Shoulder Moment Arms

While moment arm lengths may appear at first a very dry subject, there is a great deal we can learn from studying them. Some muscles have longer moment arms in a given plane than others, which tells us that they are more effective for certain actions. What’s more, muscle moment arms are different at various points in the joint range of motion, which tells us which muscles are most likely to be the prime movers when the joints are flexed, extended or in mid-range.

This study looks at the various moment arm lengths of the shoulder muscles and shows which are the largest in the main shoulder movements of abduction, adduction, flexion and extension, as well as the points in the ranges of motion where they are maximal.

The study

Moment arms of the muscles crossing the anatomical shoulder, by Ackland, Pak, Richardson and Pandy, in Journal of Anatomy, 2008

The background

The effect of a muscular contraction is to produce a turning force, or “torque” about a joint. Torque is equal to the muscular force multiplied by the moment arm, where the moment arm is the perpendicular distance between the joint and the muscle’s line of force. The moment arm of a muscle is an indication of its leverage at the joint in question. 

When a muscle has a long moment arm, it is usually capable of producing a large torque about the joint. Large muscles like the glutes and hamstrings, which are involved in powerful actions such as jumping and sprinting, tend to have long moment arms to help them increase their ability to generate torque. 

However, moment arms change depending on the joint angle. So a muscle might have a large moment arm when the joint is flexed but a much smaller moment arm when the joint is extended or vice-versa. By comparing the moment arms of various synergists at a joint, we can assess which muscle might be most important at different joint ranges of motion.

More generally, if we look at the maximum moment arm length that a muscle has across the whole range of motion at a joint, we can broadly assess the relative importance of the synergists at a given joint. But we need to be careful and recall that torque is a product of moment arm length and perpendicular muscle force production. What that means is that a large, highly activated muscle with an advantageous line of pull can produce much more perpendicular force than a smaller, poorly activated muscle with a poor line of pull. Nevertheless, the moment arm explains half of the torque-producing value, so it is still of great importance. 

What did the researchers do?

The researchers wanted to measure the moment arms of 18 different muscles and muscular sub-divisions in and around the shoulder. They decided to use the tendon excursion method to calculate these while the shoulder was (1) abducted and adducted (in the frontal plane), and (2) flexed and extended (in the sagittal plane). They wanted to see which muscles had the largest moment arms and were therefore most useful for abduction, adduction, flexion and extension.

To measure the moment arms, the researchers obtained 8 fresh-frozen, entire arms from human cadavers (4 male and 4 female) from elderly, deceased subjects who had displayed no sign of any osteoarthritis or other degeneration of the muscles or skeleton. They attached the arms to a piece of apparatus for abducting, adducting, flexing or extending the shoulder while the muscles at the shoulder joint were pinned and the tendon excursions as a result of the muscle actions measured.

What happened?

The researchers found that the most effective shoulder muscles were as follows:

• Abductors – middle and anterior deltoids
• Adductors – teres major, middle and inferior latissimus dorsi, and middle and inferior pectoralis major
• Flexors – superior pectoralis major, anterior and posterior supraspinatus and anterior deltoid
• Extensors – teres major and posterior deltoid
  
  

The following graph shows the maximum moment arms in abduction (positive) and adduction (negative) for each of the shoulder muscles measured:

The following graph shows the maximum moment arms in flexion (positive) and extension (negative) for each of the shoulder muscles measured:

The researchers also measured at what range of motion the moment arms of the shoulder muscles were maximal in abduction and adduction. They found that for the main abductors, the anterior deltoid moment arm was maximal at full abduction (120 degrees) while the middle deltoid moment arm was maximal at 80 degrees, just below the horizontal. For the other significant abductors (i.e. posterior and anterior supraspinatus), it is interesting to note that they all had maximal moment arms at very low degrees of abduction (i.e. < 10 degrees).

The researchers found that for the main adductors, the teres major, middle and inferior latissimus dorsi, and middle and inferior pectoralis major, the moment arm lengths were all maximal between 40 – 80 degrees of abduction. The teres major was maximal at the greatest degree of abduction and the pectoralis muscles were maximal at the smallest degrees of abduction, with the latissmus muscles in between. The following graph shows the ranges of motion at which the muscles had their greatest moment arms.

The researchers also measured at what degree of flexion the moment arms of the shoulder muscles were maximal in flexion and extension. They found that for the main flexors, the superior pectoralis major, anterior and posterior supraspinatus and anterior deltoid, there were large differences in respect of where each of the muscles had their maximum moment arm lengths. The anterior and posterior supraspinatus were maximal at < 5 degrees of flexion. The superior pectoralis major was maximal at 70 degrees and the anterior deltoid was maximal at 120 degrees.

In respect of the main extensors, the teres major and posterior deltoid, the researchers found that the teres major had its maximal moment arm at 56 degrees of flexion and the posterior deltoid at 30 degrees. For the less effective extensors, the researchers noted that the superior latissimus had its maximum moment arm length at 65 degrees of flexion and the teres minor at < 5 degrees.

What else did the researchers observe?

The researchers also noted some features of various key muscles, as follows:

• Latissimus dorsi – the researchers observed that the moment arm lengths of the three regions of the latissimus dorsi followed a distinctly parabolic path and were minimal at small and large joint ranges of motion but maximal in mid-ranges. They also noted that the maximum moment arms during mid-abduction were greater in the middle and inferior sub-divisions of the latissimus than in the superior sub-division. They noted that the reverse was the case during mid-flexion.
• Supraspinatus – they noted that the supraspinatus was more effective as a shoulder flexor than as a shoulder abductor and that their peak moment arms for flexion were greater than that of the middle deltoid.
  
  

What did the researchers conclude?

The researchers concluded that the supraspinatus appears to be heavily involved in early phase shoulder abduction while the deltoid muscles seem to be more involved as prime movers in mid-to-late abduction. This supports the findings of previous research.

The researchers also concluded that the middle and anterior deltoids had the greatest moment arm lengths for abduction, while the superior pectoralis major, anterior and posterior supraspinatus and anterior deltoid had the most effective moment arms in flexion. However, they also concluded that the anterior and posterior supraspinatus muscles had greater moment arm lengths than the anterior deltoid in flexion.

The researchers explain that these findings have important implications for which movements and ranges of motion athletes are likely to find troublesome after full-thickness rotator cuff tears.

What were the limitations?

The researchers commented that their study was limited in that by using the tendon excursion technique it is difficult to control and eliminate humeral internal and external rotation and glenohumeral joint translation. This may have led to errors in some of the measurements.

What are the key points?

This study provides a great deal of food for thought, with the following three key points for swimmers coming out of it:

• The most effective shoulder abductors in terms of moment arm lengths are the middle and anterior deltoids, followed by the anterior and posterior supraspinatus. The deltoid muscles are the prime movers in mid-to-late abduction, while the supraspinatus is heavily involved in early phase shoulder abduction. This may imply that this rotator cuff muscle could easily be stressed by repetitive muscular actions involving low degrees of shoulder abduction, such as the initial part of the recovery phase in freestyle.
• The most effective shoulder extensors in terms of moment arm lengths are the teres major and posterior deltoid, followed by the superior latissimus and teres minor. These muscles are particularly important for the freestyle and fly strokes. While most people will recognize the latissimus as the “swimming muscle,” it is important to realize that there are other extensors with more effective moment arms. While these muscles are clearly trained using standard shoulder extension exercises such as pull-ups, chins and rows, they can also be targeted using more specific exercises in the case of the posterior deltoids, such as the rear delt fly.
• The shoulder extensors are all most effective in the middle of the movement. This may suggest that training the shoulder extensors at end ranges, perhaps using bands or chains, may be helpful for both smooth delivery of power and for injury prevention during swimming.
  
  

By Chris Beardsley a biomechanics researcher and author of a book about scientific posterior chain training. He also writes a monthly review of the latest fitness research for strength and sports coaches, personal trainers, and athletes.