So when we're talking about exercise, there are two major types of exercise. One is called the isometric and the other one's called isotonic. This is where yoga is separated from the other exercises that you do. Isometric means that the length of the muscle does not change. So in other words, as we said, there are two components to the muscle. There's the elastic component and there's the contractile component. So what is yoga? Yoga is isometric, where the muscle length does not change. So I usually try to equate the isometric exercise as like a stretch exercise or pushing a wall that you cannot possibly move. So when you're pushing a wall, you're going to be pushing as hard as you can, but the wall clearly is not going to move. When you're doing this the muscle will be contracting and all it will do is stop when the tendon is fully tight. So that's what Yoga really is. It's mostly a stretch exercise, but when you're doing these exercises, your muscle is actually fully engaged, you just don't see a movement. So that's why we call it an immovable object exercise, or the stretch exercise, or the yoga stances. So when you're doing this, the muscle is fully utilizing its fuel and converting it to energy and of course, as we said before, when you are doing the oxidative pathway, you are going to produce a lot of heat as wasted energy. Hence, this is the reason why most yoga people, when they're doing their exercise, you'll see a lot of sweat coming out of them and of course the exercise. The other one that most of you are familiar with it's called the isotonic exercise where you're actually lifting a weight, and you can see the weight being lifted off the floor. It's called the isotonic because the shape of the muscle or the wear at the belly is always in the same shape. Tonic means shape, iso meaning same. So when you are trying to pick up an object, the muscle at rest, the tendon will be very loose and will have slack. Once you put some weight on it, the muscle will be able to lift that up. When you're looking at it in a microscopic level, in order to do this movement, you will have to have the muscle to contract first taking the slack out of the tendon and once the tendons slack has been removed, the object will be slowly raised. So this is what we call an after-loaded exercise and the type of exercise you should not be doing. The proper type of exercise you should be doing is called the preloaded exercise. So when you are picking up an object, you should actually let the weight hang a little bit. Why do you do this? So that the slack on the tendon could be lost. When you lose this, the muscle and then brain behind it is going to be calculating how much energy is required to lift the object. When it does this, the muscle becomes very efficient and the lift is very clean. So how do you equate this? When you look at somebody who is a bodybuilder or if you're a weightlifter, how do they lift things? They usually pick up an object from the floor and they let it hang a little bit. Why are they doing this? They are preloading all of their deltoids and their back muscles so they don't get hurt. Then every single time they are lifting up to a certain position to another position, they'll let it hang again. Again, there are preloading the next type of muscles that's going to be doing the lifting exercise. So your muscle is made up of lots of components. The whole muscle is covered by a covering called the Epimysium. If you slice the belly up you'll see it is made up of bundles of fascicles, fascicles of muscle fibers. In Latin, fascicle means bundle. So when you have a look at ham or if you have a look at steak, you can see that each of those things are really actually separated into nice little areas. So each one of those areas are really a group of muscle fibers. That's a Fascicle. The outer covering is called the Epimysium. Each of the fascicles are covered by something called the Perimysium. When you take a Perimysium and disassociate that, you'll see that it is made up of muscle fibers and each of the muscle fibers are also covered by another covering called the Endomysium. When you take a muscle fiber and you go down microscopically, you'll see that it's made up of protein tubes called the Myofibril. When you take a look at a Myofibril, you'll see a repeating structure of something called the Sarcomere. Sarco is the Latin word for muscle, mere being the unit. So basically, Sarcomere just means muscle unit. So the muscle units become together and you'll see when you look at it as a separate entity just concentrating on the Sarcomere component, that is made up of two components. One is the fatter interior section called the Myosin Filaments and the thinner outer layer called the Actin Filaments. So these things will stack together nicely to form a long tube called the Myofibril. These are pure proteins. So at anytime when you are healthy, you can make these proteins, so the larger the Myofibrils makes your muscles appear larger in the outside. When you don't have enough of these proteins, the muscle fibrils will get smaller, making your muscle look very thin. So case in point in this one is if you're looking at Arnold Schwarzenegger when he is in his twenties, he looks massive. How come he looks massive? Because each of these muscle fibers are filled with very large Myofibrils, because he's been loading up on proteins. When you get older what does most people look like? Their muscles become thinner as because these Myofibrils are getting smaller. So you need to keep your Myofibrils alive. So when you look at those actin filaments, it's actually made up a more complex structures. It is made up of the circular protein called G-actin or Globular actin. They have a sticky end point on it called the active site. When you take these reactants and you put them into two rows and their active sites are a 180 degrees apart and they're forming a long filament, we call it the Filamentous actin or the F-actin. When you look at the F-actin, it is covered by a broad protein called Tropomyosin, which covers the active site at rest and periodically above every seven G-actins apart, you have a small protein that links the tropomyosin to the G-actins or the F-actin complex and this protein is called Troponin. The other component which is the Myosin Filaments, is made up of two proteins basically. It's made up of an oval shaped protein called the Myosin Head, which also has that sticky end that we call the active site, and has a long cylindrical protein called the Myosin Tail. So these things are put together; two myosin heads and myosin tail are put together to form the myosin assembly. When you take each of these myosin assemblies and multiply it, you found the Myosin Complex. When you take the Myosin Complex, it's basically what I call a test tube brush or a toothbrush kind of concept. It looks like a bristle that is circular and it's got a little handle. So you get those two test tube brush looking structures and you put them with their two tails together and you form the Myosin Filament. So reviewing that, you'll see that the Myosin Filaments in the center and then you have a row of actin on top and then a row of actin and the bottom. The sliding filaments will be activated. How does your muscle work? It needs calcium. So if you don't have any calcium in your body, all of your muscle will just go limp. So when the nerve wants the muscle to be activated, it orders the muscle to release the calcium. When the calcium is released, calcium will bind to Troponin and Troponin will tug at its Tropomyosin counterpart. When it tugs at the active sites of each of the actins, it will be shown and therefore it will be priming the actin for binding. What does it bind with? It binds with the Myosin Complex. So again, at rest, all of the active sites are covered by Tropomyosin, and when calcium is available, all of the active sites will be shown to the Myosin head. When it's shown the two sticky ends, because ATP is always active and always available to a living tissue, the two sticky ends or the active sites will bind. We call this a cross-bridge formation. Once a cross-bridge formation is done, the actin and the Myosin will change its configuration. Basically, the Myosin head will tilt its head towards its tail and therefore pull the actin filament. So when you take a look at the sarcomere unit, the edges, because of its hexagonal shape, sort of gives you that Z-appearance. So the edges of the sarcomeres, we call it the Z line. When a Myosin head binds and forms a cross-bridge and it does a power stroke or a pulse, it is pulling towards the tail and the Z lines will get shorten. Does this only occur for one sarcomere? No, it appears throughout the whole Myofibril Complex. So the whole Myofibril tubal shorten and of course we said the muscle fiber is made up of multiple of these Myofibril tubes so it will all work together as a whole, making that one muscle fiber function. You thought this was very difficult at this point, but what have you really accomplished? You have made one muscle fiber contract. Can that be seen with your finger? Sometimes it's not. So sometimes, you can give pressure to your finger and you know you gave some kind of order and you can feel low tension on your finger, but your finger will not move. That is because you have activated too few muscle fibers. So when you have activated enough muscle fibers, then you can actually see a movement on your finger, and therefore, you have accomplished the movement. When your finger actually moves, we call those things an isotonic exercise. When you are just giving attention to your finger, that is called an isometric exercise.