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A. is correct for the reasons cited plus reasons related to the ability to vary rates of breathing without direct reference to stride.

However, enter the following creature in the race. A human being in all respects but one. It runs on all four.

Now who is your winner? The 4man is.

Now take your 4man and enter him into the 100 against some nasty preadators. The nasty preadators win.

Now take 4man and while chasing his pray confront him with a quick predator. Chomp he is dead.

Now take 2man and while chasing his pray confront him with a quick predator. Thrust stab the predator is kept at bay.

The compromise from four to two legs under the savanah model only makes sense if your 2man is already a tool/weapon weilder. Alternativley it makes sense in the aquatic model even if tool making/weapon weilding has yet to appear.

This line of logic, if valid, has an interesting implication. It would appear that the question of which came first, bipetalism or weapon weilding/tool making, is crucial.

Answers?

Why do you believe a 4man would win my race? As I see it, part of the efficiency of bipedal locomotion is the vertical alignment of the skeleton. In a structure composed of muscle and bone, the bone resists compressive forces, and the muscle resists tensile forces. A vertically aligned skeleton is mostly compressive...it's basically a pole balanced vertically. Very little muscle action is required to maintain that posture. Since bone resists compressive forces without any expenditure of calories, and muscle consumes much calories to resist tensile forces, vertical is superior.

By my reasoning, the worst structure would be something like an alligator. The feet are a long way from each other and the limbs are short. An alligator is not well suited to land travel. An improvement over an alligator would be something with long legs that have feet placed close together. Like a pig or goat or deer maybe. Now, a further improvement would be to somehow add springs to the feet...make the creature walk on its toes like a dog for instance. A further yet improvement would be to make the limbs very straight and make the creature walk with joints mostly in a straight position and also to stand with joints locked. Humans have this in their hind limbs. I'm not sure what 4 legged animals have this feature but I think elephants and camels and llamas do to some degree but not quite as strait and locked as humans.

Most 4 legged creatures have the joint that is equivalent to a human knee, that is the joint between the hip and ankle of the hind limb, in a very bent position. I think this is to facilitate a rapid acceleration but I'm not sure. But this definitely reduces efficiency while walking and standing.

Your first line in your post was about breathing rates and stride. I've never heard of this before. Are you saying that a four legged animal must breath in and out at specific times in its stride? I suppose this makes sense since the abdominal muscles are used more in 4legged locomotion.

The importance of efficiency though is not so much to win the race that I described. It's importance is that it allows a tribe or pack to support a much larger population on a givin allotment of food and to harvest food from a larger range. A pack of dogs beats a lepard. A tribe of humans beats a pack of dogs. Even if the individual dogs and humans are equal in size and weight and the human's only weapons are clubs, the humans still win. It takes more calories to support 100lb of dog than 100lb of human, giving equivalent activity level. Therefore the humans will generally outnumber the dogs in a confrontation and this more than makes up for the human's lack of biting and sprinting abilities. If you factor in the humans longer lifespan and larger brain, the human's ability to aquire more skills and experience in that longer life, and ability to communicate more effectively with hunting companions, the dogs have no chance. The dogs were destined to become either extinct or slaves to the humans.