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I have created a humanoid that can switch between digitigrade and plantigrade legs. It is able to do this via special bones in its legs that it can lock in place without hindering its movement but still allowing it to extend the length of its feet or legs. Depending on how they lock in place allows it to switch between digitigrade and plantigrade legs at will.

So what are the pros and cons of having the ability to switch between plantigrade and digitigrade legs?

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There are no pros only cons.

First a problem with terminology, I assume you are talking about something like this

Diagram showing digitigrade and plantigrade legs.

An image I furiously hate, digitigrade and plantigrade have nothing to do with the knee and hip.

The problem is something with pillar like legs can still be digitigrade, elephants are digitigrade, rabbits and chimps on the other hand are plantigrade even though they cannot straighten the knee. Digitigrade and plantigrade refer to the position of the foot and ankle joint, the leg posture is different. An elephant or human has pillar erect posture, the knees are locked, while a dog or horse has a flexed posture. I assume you are talking about switching from a flexed digitigrade posture to a pillar erect plantigrade posture.

Note there are not many advantages to being plantigrade, which is why it is so rare, It is good for climbing and stability in rearing and that's about it. It also spreads weight better on soft substrate but big toes do that just as well.

A human can switch to a flexed digitigrade stance, but we have to exert more effort because that is not the muscles' natural resting position. Our joints also don't handle loading as well in that position, our ankles and knees are too flexible, because they need to extend far past what a flexed leg can achieve, (a dogs leg that is completely straight is broken). Using poorly adapted joints makes those joints more likely to fail. You can't build a joint that can do both well. You can't design a leg to do both well.

Another problem is the shape of the foot a plantigrade foot is too short to get much advantage from a digitigrade stance, and a digitigrade foot is too long and becomes unwieldy in a plantigrade stance.

The advantage of a flexed leg is a longer stride, and better leverage, but the cost is less flexibility and the need for more resting tendon/muscle loading. But this is only true if the limb is adapted for this posture, likewise with a pillar erect plantigrade stance the limb has to be adapted for it to gain its advantages. If you try to do both you end up with a leg that does both jobs poorly instead one job well.

It is also worth mentioning that if you have pillar limbs, flexing them does not increase your speed (the big advantage flexed limbs give) because the stride length actually gets shorter. Flexed limb animals have longer bones to get the same length.

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    $\begingroup$ IIRC kangaroos swap between plantigrade and digitigrade stances based on how fast they’re moving: when they’re just walking along or standing, they’re plantigrade, but when they’re hopping, they’re digitigrade. $\endgroup$ – nick012000 Aug 15 at 23:44
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    $\begingroup$ actually kangaroos have a strange unique posture, they use their tail as a fifth limb to walk and hop not run. But my point still stands they are piss poor walkers without their tails they can barely manage it. $\endgroup$ – John Aug 16 at 0:12
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    $\begingroup$ @JoeBloggs except your not really reducing muscle or joint stress your just building up muscle to compensate for one but the joint stress is the same. Habitual digitigrades have interlocking joints, so joint stress is carried by bone not entirely on the ligaments, but the cost is a joint that does not have the same range of motion. $\endgroup$ – John Aug 16 at 14:45
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    $\begingroup$ @JoeBloggs no go run a mile through broken terrain like that, now do it every day, for 50 years, if you haven't snapped your ankle you have a point. I am talking about actual life stresses, not the stresses incurred by human couch potatoes (which are both of us in this scenario) doing something once in while. $\endgroup$ – John Aug 16 at 18:31
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    $\begingroup$ First please don't run with locked knees, you are doing horrible damage to your knees. I cringe just thinking about it. And I am focusing on the evolutionary adaptation to these stances, less than what you can force a limb to do for a short time. You can walk on your hands that does not means your arms are well adapted for it. $\endgroup$ – John Aug 16 at 19:05

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