The magic number?

There are certain numbers that seem to be everywhere you look. One of these is 180.
 When you make a full turn, that's 180°. The highest score in a turn of darts is also 180. And if you have an interest in running, you'll probably have heard that you should run at 180 strides per minute.
While the first two instances of the number 180 make total sense, the bit about running cadence, isn't quite as obvious. Who said that 180 spm is optimal? And why? And maybe most importantly, is it even true?

Where did the 180 strides per minute thing come from?

From what I've read, the number came from the observations of renowned running coach called Jack Daniels. He noted that elite runners run at cadences not less than 180 steps per minute. Note how I wrote "observations"? That was intentional. You see, this was not a study or anything, it was simply an observation. Also, he stated "not less than" not "exactly".
How this number suddenly became almost like a commandment in running is unclear to me. Why there is so much focus on cadence makes sense to me though. When it comes to running, there's not many things you can measure during your training. You can get some interesting numbers and feedback, but you'll need outside help. Cadence however can be measured by even the most rudimentary running watch. 

Quantifiable parameters in running

My coach has a training philosophy he calls KAPA, which in Danish stands for:

• Cadence (Kadance)
• Suspension (Affjedring)
• Posture (Positur)
• Arm swing (Armsving)

Suspension can be felt to some degree (depending on your footwear), but you can't really put a number on it without advanced equipment. Posture and arm swing can't be measured without outside help either. But cadence, CAN be measured. So it makes total sense for people to focus on cadence.

Now don't get me wrong, I agree completely with my coach that cadence is an important element in running. It's the main component in running speed even:

speed = stride length * cadence

This formula itself already shows why going for exactly 180 strides per minute is not optimal. Since a higher cadence means greater speed (provided you still travel the same or a greater distance with each stride).

But there's not just the speed component that makes cadence a relevant factor. More strides per minute require a higher leg turnover which makes over-striding (and heel landing) more difficult and unlikely. Which reduces your chances of getting injured. 

Apart from that, more elastic energy can be utilized at a higher cadence, since you'll be rapidly loading and unloading muscles and tendons. Which decreases oxygen consumption and thereby increases your running economy.
So a high cadence yields great benefit.

180 spm use it or lose it?

I personally, on average, run at around 175 spm at low speeds. During races, especially the shorter distances (10k and 5k) that number goes up significantly though, usually around 190 spm. Elite runners often run at 200+ spm even.

Your speed, the terrain, and angle of ascent/descent will all have an influence on your cadence. If I always run at exactly 180 spm, I won't be able to run as fast as I'm capable of. And depending on the terrain, I won't even be able to maintain 180 spm. When I run uphill or in loose sand/mud my cadence NEEDS to go up.
At very low speed, if I insist on having at exactly 180 spm, I'll be working a lot harder than I need to.

The point is, my cadence isn't set in stone. It's not a bad idea to know what cadence you run at though. And if it's significantly lower than 180 spm (say 150-160) then I'd definitely advise you to try to increase your leg turnover to get closer to 180. 

If you're a little under though (like I am under some circumstances) I wouldn't fuss about it. And if you run at a higher cadence than 180 spm, then there's absolutely no reason to try to get that number to go down.

So in conclusion, the number 180 is not some sort of running sweet spot that you should aim for under all circumstances. But getting your cadence at least at 180 spm is a good idea, especially at high speed or in tricky terrain.