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Are they warming up enough?
An episode with things to include in a run warm up
It could be a running technique issue.
Are they letting their heel kiss the ground?
If not their calf is contracted the entire time.
It could be a landing error.
If the person is landing on their toes vs landing ball of foot that may be the cause.
A video that addresses this http://www.youtube.com/watch?feature=player_embedded&v=23l42px5R_E