Why do you use energy when walking on flat ground? Why control of you center of mass matters.

Very clear so far. Good start.

In practice most people don't walk with the CM held at constant height. Instead the CM makes a series of (catenary?) arches like the CM of a triangular object being rolled over the ground. I wonder which is more efficient, a bouncy walk with straight knees (what I just described) or a creeping walk where the knees bend while the leg is under the body, to avoid bouncing? My guess is that the muscle strain of a creeping walk (weight is always carried on bent knees) is more tiring than the extra vertical work of a bouncing walk.

http://www.anatomytrains.com/

A human's movement over terrain is much more varied than can be defined through some "either - or" viewpoint. Every step is unique. Our bodies are stabilized and move, from head to toe, by much more than just muscles and skeletal structures.

nartreb wrote:In practice most people don't walk with the CM held at constant height. Instead the CM makes a series of (catenary?) arches like the CM of a triangular object being rolled over the ground. I wonder which is more efficient, a bouncy walk with straight knees (what I just described) or a creeping walk where the knees bend while the leg is under the body, to avoid bouncing? My guess is that the muscle strain of a creeping walk (weight is always carried on bent knees) is more tiring than the extra vertical work of a bouncing walk.

On perfectly flat terrain, human gait does tend to mimic an "inverted pendulum" motion when walking like you say...but not totally as there is some flexion in the legs, but transitioning to running the opposite actually occurs where the CM actually decreases in the middle of the stance relative to foot takeoff and contact.

You're right about bent knees requiring higher loading, but in some cases, like running, its beneficial as a way to act like a "spring" to store and return energy. I hope to learn and discuss that more. When I read the walking / running literature I just realize that some fundamentals of mechanics are ignored (many researchers aren't trained in mechanics) and it skews there interpretations, especially in thinking that measures associated with efficiency cause it.

ksolem wrote:http://www.anatomytrains.com/

A human's movement over terrain is much more varied than can be defined through some "either - or" viewpoint. Every step is unique. Our bodies are stabilized and move, from head to toe, by much more than just muscles and skeletal structures.

You're definitely right, walking on varied terrain (like hiking) is more complex. But the laws of balancing CM are fundamental and a main constraint no matter what.

I'm not sure what you mean about "more than just muscles and skeletal structures"...of course passive tissues are part of that. But orientation of your skeletal structures, and muscle activiation, are the primary choices one has in controlling motion.

Clearly it's because of air resistance. ;^)

That's why you gotta lean forward in those 60 mph winds