Twenty years after the introduction of the theory, we revisit what it does—and doesn’t—explain. by Clayton M. Christensen, Michael E. Raynor and Rory McDonald Please enjoy this HBR Classic. Clayton M.

If you’ve ever climbed stairs, carried a heavy object, hiked up a mountain, played tug of war, or done a squat, congratulations: You’ve done resistance training without even knowing it. This form of ...