The steep inclines you'll face amount to built-in intervals of high-intensity work, and the ever-changing pace of biking over difficult terrain gives you the aerobic variety an effective cardio workout should.
Navigating the ups and downs and roots and rocks of mountain biking brings almost every muscle in your body into play: the lower body muscles (calves, quads, hamstrings, glutes, and hip flexors), plus the pecs, shoulders, and even the forearms.
Amp It Up
Three-time Olympian and U.S. Bicycling Hall of Fame inductee John Howard competes with his training partners. Pick a distant landmark and race to it. Alternate between nearer and farther finish lines to vary the duration of high-intensity work. As your crew gets stronger, reduce the rest periods between races to spend more time clipped in and cranking.