In response to the q-factor debate (some people believe that narrower is always better, others feel it matters little) I often hear someone spout that our stance width when we walk (basically how widely spaced our feet are as we walk or run) is only a few inches wide, and therefore a very narrow q-factor is mechanically the best.
My response to this is very similar to one I heard from Keith Bontrager once "I'm not sure what walking and riding a bike have to do with one another." And that's true really, but let's look at the similarities and differences.
First, they are both reciprocal motions, meaning that one leg moves one direction while the other moves the opposite.
They are both weight-bearing activities. The amount of weight-bearing can vary drastically however. Walking slowly may produce ground forces equal to your body weight while running can record forces up to 9 times as high. Cycling on the other hand often runs on the range of perhaps 10% of body weight for very easy pedaling, to usually 30-40% for spinning on the flats, to perhaps at or just beyond one time your body weight (think about the times you have been pedaling uphill and your cadence slows enough, where you have to push on the pedals hard enough that it nearly lifts your butt off the seat -- this is roughly one time your body weight).
I could manufacture more similarities, like the fact that the quadriceps and hip extensors are the prime movers for locomotion in each activity, but this is a bit mundane seeing as how they are also the cause in most other forms of forward movement (skateboarding, nordic skiing, blah, blah, blah).
This is about where the similarities end.
Differences abound, and as you might imagine, I come down firmly on the skeptical side of the narro q-factor debate in cycling.
Firstly, running has significantly greater impact on the skeletal system, and most of these effects are good, since this aids with bone density. Cycling studies in the last 10 years or so have shown some scary results in nearly osteoporotic cyclists at pretty young ages, because they have years of training on the bike which is a limited weight-bearing activity.
Mechanically there is a lot going on that separates running from cycling. In running, you have one foot in contact with the ground for a short period (we call this a closed-chain position), then neither foot contacting, then the opposite foot in contact (walking has a brief phase where both feet are touching the ground at the same time). Cycling, however has both feet fixed to the pedals. In walking/running, when we have one foot in contact with the ground only, this allows the pelvis to unlock on one side, since it is not fixed in a closed-chain commitment (so we can swing our leg freely).
On the bike this doesn't happen. What about the recovery phase of the pedal stroke, you ask? (Recovery being roughly from the 6 o'clock position to about 11 o'clock) In this position the foot isn't bearing down on the pedal, you say? Well, actually, it still is. A perfect pedal stroke would have the recovery leg pulling up on the pedal to assist the other leg in it's downward force, and while it may feel like you do this when you pedal, all the research to date has shown that even the most efficient pedalers in the world do not "pull" their recovery leg up. That leg simply can't move fast enough to get out of the way, and we are left with a downward force on the pedal on the back side of the stroke, so in fact we all actually exert a negative torque on the pedals.
If you've ever been on a Compu-Trainer and used their SpinScan you will notice that with most riders if you look at the polar or the bar graph of the efficiency of each pedal at points of the stroke, that often the right leg will appear to be less efficient in it's power phase (roughly from 2 o'clock to 6 o'clock). Now this could be because of some imbalance causing weakness in the right leg, but more often than not it has nothing to do with the right leg pushing down, and everything to do with the left leg not getting out of the way fast enough.
(Incidentally, this is a very common short-coming to the Compu-trainer technology -- the SpinScan is of very limited value because of it's lack of sensitivity. Whenever I see other technology based on the Compu-trainer's I have to cringe -- the most glaring example being the Dynamic Fit Unit (DFU) from Guru. Here is a machine that costs tens of thousands of dollars and the assessment tool it uses is the 15 year old software and hardware from Compu-Trainer.)
Without getting too much into it (because it could be a post or even a book unto itself) most of us are wired to favor or be dominant on our right sides -- even if we are left handed. Some people think this is because we talk so much and our talk centers are on the left side of the brain which is also responsible for right side motor control, and the talk centers stimulate everything and keep that side of the brain "lit up" -- I think that's a neat theory but I'm not 100% sure I believe all of it. regardless, this right dominance is something I see every day. It is certainly no coincidence that about 75% of us sit slightly to the right side of our bike seat.
So we've established that there are significant differences between walking and cycling in weight-bearing (or what could be thought of as total impact), the fixed foot or closed-chain nature of cycling is different as well. A third area of discrepancy is total range of motion. Walking/running involve moderate amounts of hip extension, while cycling has none.
One of the largest differences is one that I address in every bike fit I do, and that has to do with foot and lower leg mechanics. In walking, our bodies are built to absorb impact (there's that word again) by having the midfoot move and collapse towards the ground after heel strike in order to absorb the energy of the foot making contact with the ground. Cycling has none of this. Part of that has to do with the lack of "impact" on the lower extremity, and the other half is due to only the forefoot being attached to the pedal. Forefoot mechanics factor in heavily in cycling, and they are not very well understood.
(I believe this is the case even in professionals who handle foot mechanics for a living. I have tested dozens of sets of custom orthotics on the infrared system - made by orthotists, podiatrists, doctors, and therapists - and while some didn't cause any problems, many did, and none of them corrected any alignment issues that extended above the ankle. Some actually made the foot more comfortable in the shoe, and that's great, but rarely is it necessary to go custom, and spend $400 in order to accomplish that. I have found over-the-counter inserts to do just fine in these cases)
In cycling, if you can control the forefoot you often have the ability to make corrections at the knee and the hip as well.
So what about running you ask? Haven't you expounded on the benefits of forefoot running postures?
Yes, certainly, I am a believer in...well I wouldn't necessarily say forefoot running because that prompts many people to run on their toes...I would say I'm a believer in not landing on your heels. Does this amount to a similarity between running and cycling? Perhaps, but not a very strong one due to two previously discussed facts -- the much greater impact on the foot in running really affects the demands on the foot (this makes correction of lower extremity mechanics on the bike a bit easier and more subtle), and the fact that our butt is also attached to the seat (which makes correction of lower extremity mechanics on the bike much more difficult).
So, I think it's really difficult to make determinations of cycling mechanics by referencing walking/running. In fact doing so will likely lead you astray and compund problems. You'd really be hard-pressed to find two activities that are more dissimliar in some respects. So be careful, if you are getting bike fit advice, in a bike shop or in a clinic, and the person you're talking to starts referencing walking or running mechanics or studies. Cycling is different and deserves to be distinguished.