I've been tinkering with all aspects of my stride for three years now, and if I were to pick the single most critical aspect of my stride, the part that both consumed the most time to develop and yielded the best results, it would be my arm swing.
I am convinced that arm swing is one of the most complex stride components to work on. The length of the arm bones matters (both in an absolute sense, and relative to the leg and torso lengths). The amount of arm muscle matters, as does the weight distribution between the upper and lower torso. The arm swing that works best for one runner may not work at all for another runner, even if their body builds were nearly identical.
I have several very different arm swings I use, depending on the terrain (flat, uphill and downhill), the shoes I'm wearing (shoe weight has a great effect on the stride), and my fatigue level (my best speed under the conditions).
It took me about 6 months of experimentation to not only try many arm swing variations (range, symmetry, rate, forcefulness, elbow angle, etc.), but also to keep working with the 'best' ones until they became 'natural' to me. I found it essential to record all my test runs on my Garmin Forerunner 305 so I could later compare apples-to-apples, independent of what or how I felt (except for joint discomfort). Every new arm swing variation always felt worse, or at least strange, at the start. But the numbers do not lie.
The most surprising thing I learned is that I run with my arms! When using a metronome to train my turnover rate to a higher level (I train at 190 bpm), I found it was my arm swing I had to force to match the metronome, not my legs. Whatever my arms do, my legs will follow (if they can). The reverse did not work at all for me: Trying to make my legs turn over faster was a pointless endeavor. I've recently begun looking at my leg swing, and seeing if I can use my arms to help improve it, rather than focusing only on the legs.
I also had to incorporate stretching and some light strength training to help my arms become better at doing the swings that worked best for me. In particular, my rearward swing increased quite a bit (especially uphill), and I had to increase my strength and range of motion to make it effective, comfortable and sustainable.
Right now, my best arm swing on level ground with fresh legs looks like this:
- Elbows at about 95 degrees (slightly open)
- Hands open and flat (making a fist ruins my arm swing)
- Arm swing does not cross the body (no torso twist)
- Rearward swing is slightly exaggerated (it helps me maintain my best forward lean and also helps me use my hamstrings better)
- Downward arm swing is forceful, return swing is relaxed (it basically matches what the opposite leg is doing)
Having minimal torso twist has proven to be a key component toward helping me go faster. YMMV: Many runners require some torso twist to help obtain full leg extension. I have long arms relative to my leg length (great for swimming) which seems to make torso twist unnecessary for me. (I did try bending my arms more and adding torso twist, but it slowed me down.)
The thing is, this is beginning to feel like a never-ending cycle: Every time I get faster, my stride lengthens (I keep a near-constant turnover rate), and I need to adapt my arm swing to work better with the increased range of leg motion. If you already have great cardio conditioning and good speed, you may reach your potential sooner, with less experimentation and adaptation.
I'm still trying to work my way down to an 8 minute pace, one step at a time...