Look at this picture of a horse. Can you point to the horse's knees?
Most people will quite understandably point to the knobbly bits that bend somewhere akin to where our own knees are. Like this:
Isn't it odd that horses back legs have "reverse knees?" The backwards-bend of hooved animals' rear legs allows them to accelerate quickly and gallop efficiently, for up to 3 kilometers if needed. But how did the knees reverse themselves like that? Evolution is slow. It oozes. You don't see radical redesigns like a backwards knee come out of the blue.
When you look at a horses skeleton it all suddenly becomes clear:
What we thought were the backwards knees are not knees. They're actually the horses "ankle" and "heel". The real knee, kneecap and all, is found high up inside the horses flanks. Angle your foot like this to get an idea of how a horse's back legs work.
And the points that bend in the front legs are the horses "wrists". Put your hand in this position to mimic the bone structure of a horses front legs.
This means that horses are actually running around on their fingernails and toenails. The fingernail and toenail of their middle digit to be exact. Weird!
The skeletal structure of a horse seems pretty strange, but given the horses evolutionary pressures it was clearly the best that natural selection could do with the materials it had. You see this pattern throughout vertebrate life.
Find an animal with bones, and you'll be looking at the same set of building blocks. A skull, a central spine and ribcage, a shoulder girdle, pelvis and hip bones, and 4 limbs with their telltale joints ending in finger and toe bones. Musculature is similarly uniform.
With this starter pack, natural selection goes to work. Over eons bones grow and shrink, are stretched out or compacted, and sometimes become laughably out of place, as in the hip bones of a whale.
Evolution isn't a process of careful design, it's all about randomly morphing the same structure into different configurations and seeing what works.
Once you learn the basic blueprint of vertebrate life, you see we're all pretty much the same, our muscles and bones are just stretched out in different ways to meet the needs of our environments. But all the parts are still there. Zebras have triceps. Penguins have pecs and bats have abs. Turtles have shoulder blades. Dolphins have five fingers underneath their flippers. We're all a big rambunctious extended family.
I spend a lot of time telling people that their bodies are these amazing organic wonders that we have to respect and work hard to maintain. And that's true. But it's also important to realize that your body is kind of a mess. It's not like your car or your smartphone. Every component of those modern machines was designed for a specific purpose, using the best materials for the job at hand. Your body had no designer beyond random mutation and the selective pressure of a harsh world. If something worked, it was passed on to the next generation, even if it was an inefficient and crazy way to do things.
This is why we have to spend thousands of dollars to remove our wisdom teeth. The human jaw shrunk over time, but your teeth didn't get the memo. This is why your little toe is so useless, as homo erectus became an upright walker the weight of the foot shifted towards the big toe, minimizing the outer digits of the foot. This is why our backs hurt, the spine was never intended to be a vertical column supporting the entire weight of the upper body. Which brings us to the knees.
If something worked, it was passed on to the next generation, even if it was an inefficient and crazy way to do things.
We started talking about horses' knees. The fact that horses run on the tips of their fingers and toes means that the small, delicate structures of the "wrist" and "ankle" are put under a lot of strain. And when you look at the most common injuries among horses, they are all problems in the lower extremities, with wear and tear of the bones and ligaments never meant to support a 500kg animal. Ha ha! Stupid horses!
Before you get too smug, realize that your body has some equally dumb design choices. Our own knees are nothing to write home about. With our crazy adaptation of permanently standing on two legs, we doubled the amount of weight our lower limbs have to carry. The knees are where all these forces intersect, connecting the largest bone in your body, your femur, with the two second longest bones, your tibia (shinbone) and fibula. The means this little delicate sack of fluid, cartilage, and tiny bones has to deal with the lever force produced by the longest, strongest bones in your skeleton. And it has to do it in three dimensions, twisting, rocking, hinging, and cantilevering its way through the day.
Your knees are poorly designed. This is why your ACL is so easy to tear. This is why your cruciate ligament is so easily strained. This is why your meniscus breaks and your knees get fractured, dislocated, and overextended. This is why your knees generally suck.
I use the knees as an easy to understand example, but the same is true for every part of our bodies. Pulled groins, shin splints, strained hamstrings, tennis elbow, and twisted ankles are all products of our evolutionary "just good enough" anatomical design. And of course the problems go deeper and get much more serious when we hit the defects in our digestive system, hearts, lungs, and brains. Want to have a real "WTF BODY!" moment? Watch a perfectly healthy person in their 20s keel over and die from an aneurysm.
Living a life of health and wellness requires keeping two contradictory concepts in your head at the same time. One is that your body is an amazing high performance machine. If you feed it right and work it hard it usually responds splendidly, burning through fat stores, adding new muscle, and maintaining strong bones. But you also have to keep in mind that this machine is a ramshackle collection of repurposed parts that's prone to breaking down and falling to pieces. So you need to go easy on it. You need to let it rest, recover, and understand that it's a flawed creation. If you don't respect this truth you're going to end up hurt.
Sitting around all day makes you tired and sick. But constantly pushing the envelope makes you chronically sore and damaged. Finding the middle path is the best way to squeeze as many happy, productive, and active years as you can out of the claptrap body that evolution has given you. Train hard, but be safe. Err on the side of caution when you take on a new physical challenge. Take it easy with the high impact exercises and high risk sports.
Be good to your body. It's a flawed unit to be sure, but it's also your irreplaceable, ultra-rare, one of a kind custom model!