle, with larger muscles being less easily activated than smaller muscles. We know that the triceps brachii reaches moderately high (94–96%) levels of voluntary activation, and this muscle is only slightly smaller than the deltoid. Therefore, it is quite likely that the deltoid can similarly be activated to a moderately high extent.

Although the deltoid is often considered to be a fast twitch muscle due to its key role in throwing and punching movements, research that has measured the fiber type of multiple muscles has found it to be relatively slow twitch. Indeed, other studies that have measured the fiber type of the deltoids either alone or in conjunction with a small number of other muscles have found that the muscle is either clearly slow twitch or a 50/50 mix of slow and fast twitch muscle fibers. Even so, when looking at each of the major regions separately, there seems to be a difference between them, with the posterior deltoid being the most slow twitch, then the anterior deltoid, and finally the middle deltoid.

As discussed above, the anterior and middle deltoids both reach extensively onto the descending limb of the length-tension relationship, while the posterior deltoid does not. In fact, the anterior and middle deltoids reach quite a long way onto the descending limb of the length-tension relationship, at least in comparison with many other muscles. This means that anterior and middle deltoids will experience more passive tension (and therefore muscle damage) during strength training than the posterior deltoid, as well as in comparison with many other muscles. It also seems like the middle deltoid is able to reach slightly further down the descending limb of the length-tension relationship than the anterior deltoid, suggesting that it may be the most prone to damage in this respect.

Practical implications

In practice, it is likely that we can achieve moderately-high levels of voluntary activation of the deltoid, and its muscle fibers generally reach quite a long way down the descending limb. These features make it susceptible to muscle damage Even so, it is slightly slow twitch compared to many other muscles, which is a protective feature. Therefore, it is likely that the deltoid will recover slower than some muscles and quicker than others.

The deltoids likely display meaningful differences in recovery capacity from one another. In terms of fiber type, the posterior deltoid is the most slow twitch, then the anterior deltoid, then the middle deltoid. In terms of working sarcomere lengths, the posterior deltoid does not reach far onto the descending limb, while the anterior and middle deltoids do, with the middle deltoid reaching furthest. This suggests that the middle deltoid will recover most slowly from a workout, and the posterior deltoid will recover most quickly.

What is the takeaway?

The deltoids are commonly grouped into anterior, middle, and posterior heads, although some research indicates that these heads may be further subdivided into smaller subregions. All three heads (and all but one of the subregions) are worked by shoulder abduction, but they have differing contributions to other shoulder movements. This means that the deltoids will likely benefit even more than other muscles from being trained with several different exercise variations, particularly those involving different shoulder angles, such as incline presses with varying bench heights.

The anterior and middle deltoids are more involved in shoulder flexion when the arms are above the head (as in overhead presses). The posterior deltoid is more involved in shoulder extension when the arms are elevated above the head (as in machine pullovers focusing on the top half of the exercise). The anterior deltoid is more involved in shoulder horizontal flexion when peak force is exerted when the hands are close together (as in the pec deck machine with added elastic resistance). The posterior deltoid is involved in shoulder horizontal extension during any shoulder extension exercise. Also, while the anterior and middle deltoids both contribute to shoulder abduction (and no other major muscles do), the middle head contributes proportionally more below shoulder height (such as during lateral raises).

Overall, the deltoids probably recover faster from a workout than some muscles, but slower than others. Importantly, the deltoids likely display meaningful differences in recovery capacity from one another. The middle deltoid will likely recover most slowly from a workout and must therefore be trained least often, while the posterior deltoid will recover most quickly, and can be trained much more often.