Rather than have a low rate of hard impacts, why not have a higher rate of softer impacts? Even if you don't go any faster, a more rapid cadence can make running much less stressful on the feet and legs.
A note about Cadence vs. Turnover Rate: Cadence has units of revolutions per minute (rpm), and in running it is the number of times per minute a given foot (either right or left) hits the ground. Turnover rate has units of beats per minute (bpm), and in running it is the number of times per minute any foot (both right and left) hits the ground. That is to say, cadence looks at only one leg, while turnover rate looks at both. So a cadence of 90 rpm is the same as a turnover rate of 180 bpm. Most running articles and books talk about turnover rate, but most shoe pods measure cadence.
When I started to increase my own cadence, I noticed another effect: I stopped twisting my ankles! I've always been plagued by "weak" ankles that would twist and sprain with the slightest provocation, such as stepping off a curb. I can't count the times I've "rolled" my ankles while running, where my normal pronation would keep going and I'd "run over the side of my foot".
Of course, a physical therapist would instantly identify the true cause as being chronically over-stretched tendons and ligaments combined with weak stabilization muscles. Once this state is entered, it is tough to recover without severe activity limits combined with physical therapy. As a new triathlete returning to running after a 25+ year absence, I didn't want to have to quit running so soon after returning to it!
Unfortunately, as I increased my turnover rate, I found it was almost impossible for me to maintain the higher rate: The moment my attention drifted, I would return to my bad-old slow pounding rate. And even when I did manage to maintain my focus, data from my Garmin shoe pod made it very clear that my cadence was varying wildly.
After doing some internet searches for more information, I learned that some runners had to resort to running with a metronome to maintain a desired turnover rate. Many different metronome brands and models were mentioned, but most seemed either too delicate, too heavy, too expensive, or too quiet/loud to be used for daily running.
After several shopping searches I stumbled upon the Seiko DM50, a small, inexpensive metronome with adjustable volume, and it even had a clip on it! To avoid annoying those around me, I used the quietest setting. For runs in quiet areas (away from traffic) I clip it to the waistband of my running shorts, and in noisy areas I clip it to the neck of my T-shirt.
But to what rate should I set the metronome? After more online research and some personal experimentation, I decided I wanted to use a cadence that would be just below the fastest cadence I could sustain during a best-effort 50 meter sprint. My Garmin said I was averaging a 100 cadence on such sprints, which equates to a turnover rate of 200 beats per minute. I set the metronome to a 2-beat rhythm ("beep-boop") at 190 beats per minute.
My reasoning behind selecting this setting is surprisingly simple: My research showed that all the top running authorities agree it is far harder to change to a new cadence than it is to adjust stride length. Since my goal is to run as fast as I possibly can, I decided it makes sense to learn to use that fast cadence now, then simply extend my stride length as my conditioning and skills improve. Since I doubt I'll ever be able to sprint during an entire endurance run, something below my maximum sprint cadence is indicated.
After looking at the turnover rates of many top endurance runners and triathletes (mainly done by counting the frames between footfalls in the many running videos on RunBlogger Pete Larson's site and his YouTube channel), it seemed most had rates between 180 and 200 bpm. But most top runners have fairly long legs. When I restricted my search to those with legs that looked to be like mine (32" inseam), the fastest runners tended to cluster close to 200 bpm. So it seems it was no accident that my own top sprint turnover rate was 200 bpm!
I should mention my 50 meter sprint average speed was only a 6:15 pace. Vastly slower than the top marathoners I was observing. Which is why I set my metronome to a slightly slower rate of 190 bpm (95 rpm).
One neat aspect of this number is it nicely matches my cycling cadence. My best hammering on the bike occurs at cadences between 90 and 100 rpm, depending on the terrain, the gear I'm using, and my fatigue level.
Making my legs match this rate while running has been tough! I soon found that when I did manage to complete an entire training run at this rate while averaging a comfortable 9:30 pace, my legs were much less fatigued, though my lungs were working significantly harder.
The harder breathing at the higher turnover rate clearly highlights my lack of cardiovascular conditioning, while it simultaneously demonstrates the stress on my leg muscles and joints has been significantly reduced. I suspect part of the issue is that my leg muscles will need lots of time to fully adapt to the higher turnover rate, and I expect their oxygen demand will decrease over time.
One neat side-effect has been that I've been able to reduce some of my bike training and replace it with running: Running at the higher cadence seems to complement my cycling, so less saddle time is needed to maintain my performance level. Of course, when it comes time to improve on the bike, I'll have to add that bike time back in.
I do wish it worked the other way: I'd much rather increase my biking if it would permit me to reduce my running while sustaining or improving my run performance. Biking is so much easier for me than running. The universe seems to be a one-way street in that area.
While I consistently train at a 190 turnover rate (95 cadence), I race without the metronome (mainly due to a fear of having small children point at me). So I occasionally do a test run without the metronome to see how my "free" cadence is changing. Over time, my free cadence has increased to between 80 and 85 rpm (160-170 bpm), which feels much better than my bad-old cadence of 65-75.
A gratifying change due to my training at a constant high cadence is how I handle hills. I used to suffer when encountering any terrain that wasn't flat as a pancake. Now when I go up a hill, I must shorten my stride in order to maintain the 190 turnover rate. The short, quick steps make me feel like I'm motoring all the way up! And instead of reaching the top exhausted, I'm now able to smoothly return to my normal stride length.
The best part has been the downhills. In the bad-old days, my long stride would be extremely punishing when going downhill. Within half a block I'd get joint pain, PF pain and shin splints as my heel pounded into the road and my foot slapped down. To limit impact today, I still need to shorten my stride while maintaining my turnover rate, but it now feels like I'm gliding down the hill (though I suppose I must look like a hamster in a wheel). I reach the bottom feeling fresh, ready to pour on the effort.
Unlike most runners, my downhill speed is slower than my flat-land speed. The downhills are the only place where I feel I really need all the comfort my shoes and stride can supply. Any stride fault while going down a hill instantly sends a jolt up my legs, which my spine converts to pain. My current running shoes have a 4mm heel-to-toe drop. I believe reducing that drop further, preferably to zero, will accomplish the dual effect of permitting me to increase my downhill stride length (and speed) while simultaneously giving my calves more time to absorb the impact.
That change will have to wait until my sports gear budget gets replenished. In the mean time, I'll be quite content to watch my overall pace gradually decrease, with no reduction to my running comfort.