All exercise is a form of physical stress. Done correctly, it creates better fitness by making the body adapt to that stress and become stronger. Like aerobic exercise, resistance training increases heart rate and makes the lungs work more to keep the additional blood flow filled with oxygen. But the primary benefit of muscle-strengthening activity comes from the way it taxes the muscles. Pushing or pulling against resistance generates microscopic tears in muscle tissue. These tiny tears are not really damaging: they prompt the muscle to repair itself and build more fibers to become stronger. “You’re trying to hurt yourself for an adaptive benefit,” says epidemiologist and exercise scientist Jessica Gorzelitz of the University of Iowa.