We're going to look at the Hill model of muscle contraction and the Hill model is essentially an engineering model of the essential components of muscular contraction. So you don't want to think of these as being anatomically correct, but rather we're trying to represent functional components and anatomical components of a muscle during contraction. So in this model, we have our attachment points for our muscle - would be attached by a tendon up to an anchor point up here. We've got an attachment point in this case to a resistance, a load that we're trying to move. And when the muscle contracts, of course, we would generate force on those anchor points, pulling inward from both the top and bottom in this arrangement. Now, the three main components of the Hill model are; the contractile elements, and of course, that's our tension developing molecular components of the muscle. So that would be the sarcomeres And again, any of the sarcomeres that are active during a particular contraction would be involved in this contractile element component. So this is where active shortening is going to occur. Next, we have our series elastic elements, so this is essentially the proteins and connective tissues, which are largely proteins, that the contractile elements pull on to generate tension at our attachment points. So you could think of things like the costamere proteins and the extracellular matrix proteins that form the connective tissue wrappings around individual muscle fibers or around fasicles or around the whole muscle, you can think about the tendon itself as in line with these contractile elements. And one other thing about them is that the series elastic elements are elastic. That's why we draw them as a spring here. They're kind of stretchy. And so our contractile elements pull (stretch) these series elastic elements before they can generate significant tension on our attachment points down here and up at the top. That means that short lived contractions like twitches put most of their force generation into stretching the series elastic elements. And the key here is that we have to have longer contractions to get the series elastic stretched significantly, so it becomes taut. Then we can get significant force generation at our end points. And that's why twitches have to be added together over time to develop these more prolonged muscular contractions. Now finally, the third element of our Hill model is the parallel elastic element. These are also stretchy elements, but you want to think of it as sort of like a rigid spring that both resists stretching and compression. And so it acts a bit as a buffer. It's comprised of the cell membrane and some of the connective tissues. And again, these would be things that are not pulled on directly by the contractile elements. In other words, they are not in line with or in series with - Rather, they are parallel to those contractile elements.