Straight posture would be posture in axial direction, straight as a cigar. And we are also bipedal, as are the other straight animal (penguins). How you become like that. Bipedal animals (which are bipedal at rest) are all adapted to forces that are comming from that direction. These are hopping animals. When they are hopping they sort of propel their body with hind legs. I would presume that direction of that force goes through centar of gravity. But, when they are landing, they are also using their hind legs as a spring, for that. Here again, this legs are adapted to cushion forces comming through center of gravity of the body. Since they become so adapted to acquire forces from that direction (and weight also is a force), it becomes more natural (easy) for them to hold their body in this position. All muscles become excellently adapted for this stance. So, were we a leapers?
Well, there are also other animals which are straight as cigar. Some of them are fish and aquatic mammals, reptiles, whoever is traveling through water. They have to be like that because of streamlining. When you are traveling through the water, you better be straight as cigar. There are some other animals, too. A plunge-diving birds are like this when they are in water. They need to be like this because they are (very fastly) traveling through water. When you are plunging into water, first you need to overcome a thing called 'surface tension' of water. This can be done with pointig object, a bill in birds' case. To cushion the inpact of bill, those birds use their neck as a spring (I presume, just like hopping animals are using their legs). So, here we have spring after the bill. To cushion the impact of the body, they are using air sacs. So, in their case forces are comming from the direction of bill, through center of gravity of the body. They have another problem. Because of high impact speed, there is a problem of water rushing through their nostrils. So they have covered nostrils.
So, we are also straight as cigar. Are we hopping animals, or plunge divers. Well, we do have covered nostrils, which prevents water to rush through them. To explain of what forces we are talking about, I will demonstrate a jump from 10 m hight into water. In that situation human experience forces of 20-24 G. Completly is submerged within 128-140 milliseconds after impact, by which time there has been a 53% loss of velocity. I think that we are talking about considerable forces, here, comming through axial direction. If we did plunge diving often, we could adapt our body to those forces. We could break 'surface tension' with our hands. And look at that, our hands are also straight (not bended). To cushion inpact of our head, we could break water with our noses. Paranasal sinuses (all around nose) also can have function here.
How I came to this idea. I've read an article about the weight we can carry on our head. Here is the link for it :
www.positivehealth.com/permit/Articles/Regular/joel78.htm A few excerptions :
"The weight involved can be equally impressive. For example, 66 pounds is considered the standard weight for the Sherpas of Nepal, but some, for economic reasons, can manage up to 198 pounds - a risky business, as it is difficult to be surefooted on steep mountain tracks with such a weight
hanging from your forehead. In Africa, the women of the Kikuyu and Luo tribes in Kenya can carry on their heads loads that weigh as much as 70% of their body weight."
"According to Giovanni Cavagna, studying the mechanics of walking at the University of Milan, it has to do with the way people walk when they have something on the top of their head. Cavagna compares walking to the swinging of a pendulum. A pendulum transforms kinetic energy into potential
energy and back again. If it were not for the small amount of energy lost due to friction at its point of attachment and from movement through the air, the conversion of potential energy into kinetic energy in an ideal pendulum would be 100%. A push will transfer sufficient energy into the body of the pendulum to cause it to swing freely for a very long time. During walking, the body behaves like an upside-down pendulum, albeit an imperfect one. Heglund says that we act as only 65% of a perfect pendulum, which means that, for each step we take, 35% of the energy has to be obtained from the calories we burn. Carrying a load on the head seems to trigger an energy-saving mechanism in the gait. For example, when the Luo and Kikuyu women carry 20% of their body weight, they act as more economic pendulums, achieving an average of about 80% efficiency. In other words, with this weight on their head they use no more energy in walking than is expended without such an encumbrance. The change in their gait is so subtle that it is not visible to the human eye and, unfortunately, the exact mechanism is not yet fully understood."
Another link :
www.positivehealth.com/permit/Articles/Regular/joel79.htm It became obvious that we are adapted to carry forces in this direction.
Mario Petrinovich
Yahoo! group AAT
