Tennis elbow is a repetitive stress syndrome that arises from overuse of the forearm extensor muscles. Also called lateral epicondylitis, this often occurs because of repetitive activity that involves pronation and supination, as well as extension and flexion, of the forearm muscles. This was thought to lead to the formation of microscopic tears in the tendon, at the point where it attaches to the bone. These tears were supposed to be responsible for the pain and difficulty in movement of the affected muscle.

Despite its name, tennis elbow is caused by movements of the forearm in numerous situations other than tennis. Activities which cause tennis elbow include the following:

• Tennis elbow can be caused by any activity that involves twisting the arm from the elbow, as in executing the backhand stroke in tennis. Other racquet sports which involve the same type of movement, as well as an overhead swing, may also cause an identical injury. Of note, this injury is far less frequent in top-rated athletes, suggesting the presence of faulty swing mechanics is important in its development.
• Sports which involve throwing heavy objects, such as a javelin or a discus.
• Another reason is occupational, as for instance, workers who use screwdrivers or use twisting hand tools, such as plumbers, bricklayers, carpenters, or construction workers.
• The use of a paintbrush or roller can produce lateral epicondylitis.
• The use of garden shears, which produces forceful contraction of the forearm muscles with a pronated wrist, may provoke tennis elbow.
• The use of butcher tools, as in meat shops, is another common cause.
• The prolonged use of a computer keyboard and mouse, or of scissors, can also cause the same injury due to repeated fine movements of the hand and wrist muscles.
• Playing the violin, which requires a bent forearm and wrist, may overload the elbow.
• Direct blows to the elbow joint can cause sudden and excessive loading of the joint, leading to tennis elbow, though this is a rare cause.

The basic pathology of tennis elbow involves an imbalance between the forearm muscle strength, and the load put on the forearm muscles. The symptoms of tennis elbow are now thought to be due to the degeneration of chronically injured tendon fibers, rather than inflammation, as has long been thought. Tendon tears are also more rare than expected.

Factors which predispose to this imbalance may include:

• Weak forearm and shoulder muscles
• Stiffness of the forearm or the elbow joint
• Unstable elbow joint
• Positioning the hand and arm in non-neutral positions for too much time.
• Excessive loading of the elbow joint during sports activities, by awkward technique, the use of wrongly-sized equipment, or overuse during sports activities such as tennis.
• Excessive loading with the joint in correct position during sports and other activities, due to the use of overly heavy hand-held equipment, too tightly strung racquets, or too heavy balls.
• Repeated movements of the same muscles, which causes stress injury.
• Psychosocial factors are found to be extremely important in the etiology of tennis elbow. These may include lack of support in the workplace, depression or anxiety. On the other hand, this association may not be causative, but rather the effect of tennis elbow. Studies do suggest that depression is a risk factor for this condition.