What is a Tendon?
Tendons are fibrous connective tissues that attach muscle to bone. They transmit the mechanical force of muscle contraction to the bones, which is how we are able to move our limbs.
Muscles and tendons work simultaneously, with the muscle controlling the tension of the tendon and the tendon controlling the expression of the muscle. 
Tendons are composed of filamentous collagen fibrils which merge together to form a single, dense organization of collagen fibers.
This highly structured matrix of fibers provides tendons with the stiffness and strength to withstand large tensile load with minimal deformation. The cellular structure of tendon tissues is subject to change in response to training or trauma, depending on hormones and age. 
What is Tendinosis and Tendonitis?
Tendons resist great tensile forces during repetitive activities and are therefore prone to overuse or overload injuries. Among the most common tendinopathies include tendinitis and tendinosis. These injuries are frequently confused with one another, but have a few key differences.
Tendinitis, also written as tendonitis, is the inflammation or irritation of a tendon, causing pain and tenderness near the affected joint. 
By contrast, tendinosis is a chronic degradation of a tendon and may be characterized by the accumulation of small tears in the tissue fibers. 
The most important distinction between tendonitis and tendinosis is that for tendinitis, inflammation is the source of pain, whereas for tendinosis, the pain results from chronic degenerative changes in the tendon.
Where do Tendon Injuries occur?
Tendon problems can occur anywhere in the body, but are most frequent in the shoulders, elbows, wrists, knees, and heels. In fact, some tendon injuries have become so common that they have acquired their own nicknames, like “tennis elbow”, “pitcher’s shoulder”, and “jumper’s knee”. 
Biceps tendinitis versus tendinosis
Biceps tendinitis is the inflammation of the upper biceps tendon, which connects the biceps muscle to the shoulder bones. The inflammation causes the tendon to flush red, swell, and sometimes thicken. 
Biceps tendinosis is the degeneration of the upper biceps tendon, which comes as a result of chronic overuse or repetitive overhead motion. Individuals who play sports that require frequent overhead arm movements, like swimming or tennis, will be at higher risk for biceps tendon injuries. 
Achilles tendonitis versus tendinosis
Achilles tendonitis and tendinosis account for the majority of injuries to the Achilles tendon –the cord connecting the calf muscles to the heel bone. Tendinitis is the inflammation or irritation of this tendon, and can either be within the tendon itself or near its insertion point into the heel bone.
Rotator cuff tendinitis versus tendinosis
The rotator cuff is made up of four muscles – supraspinatus, infraspinatus, teres minor, and subscapularis– and tendons that work together to stabilize the arm in the shoulder socket. With rotator cuff tendinitis, the tissues that connect these muscles to the bones become inflamed, which causes a dull ache or stiffness near the shoulder joint.
Rotator cuff tendinosis describes the overuse of these muscles and tendons, and is characterized by decreased tendon strength and tissue degeneration. 
Supraspinatus tendinitis versus tendinosis
The supraspinatus is one of the muscle-tendon components of the rotator cuff, which runs along the top of the shoulder joint and helps to raise the arm sideways. The supraspinatus is the most commonly injured rotator cuff muscle.
With tendinitis, the tendons connected to the supraspinatus muscle are inflamed, whereas with tendinosis, the tendons are damaged from degeneration and overuse.
Differences in Symptoms, Treatment, and Recovery 
Is tendinosis permanent?
No, tendinosis is not permanent. Because blood supply to tendons is generally more limited, tendinosis will require anywhere from 3 to 6 months to properly heal. A combination of rest and physical therapy exercises have been shown to accelerate this healing process and reduce the risk of further injury.
Is tendonitis or tendinosis worse?
A minority of patients will develop chronic tendinosis that can persist for a longer period of time. By contrast, recovery time for tendinitis is typically closer to six weeks and often improves with simple, at-home treatment. However, it may be misguided to assume that one condition is worse than the other, as both injuries can weaken the tendon, making it more prone to damage in the future.
What happens if tendinitis is left untreated?
If tendinitis goes untreated, you risk developing chronic tendinosis or causing permanent tendon damage. It is possible to have microscopic tears in the tendon tissue, which can worsen or cause complete rupture. Tendon ruptures are especially painful and often require surgery to repair. 
Does tendinitis ever fully heal?
With proper treatment and rehabilitation, patients with tendinitis will typically fully recover within 4 to 6 weeks. In cases of chronic tendinitis, patients may not be able to heal completely as the sheath of tissue surrounding the tendons could have scarred or narrowed. Consequently, range of motion at the joint may be severely affected and could require months of physical therapy to restore its full mobility and stability. 
How to tell if tendinitis is healing?
With tendinitis, all the symptoms come as a result of tendon inflammation. A good indication that your tendinitis is healing is if your symptoms begin to improve. A few examples could include reduced redness, swelling, and pain. If symptoms do persist or worsen, it is recommended you seek medical attention and explore other treatment options with your physician.
Does tendinitis show up on an MRI?
Magnetic resonance imaging (MRI) is a non-invasive medical technique that uses magnetic radio waves to create high-resolution images of your organs and tissues. An MRI scan may be useful in identifying tendinitis as it can reveal thickening, dislocations, and tears in the tendon. Additionally, MRIs will help assess the severity of tendinitis.
Can walking cause tendinitis?
Yes. Excessive walking, or any activity that places repeated strain on your Achilles tendon, may contribute to the development of tendinitis. Furthermore, walking in worn-out shoes with poor arch support or limited padding on the heel will increase the risk of inflammation in the tendon.
- Bordoni, Bruno, and Matthew Varacallo. “Anatomy, Tendons.” NCBI, U.S. National Library of Medicine, 7 Feb. 2021, www.ncbi.nlm.nih.gov/books/NBK513237/.
- Mayo Clinic Staff. “Tendinitis.” Mayo Clinic, Mayo Foundation for Medical Education and Research, 3 Nov. 2020, www.mayoclinic.org/diseases-conditions/tendinitis/symptoms-causes/syc-20378243.
- King, Dominic, and Jason Genin. “Tendinitis or Tendinosis? Why the Difference Is Important, What Treatments Help.” Cleveland Clinic, 7 Aug. 2020, health.clevelandclinic.org/tendinitis-tendinosis-difference-important-treatments-help/.
- Athwal, George S. “Biceps Tendinitis.” Edited by Stuart J Fischer and Michael J Wiater, OrthoInfo, American Academy of Orthopaedic Surgeons, Feb. 2016, orthoinfo.aaos.org/en/diseases–conditions/biceps-tendinitis/.
- Churgay, Catherine A. “Diagnosis and Treatment of Biceps Tendinitis and Tendinosis.” American Family Physician, 1 Sept. 2009, www.aafp.org/afp/2009/0901/p470.html.
- “The Difference Between Rotator Cuff Tears & Shoulder Tendonitis.” Ortho Bethesda, 17 Mar. 2021, www.orthobethesda.com/blog/the-difference-between-rotator-cuff-tears-and-shoulder-tendonitis/.
- Wheeler, Tyler. “Understanding Tendinitis — the Basics and Causes.” WebMD, 27 Jan. 2020, www.webmd.com/fitness-exercise/understanding-tendinitis-basics.
Elizabeth Miclau is an undergraduate at Harvard College, planning to pursue a concentration in life sciences or sociology. As a member of both Puerto Rico’s National Diving Team and Harvard’s Women’s Varsity Swimming and Diving Team, she has a strong background in elite athletics. In the past year, she has contributed to several journal publications and peer-review-funded research projects.