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humans walk upright

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walking up a hill, you realize one of the consequences of that

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we're able to walk relatively efficiently

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this has compromises for other forms of locomotion

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we're not especially good at climbing

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we've descended from a mammal that are especially good at:

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climbing

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good at walking quadrupedally

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as hominids, we took a different pathway

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which had consequences for our skeleton

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changes in our skeleton due to adaptation to bipedality

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big toes in line with other toes

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that's a specialization so that we can toe off with each step directing our energy forward

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the major element of our propulsion during our walking

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longitudinal arch

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allows us to capture forward motion energy and spring it back into the next step

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an specialized ankle

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much better for moving back and forth than making our feet close side to side to e.g. grasp around branches

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specialized knees

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instead of our femur sitting on top of our tibia as a column

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our femus is angled

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each time we put our weight on one leg, we need to put our center of gravity over the foot that supports us

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hip joint including femur and pelvis

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our pelvis has blades which wrap around and create a basin which supports our abdomen

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in our relatives, these are relatively flat

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humans have a very large muscle in the back side called the gluteus maximus

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this is actually a minor muscle in other kinds of primates

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in hominids, this muscle allows us to powerfully extend our legs

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if you are climbing a set of stairs of climbing, you'll feel those muscles working and getting tired

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in humans the change in orientation of our pelvis makes us use that muscle

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in primates, who have a very differently shaped pelvis, they use the hamstring muscles more than their gluteus maximus

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the hamstring muscles are the muscles that a sprinter uses to start in the starting blocks

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primates use it all the time because their bodies are in that stance habitually

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in hominids, we carry our weight upright

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as a consequence, the hamstring muscles are not especially effective for us unless our bodies are bending over

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but when we walk and especially when we walk upwards, we are exercising our gluteus maximus

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spine

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in order reorient ourselves to a vertical posture, we have a curavture of the lower spine

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Lumbar lordosis

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each of Lumbar vertebrae are angled somewhat to the next

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to allow the architecture of our spine to carry our body upright

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we haven't changed the angle of our pelvis relative to the ground

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what we've changed is the anatomy of that pelvis in such a way that a primate-like pelvic carriage is operating in a bipedal stance

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the head

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in the base of the head, the foramen magnum

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the hole in the base of the skull through which the spinal cord passes

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is relatively anterior, toward the front, because we have a vertical carriage of our head

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in other kinds of primates, that have a more inclined or even horizontal spine posture during locomotion, the foramen magnum is more toward the back of the skull

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bipedalism is similar and different in humans and other Hominidae

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we think of bipedality as part of human adaptation today

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if you think of chimpanzees, orangutans, and other apes, they are all able to walk bipedally

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they use bipedal locomotion as part of their repertoire when they are on the ground

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all of them can carry objects in both hands and move around this way

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but they do it a little differently than we do

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the architecture of their pelvis and the musculature of their legs are not well adapted to having their weight supported as a straight up-and-down column

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yet this is very efficient for humans

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we are able to stand for long periods of time with relatively little muscle action

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if they are going to be upright, they are going to have to have a bent knee

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they have to maintain their femura at an angle relative to their pelvis so their muscles have leverage

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they have to walk with a bent knee gait

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this gets tiring after awhile

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humans, however, are able to maintain an upward gait

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with great energy efficiency

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what led our ancestors to abandon the ability to move fast on all fours and to climb fast into through trees

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advantage of efficiency

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we are able to stand for long periods of time

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walk at a normal walking pace for long periods of time

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with great efficiency

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it doesn't take us very much muscle action in order to maintain a normal walking pace

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with each step when we push off with our toe, our leg swings forward

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that swinging is a pendulum

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gravity is doing most of the work to move our leg forward

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all I have to do for each step is to fire my leg forward a little bit to start the swing forward

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and transfer the weight of my body

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allow my body to fall forward, and then push off with the toe of the other foot

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this forward movement is not costing me much energy at all

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advantage for having head high

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having our eyes and ears higher up above the ground was an advantage

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one advantage in predation is how big you are, how tall you stand, and how big you appear

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a small bit of bluff might make a difference