Tennis elbow is the more common of the two, affecting the muscles on the outside of the forearm. The technical term is lateral epicondylitis.
Although it’s commonly referred to as “tennis” elbow, it is linked to most racket sports, including squash and badminton, as inefficient technique on backhand swings can overstrain the extensor muscles of the forearm that are employed when bringing the wrist back and turning the palm up (supination). Other factors in these sports can include weather conditions and features of the racket such as weight, size of handle, and tension of the strings.
Of course, other movements can initiate an episode of tennis elbow, typically repetitive movements as required in computer work, as well as the movements performed by some musicians and manual labourers.
On a microscopic level, some of the tendon fibres will be found to have micro-tears with subsequent inflammation, which is why the area becomes tender and pain is reproduced by movements that cause the muscles to activate.
Golfers elbow or medial epicondylitis appears as a similar condition, affecting the muscles that oppose those affected by tennis elbow: the wrist flexors, which attach to the inside of the forearm and elbow. Thus, with golfer’s elbow, flexing the wrist will aggravate symptoms.
Because a different set of muscles are affected here, different activities tend to bring it on, but this is not to say that playing tennis cannot cause golfer’s elbow or vice versa. Again, rackets the wrong size, weight, or with the wrong string tension can aggravate the wrist flexors- although forehand strokes are likely to affect the flexors more than backhand strokes. As the name suggests, golf is associated, as well as repeated throwing.
In the acute phase, golfer’s and tennis elbow have the same mechanism: microtears and inflammation, but occasionally in longer term cases there can be neurological involvement.