Intensity vs. distance

I've heard that the "magic" number is 12 - that is - at 5mph (the 12 minute mile) the calories per unit of distance (lets say 1 mile for argument's sake) will increase as your velocity increases.

So my questions are:

1) Is this true AND significant (i.e. are the increases in energy burn worth it)

and

2) Why is this happening?

I've read that you will burn more calories running/jogging as opposed to walking, because you are actually pushing yourself up from the ground at the same time you are propelling your self forward.

Now, a 12 minute mile is about the speed where I have to jog instead of power walking.  I imagine that by 6mph, no matter how long your stride, you are jogging.

I'd guess that's the reason for the "magic" number.  It's not the speed so much as the method of movement.

Going further, as your speed continues to increase, you expend more energy keeping yourself in the air with longer strides.  More upward motion along with the forward.

I'm no expert.  I'm just assuming this is the "why".  As for how significant the difference is, I have no idea.

Everyone is different, there is no "magic number"

That's interesting... I like the idea that running uses more of your body, and therefore requires more energy per unit of measure... I was thinking up all these crazy biological possibilities... and it is quite possibly only because, per unit of distance, we are using more of our body - and hence more energy. 

What part of your body does running use that walking doesn't?

Original Post by floggingsully:

What part of your body does running use that walking doesn't?

 What I was trying to say (and I know I didn't make it as clear as I could have) is that in a run, there are times your entire body is off the ground.  That is never the case in a walk.  For this reason, in a run, your muscles will have to propel your body up as well as forward, burning more calories than in a walk.

interesting..so i both speed walk and run, just to break things up.  i run at an incline of 2 or 3 and when i speed walk, it can range from incline of 2 to 11. 

why is it, even if at a lower incline, when i speed walk, my legs burn far more than when im running.  i thought that when you feel that burn, you know your muscles are going into aerobic cycle and  you are burning more cals?  am i way off?  thanks for the tips..

I believe part of the reason is related to the fact running is a weight bearing exercise - your entire body has to support the impact of your body being thrust into the air and landing over and over.  Walking (or even speed walking) doesn't invlove this kind of impact or require as much balance.

doug, the burn that is felt is actually lactic acid buildup, and is caused by the anaerobic pathway.

My guess (and this is just a hypothesis) is that when you run, your legs are moving more, which means more blood flow, both because of an increased heart rate, and because when mucles in your legs contract, they help push blood back to your heart. This would have a two-fold effect. One, more blood means more oxygen to the muscles, which means the anaerobic energy pathway is used less. Two, more blood means more waste products can be removed, such as lactic acid, which as I said, is the cause of the burn, and carbon dioxide.

When you walk on the incline, you don't get as much blood flow because your legs are not moving as fast, nor as much (i.e. a stride walking has less range of motion than a stride running), but they still require a considerable amount of energy to do what you are asking them to do, so the anaerobic pathway is used more in that case (though by no means exclusivly, it's still probably mostly aerobic, just not to the extent that the full run is, and it's enough to make you feel the burn)

I don't know if this is true, but I heard that walking and running burn the same calories *by distance*.  So, a mile walk and a mile run will burn the same calories.  It'll just take you longer to do it walking.  (Anyone, feel free to fact check me on this.)

qpx - I remember being taught the same thing in high school physics class.  At the time it made sense and there does seem to be some debate about this on the internet.

http://walking.about.com/cs/howtoloseweight/a /howcalburn.htm

"Note About the Calories Chart
You burn more calories per mile at very low speeds because you are basically stopping and starting with each step and your momentum isn't helping to carry you along. Meanwhile, at very high walking speeds you are using more muscle groups with arm motion and with a racewalking stride. Those extra muscles burn up extra calories with each step. Running may burn more calories per mile as there is an up and down motion lifting your weight off the ground as well as moving it forward."

I read someone's post that indicated that humans are optomised for walking around 3.5mph.  Anything faster or slower burns more calories, simply because it's not the way our bodies are designed to work most efficiently.

I was taught to minimize up/down motion while running, which makes sense because I'm trying to go forward, not up and down.  So, any non-forward motion is wasted energy (i.e. burned calories).  Also, minimizing up/down motion makes running a lot easier on the knees :-\