What is Tendinosis?
Journal of Sports Medicine: "Patellar tendinopathy causes substantial morbidity in both professional and recreational athletes. The condition is most common in athletes of jumping sports such as basketball and volleyball, but it also occurs in soccer, track, and tennis athletes. The disorder arises most often from collagen breakdown rather than inflammation, a tendinosis rather than a tendinitis. Physicians must address the degenerative pathology underlying patellar tendinopathy because regimens that seek to minimize (nonexistent) inflammation would appear illogical. Suggestions for applying the 'tendinosis paradigm' to patellar tendinopathy management include conservative measures such as load reduction, strengthening exercises, and massage. Surgery should be considered only after a long-term and appropriate conservative regimen has failed."
Tendons are rope-like structures that attach muscles to bones. Ligaments are similar structures that attach bones to other bones. When muscles and bones move, they exert stresses on the tendons and ligaments that are attached to them.
When your muscles move in new ways or do more work than they can handle, your muscles and tendons can sustain some damage on a cellular scale. If the increase in demand is made gradually, muscle and tendon tissues will usually heal, build in strength, and adapt to new loads. Athletes use these principles to build muscle and tendon strength with good training programs.
You can, however, do some activity that injures a tendon on a microscopic scale and then do more injury before the tendon heals. If you continue the injurious activity, you will gradually accumulate these microinjuries. When enough injury accumulates, you'll feel pain. This kind of injury that comes on slowly with time and persists is a chronic injury; acute tendon injuries are sudden tears that cause immediate pain and obvious symptoms. Tendon injuries often require patience and careful rehabilitation because tendons heal more slowly than muscles do.
Tendinosis is an accumulation over time of small-scale injuries that don't heal properly; it is a chronic injury of failed healing. Although you can't see the tendinosis injury on the outside of your body, researchers have seen what the injury looks like on the cellular scale by viewing slides of tendons under the microscope. (The microscopic changes in the tendon are described on the page The Tendinosis Injury , and the difference between tendinosis and tendinitis is described below in the section on terminology.) Tendinosis can occur in many different tendons, with some of the most common areas being the hand, wrist, forearm, elbow, shoulder, knee, and heel.
Who Is At Risk For Tendinosis?
The Bureau of Labor Statistics recorded 44,504 injuries from tendinosis and/or carpal tunnel syndrome in U.S. private industry in 1999. This number translates to about one out of every two thousand full-time private industry workers in that year. Each year, tens of thousands more U.S. workers develop these injuries. For details, see Scope of the Problem .
Tendinosis can result from long hours of activities such as playing sports, using computers, playing musical instruments, or doing manual labor. It can result from activities performed as part of your profession or recreation. Some occupations that have increased risk for chronic tendon injuries include assembly line workers, mail sorters, computer programmers, writers, court recorders, data entry processors, sign language interpreters, cashiers, professional athletes, and musicians.
Minimizing Your Risk
You can minimize your risk for tendinosis by using equipment that has good ergonomic design and is sized correctly for your body, by using good technique for your activity (whether it is sports or music or typing), by taking plenty of breaks, and by minimizing long overtime hours (easier said than done!). You can also listen to your body's pain signals. Warning signs of tendinosis include burning, stinging, aching, tenderness to the touch, and stiffness.
Don't ignore pain!
Usually tendinosis sneaks up on you. At first the pain only comes after a long or hard session of the activity that aggravates it. Later the pain comes at lower levels of the activity and it lasts longer. Finally, the pain becomes a part of your daily life and even normal activities can make it worse. Try to catch the injury as early as you can.
Terminology: RSI, CTD, and Tendinosis vs. Tendinitis
Let's get some terminology out of the way. Repetitive strain injury or RSI is a term commonly used to refer to many kinds of injuries that are caused by repetitive motion. RSI includes things like tendinosis, carpal tunnel syndrome, thoracic outlet syndrome, trigger finger, and de Quervain's syndrome. Another term that is commonly used interchangeably with RSI is cumulative trauma disorder or CTD. These injuries are also collectively referred to as "overuse injuries." The terms RSI and CTD usually refer to workplace injuries, whereas the term overuse injury is also commonly used for sports injuries.
The terms tendinitis, tendinosis, and tendinopathy all refer to tendon injuries. These terms are commonly confused and misused. The term paratenonitis refers to an injury of the outer layer of tendon; this newer term replaces the older terms peritendinitis, tenosynovitis, and tenovaginitis.
The suffix "itis" means inflammation. The term tendinitis should be reserved for tendon injuries that involve larger-scale acute injuries accompanied by inflammation. (Tendinitis is often misspelled as tendonitis, but the preferred spelling used in most of the medical literature is tendinitis.)
The suffix "osis" implies a pathology of chronic degeneration without inflammation. Doctors prefer the term tendinosis for the kind of chronic tendon injuries that most of us have. The main problem for someone with tendinosis is failed healing, not inflammation; tendinosis is an accumulation over time of microscopic injuries that don't heal properly. Although inflammation can be involved in the initial stages of the injury, it is the inability of the tendon to heal that perpetuates the pain and disability. Most of the pain associated with tendinosis probably comes not from inflammation but from other irritating biochemical substances associated with the injury (see The Pain of Tendinosis and Overuse Tendon Injuries: Where Does The Pain Come From? for more information). 
The suffix "opathy" implies no specific type of pathology, so the term tendinopathy is more general than either tendinitis (inflammation) or tendinosis (failed healing). The term tendinopathy just means tendon injury, without specifying the type of injury.
- paratenonitis (peritendinitis, tenosynovitis, tenovaginitis)
These terms all refer to injuries of the outer layers of tendons. Tendons are enclosed in a connective tissue covering called the epitenon, which contains the vascular, lymphatic, and nerve supply. The epitenon is surrounded by another connective tissue covering called the paratenon, which in some tendons is lined by synovial cells. The paratenon and epitenon together are called the peritendon. Paratenonitis is inflammation and degeneration of the outer layer of the tendon, the paratenon, regardless of whether the paratenon is lined by synovium. Paratenonitis is a general term that is now preferred to the older terms peritendinitis, tenosynovitis, and tenovaginitis. De Quervain's syndrome is one example of paratenonitis. Tendinosis and paratenonitis can occur separately or together (that is, you can have both degeneration of the tendon itself, tendinosis, and degeneration/inflammation of the tendon sheath, paratenonitis).