The periods of my training that have resulted in the most performance gain have included two things: 1) loss of excess weight, and 2) systemic training including three types of hill running. Recently, I teamed up with Jason Fitzgerald on a post for his blog Strength Running called Hill Running 101: How to Take Your Running to New Heights. If you want some tips on how to use hills to your best advantage in training, I think it's a great resources. I am a scientist by training, and approach things in a fairly methodical manner, focusing on physiology, biomechanics, and mechanism. Jason is an successful runner and experienced coach whose insatiable quest for knowledge to improve his own running have made him a great technical resource on running, and I think he's done a great job of integrating our ideas in this article. If you're looking for a few ideas on efficient training approaches you can use to take your running up a notch, check it out.
If I am looking to build strength for running, I want to make sure my form is good during a workout with good resistance but shorter repetitions. In my view, the easiest way to get resistance while running is to run uphill. The hills shouldn't be too steep, because then you're scrambling more than running and the different form required won't do much for the muscles and coordination you need for your best performance on flat ground. They should be steep enough to get some good resistance while maintaining a decent running form.
I also think it's good to work your muscles at three ranges of intensity to get the most wide-ranging benefits from hill training. You want some long steady, aerobic uphill running for sure. Trail running on hilly terrain regularly can be great for this, particularly if you have some long incline trails in your area, like we do here in Northern Colorado. My view is that you should also have some threshold-pushing hard uphills runs up moderately grades (hill repeats). Whereas the aerobic running on hills will give you a little strength, endurance, and durability the slower pace is mostly suitable for building aerobic endurance, or the ability to last long distances at a manageable pace. The repeats will work on strength, but are also run hard enough to push your into anaerobic realms. They will strengthen your legs and make hill running easier in part just from that, but largely also because of the anaerobic metabolic benefits to your running muscles.
Hill sprints are a different animal. Being on steep hills, and only lasting about 10 seconds, they aren't long enough or slow enough to be fueled by aerobic metabolism to any real extent, and so they are nearly completely anaerobic. Being highly intense in terms of effort, they will stress your muscles and tendons, but only for a few seconds at a time. They are designed purely for gaining raw power/strength.
Then, there is the downhill repeat. These are great for two reasons. The first is that they are easy to alternate with your uphill repeats. At the top of your moderately-steep quarter-mile hills, you can walk for a minute, then turn around and glide back down, focusing on a fast, smooth efficient running. Downhill repeats like this will cause eccentric contractions in your quads as you use them to absorbe the force of each footfall and then to push off as you focus on getting good hip extension when your legs fly out behind your. As your knee bends to absorb the forces of each step, your quads lengthen. This can't go unchecked, so they must also begin to contract to halt and reverse the lengthening as your hip and leg extends as you drive it out behind you. This builds strength and durability and ultimately helps you run strongly and efficiently downhill.
There are other plyometric exercises I've talked about previously to improve your downhill running performance as well. The great thing about plyometric exercises and hill running, is that if you do them right, and don't overdo them to the point of injury, studies suggest they can make your joints, muscles, and tendons more resistant to strains, sprains, and chronic inflammations (like tendinitis), over time. It's unrealistic to think that you can push yourself to train hard and reach your peak performance without ever suffering some kind of injury. The research shows that it's almost inevitable, so anything that makes the body more resistant to injury over time can only be seen as a good thing to a determined runner.
Having had some chronic tendinitis issues through much of the first half of 2011, and coming back by running mostly flat routes, I'm ready again to make a concerted effort to hit the hills, up and down. I'm looking forward to it, because smart training on hills results in faster running on all terrain. And what runner doesn't want that?
For more details on the types of hill workouts I mention above, have a look at Jason's article, Hill Running 101: How to Take Your Running to New Heights on Strength Running.