Specific populations at high risk of primary PFPS include runners, bicyclists, basketball players, young athletes and females.

In tennis players, about 39.7% have reported current or previous problems with their elbow. Less than one quarter (24%) of these athletes under the age of 50 reported that the tennis elbow symptoms were "severe" and "disabling," while 42% were over the age of 50. More women (36%) than men (24%) considered their symptoms severe and disabling. Tennis elbow is more prevalent in individuals over 40, where there is about a four-fold increase among men and two-fold increase among women. Tennis elbow equally affects both sexes and, although men have a marginally higher overall prevalence rate as compared to women, this is not consistent within each age group, nor is it a statistically significant difference.

Playing time is a significant factor in tennis elbow occurrence, with increased incidence with increased playing time being greatery for respondents under 40. Individuals over 40 who played over two hours doubled their chance of injury. Those under 40 increased it 3.5 fold compared to those who played less than two hours per day.

The prevalence of Achilles tendinitis fluctuates greatly among different ages and groups of people. However, Achilles tendinitis is most commonly found in individuals aged 30–40. A study was conducted in 1981 on recreational and competitive runners and what type of injury they were most likely to suffer from. Out of the 232 runners, 25 (11%) complained that their Achilles bothered them.

A study conducted in the city of Oulu found that a peak incidence of 18 injured Achilles occurred in 1994 and was highest in the male group aged 30–39. The study also found that 90% of the injuries occurred while playing a sport.

Risk factors include participating in a sport or activity that involves running, jumping, bounding, and change of speed. Although Achilles tendinitis is mainly diagnosed in runners, it does occur in basketball, volleyball, dancing, gymnastics and other athletic activities. Other risk factors include gender, age, improper stretching, overuse, and conditions which the individual may be born with. Congenital conditions occur when an individual’s legs rotate abnormally, which in turn causes the lower extremities to overstretch and contract; this puts stress on the Achilles tendon and will eventually cause Achilles tendinitis.

Another factor of tennis elbow injury is experience and ability. The proportion of players who reported a history of tennis elbow had an increased number of playing years. As for ability, poor technique increases the chance for injury much like any sport. Therefore, an individual must learn proper technique for all aspects of their sport. The competitive level of the athlete also affects the incidence of tennis elbow. Class A and B players had a significantly higher rate of tennis elbow occurrence compared to class C and novice players. However, an opposite, but not statistically significant, trend is observed for the recurrence of previous cases, with an increasingly higher rate as ability level decreases.

Other ways to prevent tennis elbow:

- Decrease the amount of playing time if already injured or feeling pain in outside part of the elbow.

- Stay in overall good physical shape.

- Strengthen the muscles of the forearm: (pronator quadratus, pronator teres, and supinator muscle)—the upper arm: (biceps, triceps)—and the shoulder (deltoid muscle) and upper back (trapezius). Increased muscular strength increases stability of joints such as the elbow.

- Like other sports, use equipment appropriate to your ability, body size, and muscular strength.

- Avoid any repetitive lifting or pulling of heavy objects (especially over your head)

Vibration dampeners (otherwise known as "gummies") are not believed to be a reliable preventative measure. Rather, proper weight distribution in the racket is thought to be a more viable option in negating shock.

Tendon injury and resulting tendinopathy are responsible for up to 30% of consultations to sports doctors and other musculoskeletal health providers. Tendinopathy is most often seen in tendons of athletes either before or after an injury but is becoming more common in non-athletes and sedentary populations. For example, the majority of patients with Achilles tendinopathy in a general population-based study did not associate their condition with a sporting activity. In another study the population incidence of Achilles tendinopathy increased sixfold from 1979-1986 to 1987-1994. The incidence of rotator cuff tendinopathy ranges from 0.3% to 5.5% and annual prevalence from 0.5% to 7.4%.

The cause of snapping hip syndrome is not well understood, and confusion exists within the medical community regarding causation. Athletes appear to be at an enhanced risk for snapping hip syndrome due to repetitive and physically demanding movements.

In athletes such as ballet dancers, gymnasts, horse riders, track and field athletes and soccer players, military training, or any vigorous exerciser, repeated hip flexion leads to injury. In excessive weightlifting or running, the cause is usually attributed to extreme thickening of the tendons in the hip region. Snapping hip syndrome most often occurs in people who are 15 to 40 years old.

A rotator cuff tear can be caused by the weakening of the rotator cuff tendons. This weakening can be caused by age or how often the rotator cuff is used. Adults over the age of 60 are more susceptible to a rotator cuff tear. According to a study in the Journal of Orthopaedic Surgery and Traumatology the frequency of rotator cuff tears can increase with age. The study shows the participants that were the ages of 70–90 years old had a rate of rotator cuff tears that were 1 to 5. The participants who were 90+ years old the frequency of a rotator cuff tear jumped to 1 to 3. This study shows that with an increase in age there is also an increase in the probability of a rotator cuff tear.

Extra-articular snapping hip syndrome is commonly associated with leg length difference (usually the long side is symptomatic), tightness in the iliotibial band (ITB) on the involved side, weakness in hip abductors and external rotators, poor lumbopelvic stability and abnormal foot mechanics (overpronation). Popping occurs when the thickened posterior aspect of the ITB or the anterior gluteus maximus rubs over the greater trochanter as the hip is extended.

Development of tendinitis depends on the type, frequency and severity of exercise or use; for example, rock climbers tend to develop tendinitis in their fingers, swimmers in their shoulders. Achilles tendinitis is a common injury, particularly in sports that involve lunging and jumping. It is also a known side effect of fluoroquinolone antibiotics such as ciprofloxacin, as are other types of tendinitis.

Swelling in a region of micro-damage or partial tear can be detected visually or by touch. Increased water content and disorganized collagen matrix in tendon lesions may be detected by ultrasonography or magnetic resonance imaging.

Achilles tendinitis is thought to have physiological, mechanical, or extrinsic (i.e. footwear or training) causes. Physiologically, the Achilles tendon is subject to poor blood supply through the synovial sheaths that surround it. This lack of blood supply can lead to the degradation of collagen fibers and inflammation. Tightness in the calf muscles has also been known to be involved in the onset of Achilles tendinitis.

During the loading phase of the running and walking cycle, the ankle and foot naturally pronate and supinate by approximately 5 degrees. Excessive pronation of the foot (over 5 degrees) in the subtalar joint is a type of mechanical mechanism that can lead to tendinitis.

An overuse injury refers to repeated stress and strain, which is likely the case in endurance runners. Overuse can simply mean an increase in running, jumping or plyometric exercise intensity too soon. Another consideration would be the use of improper or worn-down footwear, which lack the necessary support to maintain the foot in the natural/normal pronation.

While people with rotator cuff tears may not have any noticeable symptoms, studies have shown that over time 40% will have enlargement of the tear over a five-year period. Of those whose tears enlarge, 20% have no symptoms while 80% eventually develop symptoms.

There is no irrefutable evidence that rotator cuff surgery benefits patients more than non-surgical management and a percentage of patients never regain full range of motion after surgery.

Epidemiological studies strongly support a relationship between age and cuff tear prevalence. In a recent study the frequency of such tears increased from 13% in the youngest group (aged 50–59 y) to 20% (aged 60–69 y), 31% (aged 70–79 y), and 51% in the oldest group (aged 80–89 y). This high rate of tear prevalence in asymptomatic individuals suggests that rotator cuff tears could be considered a "normal" process of aging rather than a result of an apparent pathological process.

In most patients with PFPS an examination of their history will highlight a precipitating event that caused the injury. Changes in activity patterns such as excessive increases in running mileage, repetitions such as running up steps and the addition of strength exercises that affect the patellofemoral joint are commonly associated with symptom onset. Excessively worn or poorly fitted footwear may be a contributing factor. To prevent recurrence the causal behaviour should be identified and managed correctly.

The medical cause of PFPS is thought to be increased pressure on the patellofemoral joint. There are several theorized mechanisms relating to how this increased pressure occurs:

- Increased levels of physical activity

- Malalignment of the patella as it moves through the femoral groove

- Quadriceps muscle imbalance

- Tight anatomical structures, e.g. retinaculum or iliotibial band.

The cause of pain and dysfunction often results from either abnormal forces (e.g. increased pull of the lateral quadriceps retinaculum with acute or chronic lateral PF subluxation/dislocation) or prolonged repetitive compressive or shearing forces (running or jumping) on the PF joint. The result is synovial irritation and inflammation and subchondral bony changes in the distal femur or patella known as "bone bruises". Secondary causes of PF Syndrome are fractures, internal knee derangement, osteoarthritis of the knee and bony tumors in or around the knee.

It is an overuse injury from repetitive overloading of the extensor mechanism of the knee. The microtears exceed the body's healing mechanism unless the activity is stopped.

Among the risk factors for patellar tendonitis are low ankle dorsiflexion, weak gluteal muscles, and muscle tightness, particularly in the calves, quadriceps muscle, and hamstrings.

The injury occurs to athletes in many sports.

Workers in certain fields are at risk of repetitive strains. Most occupational injuries are musculoskeletal disorders, and many of these are caused by cumulative trauma rather than a single event. Miners and poultry workers, for example, must make repeated motions which can cause tendon, muscular, and skeletal injuries.

The literature on the pathophysiology of bursitis describes inflammation as the primary cause of symptoms. Inflammatory bursitis is usually the result of repetitive injury to the bursa.

In the subacromial bursa, this generally occurs due to microtrauma to adjacent structures, particularly the supraspinatus tendon. The inflammatory process causes synovial cells to multiply, increasing collagen formation and fluid production within the bursa and reduction in the outside layer of lubrication (Ishii et al., 1997).

Less frequently observed causes of subacromial bursitis include hemorrhagic conditions, crystal deposition and infection.

Many causes have been proposed in the medical literature for subacromial impingement syndrome. The bursa facilitates the motion of the rotator cuff beneath the arch, any disturbance of the relationship of the subacromial structures can lead to impingement. These factors can be broadly classified as intrinsic such as tendon degeneration, rotator cuff muscle weakness and overuse. Extrinsic factors include bone spurs from the acromion or AC joint, shoulder instability and neurologic problems arising outside of the shoulder.

Tendinitis injuries are common in the upper shoulder and lower section of the elbow (including the rotator cuff attachments), and are less common in the hips and torso. Individual variation in frequency and severity of tendinitis will vary depending on the type, frequency and severity of exercise or use; for example, rock climbers tend to develop tendinitis in their fingers or elbows, swimmers in their shoulders. Achilles tendinitis is a common injury, particularly in sports that involve lunging and jumping, while Patellar tendinitis is a common among basketball and volleyball players owing to the amount of jumping and landing.

A veterinary equivalent to Achilles tendinitis is bowed tendon, tendinitis of the superficial digital tendon of the horse.

In horses tendinitis is called a bowed tendon from the appearance of the affected tendon after it heals without treatment. Mesenchymal stem cells, derived from a horse's bone marrow or fat, are currently being used for tendon repair in horses.

A study containing 100 consecutive patients with a recent anterior cruciate ligament injury were examined with respect to type of sports activity that caused the injury. Of the 100 consecutive ACL injuries, there were also 53 medial collateral ligament injuries, 12 medial, 35 lateral and 11 bicompartmental meniscal lesions. 59/100 patients were injured during contact sports, 30/100 in downhill skiing and 11/100 in other recreational activities, traffic accidents or at work.

An associated medial collateral ligament tear was more common in skiing (22/30) than during contact sports (23/59), whereas a bicompartmental meniscal lesion was found more frequently in contact sports (9/59) than in skiing (0/30). Weightbearing was reported by 56/59 of the patients with contact sports injuries whereas 8/30 of those with skiing injuries. Non-weightbearing in the injury situation led to the same rate of MCL tears (18/28) as weightbearing (35/72) but significantly more intact menisci (19/28 vs 23/72). Thus, contact sports injuries were more often sustained during weightbearing, with a resultant joint compression of both femuro-tibial compartments as shown by the higher incidence of bicompartmental meniscal lesions. The classic "unhappy triad" was a rare finding (8/100) and Fridén T, Erlandsson T, Zätterström R, Lindstrand A, and Moritz U. suggest that this entity should be replaced by the "unhappy compression injury".

Though TTS is rare, its cause can be determined in 70% of reported cases. In the workplace TTS is considered a musculoskeletal disorder and accounts for 1.8 million cases a year, which accumulates to about $15–$20 billion a year New studies indicate an occurrence of TTS in sports placing high loads on the ankle joint (3). This can be seen in figure 1. TTS occurs more dominantly in active adults, with a higher pervasiveness among women. Active adults that experience more jumping and landing on the ankle joint are more susceptible (see figure 2). Though athletics and sport are correlations, cases are individualistically assessed because of the oddity.

In 1997 Morrison et al.

published a study that reviewed the cases of 616 patients (636 shoulders) with impingement syndrome (painful arc of motion) to assess the outcome of non-surgical care. An attempt was made to exclude patients who were suspected of having additional shoulder conditions such as, full-thickness tears of the rotator cuff, degenerative arthritis of the acromioclavicular joint, instability of the glenohumeral joint, or adhesive capsulitis. All patients were managed with anti-inflammatory medication and a specific, supervised physical-therapy regimen. The patients were followed up from six months to over six years. They found that 67% (413 patients) of the patients improved, while 28% did not improve and went to surgical treatment. 5% did not improve and declined further treatment.

Of the 413 patients who improved, 74 had a recurrence of symptoms during the observation period and their symptoms responded to rest or after resumption of the exercise program.

The Morrison study shows that the outcome of impingement symptoms varies with patient characteristics. Younger patients ( 20 years or less) and patients between 41 and 60 years of age, fared better than those who were in the 21 to 40 years age group. This may be related to the peak incidence of work, job requirements, sports and hobby related activities, that may place greater demands on the shoulder. However, patients who were older than sixty years of age had the "poorest results". It is known that the rotator cuff and adjacent structures undergo degenerative changes with ageing.

The authors were unable to posit an explanation for the observation of the bimodal distribution of satisfactory results with regard to age. They concluded that it was "unclear why (those) who were twenty-one to forty years old had less satisfactory results". The poorer outcome for patients over 60 years old was thought to be potentially related to "undiagnosed full-thickness tears of the rotator cuff".

Patellar tendinitis (patellar tendinopathy, also known as jumper's knee), is a relatively common cause of pain in the inferior patellar region in athletes. It is common with frequent jumping and studies have shown it may be associated with stiff ankle movement and ankle sprains.

The exact etiology of tendinopathy has not been fully elucidated and different stresses may induce varying responses in different tendons. There are multifactorial theories that could include: tensile overload, tenocyte related collagen synthesis disruption, load-induced ischemia, neural sprouting, thermal damage, and adaptive compressive responses. The intratendinous sliding motion of fascicles and shear force at interfaces of fascicles could be an important mechanical factor for the development of tendinopathy and predispose tendons to rupture. Obesity, or more specifically, adiposity or fatness, has also been linked to an increasing incidence of tendinopathy.

The most commonly accepted cause for this condition however is seen to be an overuse syndrome in combination with intrinsic and extrinsic factors leading to what may be seen as a progressive interference or the failing of the innate healing response. Tendinopathy involves cellular apoptosis, matrix disorganization and neovascularization.

Tendinopathy can be induced in animal models by a surgical injury to the tendon. In both sheep shoulder (infraspinatus) and horse forelimb (superficial digitor flexor) tendons, a mid-tendon transection caused pathology in the entire tendon after four and six weeks respectively.

Quinolone antibiotics are associated with increased risk of tendinitis and tendon rupture. A 2013 review found the incidence of tendon injury among those taking fluoroquinolones to be between 0.08 and 0.2%. Fluoroquinolones most frequently affect large load-bearing tendons in the lower limb, especially the Achilles tendon which ruptures in approximately 30 to 40% of cases.

Enthesopathies may take the form of spondyloarthropathies (joint diseases of the spine) such as ankylosing spondylitis, plantar fasciitis, and Achilles tendinitis. Enthesopathy can occur at the elbow, wrist, carpus, hip, knee, ankle, tarsus, or heel bone, among other regions. Further examples include:

- Adhesive capsulitis of shoulder

- Rotator cuff syndrome of shoulder and allied disorders

- Periarthritis of shoulder

- Scapulohumeral fibrositis

- Synovitis of hand or wrist

- Periarthritis of wrist

- Gluteal tendinitis

- Iliac crest spur

- Psoas tendinitis

- Trochanteric tendinitis

In medicine, an enthesopathy refers to a disorder involving the attachment of a tendon or ligament to a bone. This site of attachment is known as the entheses.

If the condition is known to be inflammatory, it can more precisely be called an enthesitis.

Anything compromising the tunnel of the posterior tibial nerve proves significant in the risk of causing TTS. Neuropathy can occur in the lower limb through many modalities, some of which include obesity and inflammation around the joints. By association, this includes risk factors such as RA, compressed shoes, pregnancy, diabetes and thyroid diseases

Supracondylar humerus fractures account for 55%-75% of all elbow fractures. They most commonly occur in children between ages 5–8, because remodeling of bone in this age group causes a decreased supracondylar anteroposterior diameter.