 Hey everybody, Dr. O here. In this video I want to talk about some terminology, some terms that are going to be very important if you understand how muscles work. I want to talk about the all or none principle. I want to talk about what motor units are and I want to talk about the idea of recruitment. So individual muscles are going to be made up of individual muscle cells, also known as muscle fibers. Each of these muscle fibers is either relaxed or fully contracted. And that's the idea of the all or none principle. We talk about it a lot more with nerves, with neurons, but the all or none principle works here as well. So not a muscle, but an individual muscle fiber is either fully contracted or it's not. So we'll talk about, so how can you lift light things and lift heavy things? Well it's not the amount of force you can generate with a muscle fiber that changes. So we'll come back to that. So the all or none principle says that a muscle cell, a muscle fiber is either fully contracted or it's relaxed and there's no middle ground. You can't partially contract a individual muscle fiber. I think of a good analogy being like a gun. Like for a gun to fire, I have to pull the trigger with enough, squeeze the trigger with enough pressure. I don't shoot, so I don't know, pull or squeeze, but I have to squeeze the trigger enough to cause the bullet to come out of the gun. So I can't make the bullet fire faster out of the gun. I can't make it fire slower out of the gun as long as it's either all or none. The gun is either not firing or it's firing. That's how I look at muscle fibers. So how much tension a muscle will actually produce depends on the total number and type of motor units that are stimulating the muscle fibers of that muscle. So let's go ahead and look at what a motor unit is. A motor unit is a motor nerve, a motor neuron, plus all the muscle fibers that it innervates or controls. So I like to think of it this way. We have small motor units and we have large motor units. Small motor units are going to control as few as three or four individual muscle fibers. So small motor units are not going to generate a lot of force because they're only stimulating a few muscle fibers. Think about fine motor control, playing a piano, playing an instrument, writing, things that don't take brute force but take manual dexterity. These types of things are usually going to be controlled by small motor units. Large motor units, they can control hundreds or even thousands of muscle fibers. So power generation. So if I'm going to stand up from this chair, I'm going to run, then I need to generate a lot of force. I'm going to use large motor units. So small motor units are used for fine motor control. Large motor units are going to be used to generate power. And that also ties into the last point I want to cover in this video which is the idea of recruitment. So you see the guy here trying to open this pickle jar. So if muscles, if muscle fibers can only be fully contracted or not, then if I need to generate more force, I do that by calling in more muscle fibers. So I can't make a muscle fiber do more work. I need more muscle fibers to do the work. So if I'm trying to open this jar and I can't get it to open, then recruitment is going to call in more motor units to stimulate more muscle fibers, or it's going to call on larger motor units to stimulate more muscle fibers. And that's how you can generate more force, whether it's lifting a heavier weight or whatever you're doing. So the all in on principle says that a muscle fiber is either contracted or it's not. A motor unit is a motor neuron, a motor nerve, and all the muscle fibers that it innervates or controls. And the recruitment is the process of slowly increasing the size and number of motor units that are used to generate the correct amount of force. So within an individual muscle, you're going to have lots of motor units controlling individual muscle and only parts of a muscle are going to be working at one time. So unless you reach this idea of tetanus, a muscle is never going to be fully contracted. There's always going to be some motor units that are resting, allowing muscle fibers to rest in a rotation and they'll come back to work if you need a sustained contraction. All right, just some terminology that I think is going to help you as you move forward. I hope this helps. Have a wonderful day. Be blessed.