Until I attended a POSE Method running clinic with Dr Romanov in Odense (Denmark), I subscribed to the common misconception that one should take bigger strides when running down hill and shorter strides when running up hill. As we will see, this is in fact the opposite of what you should do.
The confusion centers around how we think of stride length. Our idea of stride length derives from our perception of how displaced our legs are from each other as this is the aspect we can sense and control. I'm going to call this relative stride length. However, what we are interested in is how this translates into running speed - in other words what is our absolute stride length as measured between footprints. It is clear that, as we tend to run much faster downhill than uphill, our absolute stride length is indeed longer when running down hills. Nevertheless, we tend to also increase our relative stride length and this, coupled with the increased speed and extra distance that we fall in each step, means that we land with incredible force. This is bad. All too many of us know how easy it is to injure yourself running downhill but I have yet to hear of someone who has injured themselves running uphill.
The theory behind the POSE Method can be just as easily applied to running up or down hills. As I have discussed in previous posts, the POSE Method hinges on the idea of running being a sequence of "poses" (being in balance on the ball of one foot), in between which you "fall" forward by leaning forward from your ankles and you "pull" your foot from the ground. Relative to your body you always pull your foot directly along the line directly between your foot and your butt (actively using your hamstrings as opposed to quadriceps or hip flexors). The idea is that this positions it optimally to fall back to the ground directly under your center of mass (in other words, you avoid over-striding). The only degree of freedom you have to control your absolute stride length is how much you lean and what Dr Romanov refers to as the Range of Movement (ROM), or how far you along the foot-butt line you pull your foot. If we are relaxed, then we tend to find an optimum ROM for a given speed automatically: too much ROM and we waste energy moving our legs excessively; too little ROM and we take too many little steps. I discussed this in a fair amount of detail in my post on rhythm in running. In practice, I never actively adjust my ROM but I sometimes make a mental note of how high I am pulling my feet as an indication of how fast I am running: the faster your run, the higher you pull your feet.
So, running down hill you are running faster and should have a greater ROM? NO! The ROM is the height you should pull your foot so that it falls back to the ground exactly in time to make ground contact. Why do we pull our foot at all? The reason is clear if you try to run with straight legs - apart from them getting in the way, it is also extremely tiring. By folding your leg up, you reduce its angular momentum, or the energetic cost of it rotating, rather like a shorter pendulum. (By the way, the POSE Method stresses the point that this rotation is performed by gravity and should not be achieved via an active use of the hip flexors.) The long and the short of it (excuse the pun) is that it rotates more quickly allowing you to lean further forward which, in turn, means that you get more benefit from gravity pulling you forward so you go faster! Now, if you are running downhill, the distance your foot falls to the ground is a bit greater than it would have been on the flat, therefore you don't need to pull your foot so high. This means that your ROM should be less running downhill. This idea doesn't exactly correspond to the relative stride length I described earlier, but it certainly feels like we are "shortening up" as we run down hill. Running uphill is just the opposite: we have to increase our ROM because our foot makes ground contact sooner than it would otherwise and we have to pull it higher as a result. It is actually much easier to run uphill than it is downhill from a technical point of view. Dr Romanov said that "Everyone runs POSE uphill, the trick is to be able to run POSE on the flat and downhill".
Another difference is that, relative to the ground, you lean less when running downhill and more when you run uphill; relative to the vertical, the amount of lean is more or less constant. This makes sense: uphill you need more "help" from gravity and downhill you are limited by your ability to turn over your legs quickly enough.
At the end of the POSE clinic, Dr Romanov had us run downhill blindfolded (guided by a non-blindfolded partner!). Interestingly enough, we all ran downhill naturally shortening our ROM. We then ran uphill, also blindfolded, and it didn't feel as though we were making any more effort than running on the flat.
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