A shaft structure with adjustable and self-regulating stiffness is provided for golf clubs, fishing rods, and the like. The shaft structure employs in a presently preferred form a hollow tube in which a piston and a longitudinally spaced platen, each longitudinally slidable, are incorporated. The piston and platen are separated from one another by a spring biasing member. The platen is position controlled longitudinally by a longitudinally extending jackscrew that is rotatable and threadably associated with that platen, but that is longitudinally fixed relative to the tube. The chamber defined on one side of the piston contains a fluid which can be a gas or a liquid, and the platen assumes a position along the shaft which to corresponds to a location where forces on each side of the piston are equalized. Rotating the jack screw causes the platen to move longitudinally, the direction depending upon the direction of jackscrew rotation. This movement causes the piston to relocate to a position where the respective pressures on each side thereof remain equalized.