When runners and triathletes think "hill repeats" they generally assume that the workout is designed for building strength.
Although training on hills can significantly improve strength and power, there are a few other (not so) hidden benefits.
Of those benefits, an improvement in speed and economy and reduced risk for injury are some of my favorite reasons why I like to head to the hills (and off the track) for "speed" work. When I want to stress my cardio system (elevate heart rate) while also improving muscular endurance, it's much safer and effective to do hill repeaters.
After a few weeks of priming my system with 10-12 minute 1-minute building efforts (thanks Grace A for the awesome workout idea) on gently rolling terrain, I was excited for more specific hill work. Yesterday my morning run workout was:
WU:
~30 min EZ (gently rolling hills)
MS:
6 x 2 min controlled strong uphill efforts w/ EZ jog down (~2:30) + 20 sec rest
Post set:
EZ home (more hills)
Total: 1:14 (9 miles)
The hill was ~4% grade and 0.3 miles. I had a start and finish point so that I didn't have to look at my watch, except for the 20 sec reset at the bottom.
When running uphill, the body has to recruit a lot more muscle fibers than running on the flat. Because the incline requires you to work harder with each step forward, every workout can translate to a more powerful running stride when you take your workouts to flat ground or rolling hills. When you run on flat surfaces, specifically speedwork on the track, running biomechanics are extremely important. This is why track athletes spend almost more time warming up compared to their actual workout. To drive you forward at a fast pace with an elevated heart rate on flat surfaces, your legs need to turn over faster and there is a change in the muscle fiber tiring and biomechanics. There are so many factors that can increase the risk of injury on flats. But when running uphill, the propulsion forward is much safer for the body as you are less likely to overstride. And since you are not running as fast, athletes may find it safer to increase heart rate than on the flats. This isn't to say that there isn't a risk for injury when running hills (especially the eccentric action of running downhill) so like any run training plan, proper warming up and progression is key.