I was talking with someone about walking vs. bike riding, and had to explain why walking is far less efficient: every time your foot hits the pavement, bam, it's a dead stop; large kinetic energy loss. It's remarkable how many people don't get this. (another fun question: why does a car get better highway, than city, fuel mileage?)
Now, the hard part: how inefficient is it? What is the energy loss? It's a tough chestnut, on account of ankle flexibility, which permits some continuous motion of the body.
Suppose you were given this project, with adequate funding: quantify it. i.e. how much energy is lost on each foot step? How would you determine this? What experimental apparatus and measurements would you need? How accurate would be the results?
Bonus credit: discuss the relative efficiency of bicycle vs. roller skates.
I think you'll find that bipedal locomotion is remarkably efficient, though muscles aren't all that efficient. There is a lot of information on this in robotics papers, if you care to chase it (try doing a search on "efficiency + human + walking (or locomotion)".
Some? The body doesn't come to a stop when walking.
only quickly perused it, but there seems to be a lot of methodology in there.
Every time a wheel hits the pavement, bam, it's a dead stop. Check it out:=20
It's not really remarkable how wrong you can be.
Time spent braking, idling and gear changing.=20 Braking is highly inefficient, KE is wasted as heat. Predominantly wind resistance is overcome on the highway, heating the air with friction, not the brakes.
Oh, we are back to walking again.=20 Bike riding is inefficient, when the pedal gets to the bottom having been pushed *down*, bam, it's a dead stop. Oddly enough pedals push the other foot up to the top=20 until bam, it's a dead stop. Walking is inefficient if you swing your arms. Riding a bike is little like climbing a hill, you lift yourself as=20 you step hard on the pedal, but having reached the top of=20 the hill you can glide down again.=20 =20
I'd need to know food mileage. I'd starve you for a week,=20 feed you on one can of beans then put you on a treadmill=20 until you dropped, measuring weight change and calculating calories from beans.=20 You can hold the side rails if you want to. Resting is allowed.
Then I'd repeat, starve you for a week, feed you one=20 more can of beans and put the bike on the treadmill.
I anticipate about the same food mileage. With adequate funding I'd pay another a bonus if he got greater food mileage from walking=20 than you get from cycling to avoid any cheating. Since you would have an axe to grind you'd get no bonus, so I'd give him an axe too to level=20 the playing field. =20 =20
Roller skates have smaller wheels which turn faster, creating greater friction in the bearings.
Another week of starvation, another can of beans. I anticipate less food mileage than walking.
Neat, but wrong. It is physiological. Your leg can produce far more power in the cycling motion where essentially the leg is straightening out to produce the force vector. (With racing pedals you also produce force by bending the leg, pulling the pedal up.) In walking, you pull your body forward with an already extended leg, which stays mostly extended for the full cycle. Different use of the muscles, different angles for the force vectors.
Both cycling and walking produce full stop at some point in the cycle, but that full stop is the change of direction of the limb, not the body. The body does not come to a full stop. There is not _large_ kinetic energy loss of the whole mass stopping.
In addition, with a racing bike, the recovery stroke produces useful force as you pull the pedal up - in walking, the energy expended in the recovery stroke produces no useful force.
Well, using ~world record time for the Marathon (42.2km) an equivalent distance for a cyclist would be 96.2km. So that's about 2.28 times more efficient for the world class cyclist vs the world class runner. We can assume each one used all they had, so the energy consumed is assumed to be roughly equal.
That's "soft", just intended to put a stake in the ground.
I think that the premise may contain an invalid assumption. Say you devise the perfect measuring setup that determines the exact energy consumed. The work performed is easy - the mass is easily measured, and the distance fixed. Ok, you can tell how much work was done and how much energy expended. The problem is that the result applies only to that person for that particular test. The next time the test is done, the energy used will almost certainly be different, even if every external factor (temperature, humidity, wind etc) is kept the same. A world class athlete would doubtless perform more efficiently than a couch potato. So whatever efficiency (or inefficiency) number you arrive at would itself be "soft". Bottom line, you cannot establish the efficiency (or inefficiency) of _walking_ - what this setup would measure is the efficiency/inefficiency of a human at the particular task, not the efficiency of the task itself.
Ok, again using the theoretically perfect setup: Build two perfect machines - one that walks, the other that rides a bicycle. Measure the work/energy and determine efficiency/inefficiency of walking and cycling. That should result in "hard" numbers.
Why? And why the first question about walking? Why did you _have_ to explain why walking is less efficient than cycling? And why not the simple answer that it takes more energy to cover the distance walking than it does cycling? If the question is really a physics question, you need to remove the human from the issue. On the other hand, if the answer really needs to cover more than physics, you need to mention that, too.
For example: bike vs roller skates. The perfect experiment is equal mass, equal wind resistance equal force, equal friction - everything equal. Guess what? There is no difference in efficiency. Now add a human to the mix, and it becomes roller skatING vs bicyclING. And that automatically involves physiology. If your training and education is in that area, perhaps you are qualified to answer.
A rough calculation is not difficult: measurements have been made of 'x calories per hour of  activity'. You may choose to peform better measurements, but let's start with walking at 100 kcal/hour for a 150 pound person walking at 5 mph. From this you can calculate the power expended, and compare that against other processes. Before you make the problem intractable by anatomical considerations, you may be better off discussing the role of terrain.
I believe cycling can be much more efficient than walking, but the collisions with the ground are probably rather elastic, with energy being stored and released by the springy tendons of the (youthful) legs. That being said, tendons are not perfect springs and muscles have to be stretched, so there are significant losses occurring with every step. Meahwhile a chubby cyclist coasts by. I read somewhere that cycling uses just 1/5 of the energy of walking!
That's an optical ilusion. If it went BAM, you'd hear it. But you don't. Seen from the viewpoint of the axle, which after all is where the power is applied, the entire wheel rotates smoothly and continuously. And the pavement flows by smoothly. Nothing goes BAM.
Several sources, easy to find with google, cite bicycling as 3 or 4 times more efficient than walking. A common-dense mental experiment will show that bicycling is much more efficient.
Just imagine walking at some nice rate, and how much your breathing and heart rate would be elevated. Now imagine pedalling a bike at that same level of exertion. Pretty obviously, you'd be going a lot faster on the bike.
You could quantify this by equalizing heart or respiratory rates between walking and biking (you need only a wristwatch and some patience), or more precisely by measuring CO2 production.
Your center of mass never comes to a dead stop when your foot hits the pavement, so I think your argument is bogus. And when one foot stops, the other one is usually already in motion, so you might want to rethink all of this.
Good answer- I have mucho experience with road cycling, running, hiking, and now rowing, in the outdoors and on the machines that estimate all kinds of parameters like speed, cumulative calorie burn, and peak output power. It is pretty obvious to me that the man made machines significantly boost our ability to travel farther, faster, and longer, when used correctly that is. For example, it's kind of difficult to substitute walking for rowing when attempting to traverse a body of water, and similarly, that boat or bicycle is not much good in mountainous terrain. Of the three methods to be used as a means of improving cardio/respiratory physical fitness, my vote goes to the road bike on the indoor trainer for highest sustained power for longest duration at a constant rate. I can pull 1000W peak on the rower, but it's a jerky kind of repetitive pulsed type of output and relatively slow. You just can't duplicate those kinds of peak powers with hiking unless maybe you sprint at intervals or something like that, but generally you will be carrying a load making that impractical. The OP is sort of correct about the walking inducing more joint impact than the others, you would have to do a 50 miler carrying a 75 pound load to fully appreciate how this adds up. Anyway, forget the leg powered stuff, I'm working on doing a handstand walk for 100 meters, the ultimate in inefficient locomotion.
That's not how cycling is done, it is a rotary motion which means there are no "bam" types of dead stops in the wheels or the legs. Duh, that's why it's called a bi-*CYCLE*. As for the leg motion, the seat height is adjusted so that the knee is fully extended when the pedal bottoms. The leg is used to apply peak force on the pedal only through the last 20o of pedal rotation at the bottom, the remaining portion of the cycle is all momentum. The opposite leg pulls up and is definitely not pushed up by the stroking leg. For most people, a constant pedal cadence of 90RPM is optimal so as to assist the blood return to the heart, change gears as necessary to maintain this cadence.