Day 2 – Latissimus Dorsi, Teres Major
Latissimus Dorsi Muscle
The latissimus dorsi muscle is a broad, flat and superficial muscle like the trapezius. It covers most of the lower thoracic and lumbar regions of the back. The latissimus dorsi, or “lats” as they’re sometimes called, is the most superficial muscle of these regions. The only exception to this is the upper parts of the lats near the midline (superomedial part), which lie deep to the trapezius. Muscles that are deep to the latissimus dorsi include the serratus posterior inferior, and the lower sections of the spinalis, longissimus, iliocostalis and multifidus. The name latissimus dorsi in Latin means “broadest back muscle”. It is the largest muscle of the upper body.
The primary functions of the latissimus dorsi are extension, adduction and medial (internal) rotation of the humerus. It also performs horizontal abduction/extension of the humerus, and functions as an accessory muscle of respiration. The teres major muscle is a synergist of the latissimus dorsi because they share an insertion point on the humerus, and have similar fiber directions, or lines of pull. However, the teres major is a much smaller muscle. Both of these muscles, as well as the pectoralis major, perform extension and internal rotation of the humerus at the shoulder joint. These muscles are particularly important when performing functional activities such as climbing, swimming, and pushing up from the armrests of a chair to stand up. Dysfunction of the latissimus dorsi muscle can be associated with clinical conditions such as chronic back pain and chronic shoulder pain.
Latissimus Dorsi
Origin: Spinous processes of T7-L5 vertebrae, sacrum, thoracolumbar fascia, posterior iliac crest, and lower 3 or 4 ribs
Insertion: Medial lip of bicipital (intertubercular) groove of humerus
Action: Extension, adduction and medial rotation of humerus at glenohumeral joint
Innervation: Thoracodorsal nerve (C7,8)
Teres Major Muscle
As mentioned above, teres major a companion muscle to the latissimus dorsi. In fact, some fibers of both muscles converge into a single tendon that inserts on the medial lip of the bicipital groove of the humerus, which is located on the proximal and anterior aspect of the humerus. Teres major, along with the latissimus dorsi and subscapularis form the posterior border of the axillary region (also called axilla or armpit), which is an area of caution for massage. The teres major muscle is NOT one of the rotator cuff (RC) muscles. Teres major is larger and thicker than the teres minor, which is one of the RC muscles and performs external rotation of the humerus. The origin and belly of the teres major are easy to palpate.
The primary functions of teres major are extension and internal (medial) rotation of the humerus at the glenohumeral joint. This muscle is prone to muscle hypertension and developing trigger points. It is a common contributor to myofascial pain syndrome. And because it is a relatively short muscle, it is susceptible to strain injuries, especially if not properly stretched before physical activity. Trigger point therapy (neuromuscular therapy), myofascial release and assisted stretching are good techniques for clients with tight teres major muscles. Massage clients can also benefit from instruction of how to stretch the muscle on their own (self-care).
Teres Major
Origin: Inferior angle of the scapula (dorsal surface)
Insertion: Medial lip of bicipital (intertubercular) groove of humerus
Action: Extension, medial (internal) rotation, and adduction of humerus
Innervation: Lower subscapular nerve (C5,6)
Learning in Action
- Demonstrate the actions of each muscle.
- Locate the origin and insertion of the latissimus dorsi and teres major muscles on a study partner.
- Palpate each muscle from origin to insertion. As you palpate, have your study partner alternately perform a gentle contraction and relaxation of the muscle to help you differentiate this muscle from the surrounding tissues.
- Find the following bony landmarks and say the names of structures (out loud) as you palpate them. Also say how each feature relates to today’s two muscles. For example, “This is the inferior angle of the scapula and is the insertion point…”.
- Inferior angle and axillary (lateral) border of the scapula
- Spinous process of T7-L5 vertebrae (*the spinous process of T7 is about even with the inferior angle of the scapula)
- Lower ribs (posterior)
- Sacroiliac joint (SIJ)
- Iliac crest
- Bicipital groove of the humerus
- Draw the muscles.
- Study the latissimus dorsi muscle image for ~1 minute.
- Sketch the latissimus dorsi and surrounding structures on a sheet of paper using the image as a reference. Make this sketch as accurate as possible but complete this in 1 minute or less. Compare your sketch to your reference image.
- Now turn your paper over and sketch the muscle without looking at any other images. Include as much detail as you can remember, but complete this in 1 minute or less. Compare your sketch to your reference image. Note any errors in your mental image of the muscle, especially where the muscle attaches to bone, and the size and position relative to other structures around it.
- Go back and label the origin, insertion, and any key features of this muscle or surrounding structures. Add more detail if you need to.
- Repeat this process for the teres major muscle.
- Create your muscle flashcard for the latissimus dorsi and teres major muscles.
Spaced Repetition
Spaced repetition is an effective study technique that is based on the spacing effect of learning. Learn Muscles in 60 Days is based on the principle of spaced repetition (as well as other principles of learning). Spaced repetition goes by other names including distributed practice, spaced rehearsal, and spaced retrieval. The spacing effect is a principle that states ‘learning is more effective when the study sessions are spaced apart rather than crammed into one session’. This learning technique is backed by evidence and has been researched since the late 1800’s. It has been proven to increase the rate of learning and increase information retention or recall. The spacing intervals get progressively longer, which decreases the probability of forgetting the information. This is helpful for learning things that you need to remember indefinitely, such as learning another language, or a massage therapist learning muscles. This strategy is much better than trying to quickly memorize something for an exam and forgetting it later.
The spaced repetition technique can be applied by using different types of study materials such as flashcards or study notes. Retaking practice quizzes is also a great tool to apply this technique, plus it provides provides feedback (score) and incorporates principles of active learning.
Spaced Review
After taking today’s quiz, go back and review the muscles covered on Day 1 and retake that quiz. If you are having a hard time remembering those muscles, then repeat the Learning in Action steps too.