The trapezius is a large, triangular shaped muscle that covers the back of the neck, shoulders and thoracic spine. The muscle is usually described as having three sections: the upper, middle, and lower portions. Physical therapists often talk about an imbalance in the strength and activity of the different sections of the trapezius contributing to shoulder problems. Consequently specific exercises are used to isolate the different portions of the muscle.
One question I think deserves asking: are there really three different parts of the trapezius?
The trapezius is a large muscle and is undoubtedly involved in many movements. The muscle spans from the base of the skull all the way down the back to the 12th thoracic vertebrae. The muscle inserts on the shoulder girdle at the distal portion of the clavicle, the acromion, and the spine of the scapula.
The imbalance most commonly noted as causing shoulder problems is an increased ratio of upper trapezius activity in relation to lower trapezius activity. Claiming an imbalance implies that the upper and lower portions of the trapezius are capable of contracting with some degree of independence.
In some animals there are three distinct trapezius muscles, but in humans they form one continuous muscle. Since all of the fibers of the trapezius are thought to constitute a single muscle, it’s a valid question to ask if this independence is possible. Can one portion of the trapezius be stronger, more active, and be contracted preferentially from the other portions?
The Trapezius As One Muscle
Anatomy textbooks typically denote the separate roles of the different sections of the muscle as:
- the upper trapezius elevates and laterally rotates the scapula
- the middle trapezius retracts the scapular
- the lower trapezius depresses and medially rotates the scapular
These actions make it seem as if the trapezius is made up of three distinct muscles. There are even some highly questionable manual muscle tests designed to measured the relative strength of each of the sections.
The function of different sections of the trapezius has recently been called into question, particularly in regard to the upper trap elevating the scapula as detailed in this excellent article by Adam Meakins.
Contrary to previous thinking, it’s becoming better established that the upper trapezius is not the main elevator of the shoulder. The fibers of the upper trap are small and badly positioned for this role. As Adam also points out, the trapezius does not work in isolation. Rather, it works in synergy with less superficial muscles.
Because of its multiple insertions and fiber orientations, the trapezius will certainly influence movement in different ways depending on the position of the scapula relative to the spine. There may also be separate motor nerve branches supplying each section. The main nerve supply comes from the spinal accessory nerve. There is some question whether other nerves from the neck also supply the muscle.
I’m curious why anatomists chose to divide the muscle into these sections. It’s hard to believe that different sections of a continuous muscle can be activated separately. I have not seen evidence that the upper, middle, and lower traps function independently.
Several studies have looked at the supposed imbalance in the trapezius, including this one which appeared to find a higher ratio of upper trapezius to lower trapezius activity in subjects with shoulder impingement. These studies are based on the assumption that there are indeed three separate functioning parts of the trapezius.
Most studies on trapezius activity also rely on surface EMG measurements. Eletromyography (EMG) involves places electrodes on the skin overlying the muscle and recording electrical signals. EMG is useful, but it has limitations.
- One problem with EMG is crosstalk, or picking up signals not just from the muscle being studied, but also the activity of muscles nearby. For example, attempting to measure upper trapezius activity during a shoulder movement might also measure levator scapulae activity.
- A second problem is that EMG activity can be influenced by the velocity of muscle contraction.
Other factors that can influence EMG readings include:
- subcutaneous tissue
- changes in electrode spacing with movement
- muscle movement relative to the electrodes
Because of these limitation, EMG studies need to be interpreted with caution. What appears to be an imbalance in electrical activity in the trapezius may not actually mean a difference in activation of fibers at either end of the muscle.
Can Exercise Target One Portion of a Muscle?
Assuming the trapezius is in fact one muscle, then fundamental to answering the original question is determining whether parts of a muscle can be preferentially activated. Does one portion of a muscle get activated more than another part for a particular movement?
Different exercises vary the amount of tension and stress on different parts of a muscle. A single muscle can have multiple never suppliers as well. Whether this increases the recruitment of motor units in the part of the muscle best positioned to assist with a certain movement or exercise is not clear. The neuromuscular system is complex, so it’s possible. This is not something that should be assumed however.
This issue is similar to the debate over whether it’s possible to isolate the vastus medialis obliquus (VMO) portion of the quadriceps for knee rehabilitation. The quadriceps muscle group is actually four separate muscles that form a common tendon and work together to extend the knee. It was once thought that a weaker VMO in relation to the vastus lateralis (VL) part of the quadriceps was the cause of patellar tracking issues. A systematic review on the subject that included 20 different studies concluded that the VMO cannot be preferentially activated and strengthened. In other words, the quadriceps can only be strengthened as a whole. It is not possible to target a specific part.
One of the most debated cases of this is with the rectus abdominis. Upper and lower abs. There is a lot of conflicting evidence on whether exercise can target the upper or lower part of the rectus abdominis, and if differences do exist they may be small.
Bodybuilders frequently talk about doing exercises to target the upper chest. Anatomically, this refers to the upper portion of the pectoralis major muscle that attaches to the clavicle. The pectoralis major has innervation by both the medial and lateral pectoral nerves. Contrary to popular belief though, incline bench pressing does not appear to increase activation in the upper portion of the pectoralis major.
The deltoid muscle of the shoulder may be the most relevant to this discussion as it is also commonly said to function in parts, with each part performing a different movement. Anatomically, the deltoid is said to have three heads: anterior, medial, and posterior. The anterior and middle deltoid are innervated by the anterior trunk of the axillary nerve. The posterior deltoid is innervated by the posterior trunk of the axillary nerve. Studies show that there may be different levels of activation in the different regions of the deltoid depending on the shoulder movement being performed. Again, the conclusions that can be drawn from all these studies are subject to the limitations of EMG measurement.
Most importantly, none of the studies show that you can activate one part of a muscle while the rest stays dormant. This is true for the quads, abdominals, pecs, and delts. Regardless of the movement, when a muscle gets used the whole muscle is activated. This holds true even for muscles with separate heads and multiple nerve suppliers. It’s not possible to switch on one portion of a muscle, while switching off another, as is often attempted with lower trapezius strengthening exercise.
It’s unlikely that the lower trap can be activated without activating the upper and middle portions of the muscle as well. When we talk about targeting the lower trapezius, we really mean improving the strength and synergy of the group of muscles that control motion of the shoulder blade.
The trapezius acts to stabilize the shoulder and transfer mechanical tension. It holds the shoulder girdle up like a suspension bridge and controls the position of the shoulder blade when the arm is moving. This stabilization occurs through contraction of the entire muscle, not just a portion. Problems like shoulder impingement should probably not be attributed to a trapezius imbalance or “weak lower traps”. Likewise, an “overactive upper trap” is likely not the cause of neck tension or an elevated shoulder girdle. We can still work on improving scapular depression and rotation, but the explanation for how we do this needs to change.