bfloyd

So I've read and heard that spinning is better on the knees. But, how is it better on the knees? Is there proof that spinning is a more efficient way of riding? What muscle groups are used when spinning (as I've read that different muscles are used than mashing)? When spinning is concerned, are they reffering to 100 rpm, 120 rpm, higher? Thanks.

DannoXYZ 

It comes from physics and how power is generated.

Power = (force x distance) / time

making specific to cycling we get...

Power = PedalForce*RPM

To generate equivalent power & speed at two different cadences.. say... 60 rpm vs. 90 rpm we have:

Power1 = Power2
PedalForce1*RPM1 = PedalForce2*RPM2
PedalForce1*60rpms = PedalForce2*90rpms
PedalForce1 = PedalForce2*1.333

Basically mashing your gears at 60rpms will require 33% more pedal-force than spinning at 90rpms for the same power & speed. This generates a lot more stress and strain on your knees at the lower RPMs. Especially when you try to go faster and start pushing harder and harder to keep up with the guys that are spinning. At 20mph average, mashing at 60rpms will feel like a lot of work, while spinning @ 20mph is a piece of cake. Forget it at 25mph..

An even bigger disparity occurs when you're going for maximum-speeds like in a sprint where you're pushing on the pedals at 100%. If you're trying to push a big gear at 100% pedal-effort at 60rpms, an equally-strong guy pushing on his pedals with EXACTLY the same force, but using a lower gear for 120rpms will generate TWICE the power (+100% more). He'll end up going 25% faster than you for the same strength, heh, heh...

Aside from fit issues, which should be optimized anyway, knee pains are often caused by low-RPM mashing. Even if you don't have any knee issues now, it's best to be preventative and spin to apply as low forces on the knees as possible to ward off future injuries. Plenty of people have posted about knee pains and going to higher-RPMs with easier gears alleviated that problem.

cyclezen

aside from all the stuff DannoXYZ has noted, which is all important, another aspect of high cadence riding and training is that it trains your muscles to work more efficiently. The entire pedal stroke causes your muscles to work against each other to complete the pedal revolution. A huge part of cycling is creating a more efficient stroke.
This inefficiency is exhibited when a rider goes to their upper cadence limits and they start bouncing on the saddle (caused by opposing muscle action). As one approaches the higher cadence limits a lot of force is surrendered to these inefficiencies, to the point where you can no longer increase rpm.
Training muscles to be efficient at 120 or higher rpm means they will be hugely more efficient at 90 rpm.
Ask A Trackie...
Trackies absolutely need to be able to hit 140 or higher to be anywhere near competitive.
To become greatly more efficient at 90 to 100 rpm the formula is simple, become more efficient at 120 & 130.
Ultimately its also a key to adding 'speed' into your riding, not just efficiency.

HardyWeinberg

I don't know if this is tangential or not but I was getting physical therapy this morning from maniacal triathlete and she told me to focus on pedalling in terms of back and forth, steam locomotive style, rather than up-and-down (which seems like the default mashing mindset that I haven't really had for a long time) or round-and-round (which I've considered but never really fully internalized). From her standpoint, the back-and-forth approach is less prone to skewing which muscles get used.

DannoXYZ 

It's the illusion of back&forth, yes. Your feet will always move in circles, but you can activate the muscles differently around that circle. The downstroke is natural due to gravity. By focusing on back & forth you tend to balance out that dominant downstroke. Lemond advocates a "scrape mud off bottom of shoes" analogy that does similar things.

Nice thing about getting more even pedal-stroke is that higher-RPMs gets easier. Due to gravity and the relative size differences in the muscles, you can never get even force all teh way around, but you can certainly make it less lobsided. Just getting the upstroke leg out of the way so that it carries its own weight will free up a lot of power from the downstroke leg. You'll notice that when you start concentrating on pedaling smoothly, you automatically pick up RPMs and speed?

So the goal really isn't about high-cadence, it's about effective pedaling-form. When you've got that down smooth, the cadence automatically picks up.

bfloyd

Thanks everyone. The true circular pedal stroke makes much sense that the higher rpm will naturally follow. So, should one start with an extremely easy gear for this practice (say a 42 x 19 or such)? Would it be beneficial to change to a shorter crank length (such as changing from a 172.5 to a 170 or even a 165)?

ericgu

For cadence drills you need a little resistance, but not so much that you get out of breath quickly. You may find this hard to do initially but it will come over time.

Initially, start at what you would consider a normal cadence, then smoothly increase your cadence over 30 seconds until you reach your maximum comfortable cadence. If you start bouncing, back off a bit. Continue at that cadence for 30 seconds, and then slow back down.

Repeat it a couple of times. Over time, increase the time you're spending at a higher speed for up to about 90 seconds.

One-legged drills can also help a lot on pedal stroke...

Crank length doesn't matter.

RiPHRaPH

To answer the original question, it is very difficult to both mash down AND pull up with equal efficiency. The name of the game is balance, as in balanced muscle groups. Mashing causes you to really use the quads and not use the calves as well.

Over time this can cause an unbalanced muscle grouping that can cause your hinges (knees) to have a different set of stresses.

HardyWeinberg

I wound up just using the lowest gear I could stand. Eventually (like a year of commuting later) I was back to using the whole gear range (in a pretty high cadence).

A coworker (who refuses to get off his big chainring) mocks (good-naturedly) my flying legs, but he's about to buy a car 'cause his knees have about had it. He got several good decades out of them at least. He's a great guy, but been a good role model on how I don't want to bike.