The teres major (TM) is a small muscle that runs along the lateral border of the scapula. It is one of seven scapular muscles that move around the glenohumeral joint to facilitate shoulder movement .  It is sometimes called Rat’s little helper because of its association with Widest back. 
Image 1: Red shows TM
Origin: Posterior surface of the mandible
Insertion: The medial lip of the intertubercular groove of the humerus.
- It is associated with the latissimus dorsi, which surrounds the lower border of the teres major. The teres major tendon lies behind the latissimus dorsi tendon with a bursa in between.
Motor nerves: lower limb (C5 C6) .
Arterial supply: thoracodorsal branch of the subscapular artery posterior to the circumflex humeral artery
Image 2: Teres major muscle (highlighted in green) – posterior view 
This 2 minute video summarizes the teres major well. 
The TM muscles function together with the latissimus dorsi (LD), working synergistically to extend and internally rotate the shoulder. For example, the TM and LD muscles function during the acceleration and following phases of a baseball pitcher’s throwing motion. ; assist The latissimus dorsi are used, for example, in climbing, tennis, splitting, swimming, and rowing. 
Figure 3: The teres major and latissimus dorsi function during the acceleration and throwing phases of baseball pitching.
Etiology – Injuries
Stretch or impingement injuries to the teres major muscle have been associated with the evolution of teres major injuries in sports or motor vehicle accidents and in falls onto the lateral scapula.
In addition, repeated microtrauma secondary to upper and lower arm movements (e.g., bag retrieval from the back seat of a car being dropped and other sports injuries) can produce myofascial pain in the teres major in the nervous system.
Teres Major injuries cause pain and discomfort during activities that require arm movement to the side or back.
- Isolated tears of the teres major are extremely rare but can occur in baseball or cricket players especially batsmen and fielders.
- The main symptom of a teres major tear is sudden, sharp pain in the upper shoulder and armpit. This usually arises if the muscles are not rested and no treatment is done.
- The pain is usually dull and intensifies with physical activity and movement of the back of the hand, such as reaching into the back pocket while playing tennis and hitting a baseball freestyle.
- Swelling may occur due to inflammation in the affected muscle area. The swelling is not visible externally but can be felt when the area is palpated (near the base of the shoulder blade). 
- Discomfort and pain in the upper arm can reduce the patient’s range of motion in the arm.
- The jumping symptom – the involuntary withdrawal of the moving muscles – is also a common symptom.
- Teres Major Strain usually occurs when adequate warm-up is not done prior to exercise.
- Teres major is overactive and short in people with upper crossed syndrome (UCS) see link.
Teres major is one of the four rotator cuff tendons involved in many shoulder disorders including quadrangular space syndrome.
Figures 4 & 5: Sports associated with TM injuries.
Figure 6: Reaching into the posterior pocket causes pain in a TM injury
Figure 7: Illustration of the four-dimensional space
- With the patient lying on the floor and arms off the side of the table hold the latissimus dorsi between your wrist and thumb
- Move your fingers and thumb medially to the posterior boarder of the scapula
- The large muscles of Teres lie medial to the latissimus and attach to the posterior border of the scapula
- Follow this line upwards to the axilla where it fuses with the latissimus dorsi.
Figure 8: Posterior part of the shoulder – more than 5 thirds; 3 posterior widest; 6 small slices; 7 on the spine; 8 infraspinatus; 13 triceps arm length head
Length Tension Testing / Stretching
- While the patient is in supine flex the patient’s shoulder up to 180 deg by one arm by holding the other arm
- Keep the scapula against the posterior boarder with your other hand
- Rotate with the hand holding the forearm
- Both the moving hand and the hand stabilizing the scapula are used to sense muscle tone and barrier size 
Most cases of teres major injuries heal successfully without surgery. treatment includes
- POLICE principle
- Warm Up: Major injuries from Teres workouts can often be healed with a proper warm up before the workout. Warming up helps relax muscles and tissues so they are less tight. The flexibility of muscles helps them to stretch and manipulate more easily.
- Strengthening exercises: Perform shoulder stabilization compound and isolated teres major exercises, such as the seated row with resistance bands Lat pulldown Chin up Start at an appropriate level without causing pain. Avoid vertical press exercises, such as overhead presses, if you have limited overhead range of motion. Do Switch to high-incline press exercises (e.g., shoulder presses on a 60-75° incline)
- Stretching exercises such as lateral stretches; kneeling stretches (arms on the floor or arms raised); overhead lats stretches.
- Posture: Avoid hunched over and internally rotated shoulders. When sitting for long periods of time, get up and take quick breaks at least every half hour to spread out the time spent in the position. Walk around or stretch your teres major.
- Trigger point release: TrPs in the teres major typically involve pain in the posterior deltoid region and long head of the triceps, and posteriorly of the shoulder joint occasionally involving the dorsal side of the forearm and rarely involving the scapula or elbow. 
- Myofascial Release Techniques
- Radiopedia Teres Major Available: https://radiopaedia.org/articles/teres-major-muscle(accessed 13.1.2022)
- Biel A (2005). Trail Guide to the Body (2nd ed). Boulder, CO: Books of Discovery.
- Teres major muscle (highlighted in green) – posterior view image – © Kenhub https://www.kenhub.com/en/library/anatomy/teres-major-muscle
- Teres major muscle video – © Kenhub https://www.kenhub.com/en/library/anatomy/teres-major-muscle
- Fitzpatrick D, Cagle P, Flatow E. Isolated Teres Major Rupture: A case report with a suggested dedicated imaging protocol and review of the literature. Journal of radiology case reports. 2016 Apr;10(4):31. Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4861626/(accessed 13.1.2022)
- Travell JG, Simons DG, Simons LS (1998). Travell and Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 2: The Upper Half of Body (2nd ed). Baltimore, MD: Williams & Wilkins.
- Ehealth star Teres Major Muscle Available:https://www.ehealthstar.com/anatomy/teres-major-muscle (accessed 13.1.2022)
- Rehab my patient Teres Major Available:https://www.rehabmypatient.com/shoulder/teres-major (accessed
- Prime health channel Teres Major Available:https://www.primehealthchannel.com/teres-major.html (accessed 13.1.2022)
- Dalagiannis N, Tranovich M, Ebraheim N. Teres minor and quadrilateral space syndrome: a review. Journal of orthopaedics. 2020 Jul 1;20:144-6.Available: https://pubmed.ncbi.nlm.nih.gov/32025138/(accessed 13.1.2022)
- Sanzo P, MacHutchon M (2015). Length Tension Testing Book 2, Upper Quadrant: A Workbook of Manual Therapy Techniques (2nd ed). Canada: Brush Education.