It seems people are always wanting more flexiblity. They stretch, do yoga, etc. to achieve this. If athletes do not stretch enough, they can injure themselves.
So what is the natural benefit of shorter tendons vs. longer tendons and why hasn’t natural selection caused the human body to be very limber?
The human body is as limber as it needed to be to survive. That’s all that evolution ever does.
If people now want to be more limber that probably has everything to do with our basically sedentary lifestyles and nothing to do with evolution.
In general terms, joints that need to be exceptionally strong have very little mobility. An example are the suture joints between the various bones of the cranium.
Very strong joints have more mobility, but movement is usually mostly in one plane with very limited movement in other planes. Your knee is an example…it moves more or less in one plane, but is very strong. There are enormous stresses on the knee joints when running and jumping, which it tolerates pretty well. Of course forces from the side/front can damage the very strong ligaments that bind the bones together.
Joints that are not very strong often have lots of mobility in more than one plane…think of the shoulder joint…it can rotate and move in many directions, but is fairly easily damaged…examples are dislocations, impingements, and rotator collar injuries.
So, in evolutionary terms, joints that need lots of flexibility are going to have somewhat weak and stretchy ligaments to allow that flexibility. Joints that must be strong require strong and non stretchy ligaments.
Evolution simply reflects the physics involved in stress and flexibility.
I thought this was going to be about adaptability and I was all eager to come charging in with “Whaddya mean? Humans can survive in scorching heat, freezing cold and even outer space!”
Anyway, if environmental conditions were such that we needed more physical dexterity, i.e. we were still living in trees, no doubt it would come at the expense of some other attribute and the thread would be something like “Hey, I fell off my branch the other day while watching Tree’s Company and shuffled around on the ground and got a big thorn in my back left foot! What’s the dealio?!”
So why do athletes need to stretch all the time? They don’ t lead sedentary lifestyles. Their joints still need to be strong.
It seems, then, we’ve developed the ability to stretch, which allows us to achieve greater flexibility.
Last I heard, athletics was not a necessary survival trait.
No, we can’t. Humans plus clothing (anything from bearskins to spacesuits, as appropriate) can.
Aren’t we already among the more dextrous of creatures? I’m thinking of our “put the arms over the head” capability.
Athletes are trying to stretch their bodies to their physical limits. The rest of us are trying to get by. Our bodies allow us to do fine when we get a normal amount of physical exercise. Getting those last few percent out requires some extra effort.
Our joints are no different from our muscles. Except for a very few at the extreme ends of the bell curve, any set of muscles will carry our weight throughout the day. To do certain feats of physical exertion require greater muscle growth, or at least more specific muscle growth. We all can do this, but it doesn’t exhibit itself naturally and it requires constant attention to maintain.
It’s a reproductive advantage, though. 
So? We have clothing, beavers have dams. Is one a product of evolution but other, not?
Unneeded physical traits and capabilities tend to disappear in nature. We lost (most) of our ability to swing by our hands through the trees when we started living on the ground. We lost our overall strength as our intellect grew and our skill at toolmaking and hunting improved. Our feet lost their prehensileness as we evolved to walk erect. I think it’s safe to say that all creatures are a mix of strengths and weaknesses produced by their own particular evolutionary journeys. One might wonder why we have penguins and pelicans, instead of one bird that do all the things that both birds can, but evolution doesn’t seem to work that way.
Yes. Unless you’re a proponent of ID.
The ability to make clothing and/or dams is a product of evolution.
Neither clothing nor dams themselves are.
Perfect timing.
While googling around for that thread on treadmill vs outdoor running, I stumbled across research that indicated less flexibility increases running economy:
http://www.thieme-connect.com/ejournals/abstract/sportsmed/doi/10.1055/s-2002-19271
For the same reason we didn’t evolve wings or venom fangs. Or gills. Or parthenogenesis.
Are you implying that the entire human species needs to acquire additional flexibilility, to accommodate the special needs of athletes? Should we also deform our legs and feet, to accommodate the special needs of ballerinas?
Ballerinas are another group that is always stretching. Has a medical professional ever advised a normal healthy person to *not *stretch?
What does that have to do with evolution? People who don’t stretch are just as capable of reproducing as those who don’t. That’s all that matters.
Athletes are always stretching and working out because they are competing with each other, a group that consists of the very best physical specimens of our species–they are pushing their bodies to the limits. Not to say that our ancestors a million years ago or whatever were lazy bums, but I would submit that hitting a rabbit/any one of several species of small furry creatures, with a rock from a sling takes much less flexibility than pole vaulting 6 meters in the air. Or even eluding predators–we never ever had even the slimmest glimmer of hope of outrunning/outclimbing/generally outlimber-ing any self respecting large carnivore, so I’m guessing we mostly survived by ganging up on them, beating them with blunt objects en masse. Maybe similarly adrenaline inducing, but not as physically demanding as olympic wrestling.
ETA that I like this explanation even better: