Medical Central: Muscles

Today's subject is literally connected to the Skeleton... Muscles!!!

Muscles
Like I said above, Muscles are connected to the Skeleton. This is because muscles are what move the Skeleton (and, therefore, the rest of us). They're connected by hard fibers called tendons (the biggest tendon in the body, the Archilles [sp?] tendon in our leg's back, feels like bone, but can bend if pushed hard enough). Tendons also help a weird fact about muscles... muscles that are a good (though reasonable) distance from their targets can still move them! For example? The Deltoid (sp?) muscles is responsible for a lot of shoulder movement, but is directly on the shoulder, but some shoulder movements are controlled by muscles in our back! Since tendons can be long and they "stretch" out from the muscles, it explains this phenonemon (sp?).
Did you think that we had a lot of bones in our Skeleton? Well, get this... there are more than 600 muscles in our body!!! And that's just the controllable skeletal muscles! In fact, there are so many muscles, they take up around 1/2 of our body weight! Yikes! But they do many jobs... some muscles even protect abdonminable (sp?; in abdomen) organs that are in the abdomenal (sp?) cavity. And some of these jobs have different amounts of muscles doing it... making faces can take a lot of muscles! At least 11, but then the numbers can do a good amount of jumping from low to high and back again!
Despite having many jobs, muscles can be divided into three categories... skeletal muscles (the ones we can control), the smooth or involuntary muscles (the ones controlled without us thinking about it; also looks like strings with pointy ends while skeletal apparently have stripes), and the cardiac muscles (AKA, heart muscles). And there's an exception or two... lungs can be controlled by thought, but most of the time, the brain controls it. And sometimes there's not... certain organs controlled by smooth muscles keep on keeping on without any help from our thoughts, and the cardiac muscle keeps its beat no matter what, without rest or a break.
But no matter what, muscles are controlled by our brain in some degree. Smooth (or involuntary) muscles and cardiac muscles are controlled by the brain constantly. And even if we control the movements or actions, it requires the brain to send messages down the nerves in our spine, out the nerve endings, into the muscles, and then the muscles reacting. That may sound long, but flex your finger or roll your eyes. That whole process just happened! Probably even a few times! But it happened in less than a second. Yikes once again!
There are some muscles that will surprise you. You know about the colored ring around the pupil of our eyes. Get this: it's a smooth muscle! The brain is in total control of that one... we have no-go on it! And the esophagus also has a muscles all around its wall that does a lot of pushing after we swallow (in order to get it to our smooth-muscled stomach).
Muscles work by contracting. Muscles are made up of muscle fibers, which are made of even smaller fibers, which are then made of two other kinds of fibers! When they get the signal to work, these two types of fibers pull and get together, making the muscle contract. When the muscle relaxes, they ease off each other (though they stay pretty close to each other).
The relaxing part may have confused you a little. But when the muscles stretches out, they relax. But muscles only pull... never push. So how do they stretch out? Muscles work in pairs! When it's time to stretch, the other set of muscles is contacted, and it pulls. That makes the part of the body move the other way, and the other muscle relaxes. Take the arm for example... when you bend it up, the front of your arm's muscle pulls. When you bend it down, the muscle in the back of your arm pulls on the elbow's end, and the arm's front muscle that was pulling before relaxes and stretches out.
Muscles, of course, need energy. Everyone knows this. Where does it get energy? You know the answer... from the food we eat, and from the air we breathe, of course! Muscles need energy from food and oxygen to go. The energy made is stored in a chemical called ATP in our cells. However, when we are working really hard, this energy can be used up. And while the muscles can make energy without ATPs for a while, it doesn't last long. This process makes lactic acid, and, like any of your would expect from something called  "acid", it hurts our muscles! That's when the body screams at us, "STOP! IT'S TIME FOR A BREAK!!! YOU NEED TO REST, RECOVER, AND GET SOME EXTRA OXYGEN!" Of course, you would already be breathing harder and faster at this point to get that extra oxygen, but it's not really helping at this point. That's usually why athletes need rest after their sport... sprinters, who really push it, for example!