Cross-country skiing is such a lung-busting workout that an entire sport, the biathlon, was designed to test an athlete's ability to fire a gun – and hit a target – during a cross-country race. (No surprise, then, that it's such a calorie inferno.)
Because the sport involves moving your extremities in relationship to your core rather than to gravity, your shoulders, core, glutes, hamstrings, and calves work constantly.
Amp It Up
Champion skier and coach Nathan Schultz suggests counting your strokes across a four-minute course. Every time out strive to reduce your stroke count. You can also try skiing without poles to focus on your legs and core.