Running is generally viewed as the “default” form of cardio. There are good reasons for that: It’s the form of cardio that comes most naturally to humans, and it’s easy to do just about anywhere. And, of course, running is the backbone of every Spartan race. But there’s more to running than just a steady-state slog. Switching up speed, duration, and incline all have benefits. And not just on flat ground—there’s a lot to be said for running stairs. Well, primarily for running up stairs, not down them. Here’s why.

It’s Easier On Your Joints

Running-related injuries are more common than many people realize. The most common acute injury is a rolled or twisted ankle, however, most running injuries are due to repetitive stress and chronic overuse, according to recent research published in the American Family Physician.

The reason running is so hard on your joints is because, in essence, you’re repeatedly slamming your foot into the pavement. Each step sends a shock wave of force through your foot and up your leg. That force is proportional to both how fast you’re running, and how far your foot has to drop from the top of its movement arc to the ground.

That means there are three ways to reduce the force acting on your legs: run slower, be shorter, or run upwards so that your feet don’t have to go down as far as they go up on each step. Running either uphill or up a flight of stairs will both shorten the distance your feet have to drop on each step, and slow down your running speed—but running up stairs is better due to the steeper angles involved.

Needless to say, running down a flight of stairs exerts more stress on your legs, and should be avoided if you’re trying to minimize joint stress.

It Builds Muscle Mass

Running can build some muscle mass if you have no prior history of training your legs. However, the gains quickly level off because, unlike with weight training, you’re not contracting your muscles against resistance.

Running upwards adds some resistance since you’re contracting your muscles against gravity; the steeper the angle, the more gravitational resistance you have to overcome. This is still no substitute for lifting weights, but you can raise the resistance somewhat by wearing ankle weights, and raise the metabolic intensity by running faster.

Aside from offering more resistance, running up stairs also works your muscles that is qualitatively different from level running, because…

It Works Different Muscles

Running stairs moves your legs at a different angle from regular running, and that means it’s dependent on different muscles.

Level running primarily relies on the calves and hamstrings, which is why those two muscles tend to be the ones that get the most sore after running. It’s also why shin splints and pulled hamstrings are so common in runners. As you increase the angle at which you’re running, the load gradually shifts from your calves and hamstrings to your quadriceps and glutes.

This is why stair-climber machines tend to be a staple of women’s glute workouts—at a steep enough angle, running stairs resembles something halfway between running and Bulgarian split squats.

You can adjust the degree to which you work the glutes and quads versus the calves and hamstrings by adjusting the angle at which you’re running. A steep angle is ideal for building your glutes and quadriceps, while a more modest angle will spread the workload more evenly throughout your legs for a more balanced workout.

It’s Just as Healthy As Running

Aside from all the specific advantages that stair climbing offers in terms of muscular development, athletic performance, and injury prevention, it’s also great for your health. Stair climbing significantly improves your balance, endurance, and resting heart rate, according to research published in the Scandinavian Journal of Medicine and Science in Sports.

In terms of cardiovascular fitness, running stairs about as good as running or using an elliptical machine, a study published in the Journal of Sports Medicine and Physical Fitness found, so there’s no need to favor one form of cardio over another on that account.

The improvement in balance, however, stands out as something that’s likely unique to stair climbing due to the greater instability and higher demands on your muscles to keep you upright and in alignment.

It Makes You a Better Runner

It’s no secret by this point that combining different exercise modalities can make you a better athlete. Cross-training is all the rage these days, for the simple reason that it works.

More specifically, practicing running stairs improves your treadmill and track running performance, as measured by both run time and VO2 max, reports a study published in the journal Medicine & Science in Sports & Exercise. To be more specific, it improves them about 75 percent as much as you would get from actually running. So while you still need to run to become a good runner, the degree of carryover is greater than most people would expect.

Unfortunately, this study didn’t test a combination of stair climbing and level running. But most champion runners these days do mix in two or three sessions a week of some other form of cardio, like swimming or stair climbing.

Based on the habits of champion runners, along with this study, it makes sense to do one or two stair running sessions a week—or one stair climbing session per two to three running sessions—if you want to be a better runner.