My 11 year old son is complaining of a pain on the left hand side of his chest whenever he jumps or bends down?

See a Trusted MD: Although it is sometimes difficult to evaluate a complaint in a child, I would have your son seen by a trusted MD who can listen to the lungs and order further tests.

Muscle strain : This is likely a muscle strain but he needs to be examined to be sure. Please see his doctor so all other causes can be ruled out & he can have the proper treatment recommended.