When running, three variables determine intensity, says David Siik, creator of Precision Run and LA-based senior manager of running for Equinox: speed, incline, and duration.
While designing Precision Run’s programming, he spent two years studying how these factors affect your body and your workouts. The findings: If you sprint at inclines higher than five percent you’re significantly more likely to get injured, a risk that far outweighs the potential benefits you’d get from upping the intensity in this way.
To sprint safely, Siik suggests first determining the fastest speed you can maintain for 30 seconds on a flat road. You’re cleared to run at that pace on inclines ranging from zero to five percent. If you go any steeper, reduce your speed by 0.5 to one mile per hour for every two-point increase in incline. (So if your max speed is 10 miles per hour, you’d run between nine and 9.5 mph on a seven percent incline.)
The bottom line:
Even when you follow these rules, limit all uphill sprints to 60 seconds, Siik says. Make your recovery periods two or three times as long and complete them on a flatter incline.