“Tennis leg is a general term used to describe pain in the leg caused by a tear of the inner head of the big calf muscle, the plantaris muscle or sometimes both.”

The injury most frequently occurs due to a force or trauma to the leg whilst the knee is straight. Movements such as jumping or pushing off may also cause tennis leg. Both movements are frequent in tennis, hence the development of the term tennis leg to describe this injury.

Ref:  http://www.sportsinjuryclinic.net/sport-injuries/ankle-achilles-shin-pain/tennis-leg

http://www.scientificanimations.com/tennis-leg-achilles-tendonitis-two-canbe-dangerous/

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