Method of filling an athletic bag with air and liquid

A method for preparing for use an athletic bag of controllable blow responsive characteristics, suitable for being struck by a part of the body of an athlete during practice and for controllably absorbing the force of such blow, said bag including a container for holding fluid under pressure, an opening in said container and a push-pull valve for communicating with such opening and selectively permitting or preventing movement of fluid through it, which container includes a flexible substantially non-elastomeric cylindrical outer cover and an elastomeric fluid-tight inner bladder of essentially the same size and shape as the cover, capable of holding liquid and gas under pressure, which comprises adding liquid to the interior of said bladder through said valve to fill said bladder to 75-99% of its volume, and raising the pressure in the bladder to from 0.01 to 0.4 kg./sq. cm. gauge by blowing air through said valve into the bladder, and closing the valve, with the bladder, under such pressure, being restrained by the cover against expansion and exerting a controllable pressure on said cover.