We're going to learn a little muscle functionality background here. If you don't know this information, reading the other exercise sections will end up being kind of problematic. We'll begin with the "positive" and the "negative" parts of the motion. These are actually called concentric and eccentric. Stop using the words positive and negative. The best indicator for a person who knows nothing about working out, is the use of those two words. We'll put it into an example: standing barbell curls. You curl the weight up. That's concentric. You lower the weight back down. That's eccentric. Both of those combined is called isotonic. And if you just hold the weight steady, not moving it at all at some point in the range of motion, that's isometric. The same rule applies to every exercise- rows: you row it in, concentric, you let the weight back, eccentric. Essentially the hard part is concentric and the easy part is eccentric. It's not very complicated. The reason this is important though, is that concentric, eccentric, and isometric muscle activation techniques are all physiologically different in terms of the way the muscle flexes- and this is a huge factor of your muscle development.
Let's consider the anatomy of the muscle before we talk about the physiology. All muscles work in the same way, but we'll stick with our biceps example for the sake of simplicity. We'll stay on the biceps as our example. Extend your arm out all the way. Now flex it all the way. Notice how your forearm and upper-arm got real close together? That's the purpose of the bicep. That's what your muscles do at every joint in your body. In this example, your biceps muscle crosses your elbow joint- connecting both above and below it. So when the muscle shortens, it brings the bones associated with the joint closer together. All movement in your body is this exact same thing on the macro-scale. Every skeletal muscle works like this. This means that the concentric portion of every exercise, is the shortening of a muscle, which brings a couple bones closer together. The eccentric portion of an exercise, is the resisted lengthening of the muscle which causes the bones associated with the joint to move farther apart. So the concentric is your muscle shortening and the eccentric is lengthening. Still flexing, but lengthening. If you were doing those bicep curls, and slowly letting the weight down, and i really quickly took the weight out of your hands, your arms would come flying back up concentrically. So you're still flexing, it's just that the muscle is lengthening while you're flexing. It's really not all that difficult, but physiologically this plays out into some of the most important characteristics of muscle growth and development.
Okay, moving into physiology. Your muscle is made up of what's called contractile proteins. The two contractile proteins are called actin and myosin, and are what actually makes your muscles flex. So while you're relaxed, the actin and myosin aren't really doing anything. But during the shortening phase of your muscle (concentric), actin and myosin link up together. It's almost like they just reach out and hold hands. That's actually kind of what it looks like microscopically. So you have millions of these little actin-myosin "cross-bridges" (that's what their little linkage is called), and when you begin shortening your muscle, they basically use their hand-holding to pull towards each other, shortening the distance between their combined boundaries (called a z-line). For each little group of actin and myosin pulling closer to each other, you have a tiny little microscopic amount of shortening in your muscle- so when you have a million of them doing it, your entire muscle actually shortens on the large scale. The more actin and myosin linking up and pulling, the greater the strength of your muscle contraction. So if you have two million cross-bridges pulling, you're flexing your muscle with twice as much strength as if you only had one million cross-bridges. And you have way more available cross-bridges in each muscle than you can use at one time. So training your body to recruit more will make you exceedingly stronger. Training your body to make the cross-bridges cause muscular damage will make them grow, making you
exceedingly larger. We'll get into the application methodology soon- little more physiology first though.
Okay, you've heard of muscle fibers I'm sure. Essentially, muscle fibers are just groups of tons of actin and myosin. One muscle fiber stretches the entire length of your muscle, so they're actually really long. Hence they have the capacity to house tons of actin and myosin. And when you flex your muscle, your brain sends an electric impulse to recruit a certain amount of muscle fibers. When you do that, every actin and myosin in each muscle fiber activated do their cross-bridge-link-up-hold-hands-and-pull thing. So basically, every muscle fiber that's activated, contracts to its maximal potential, and the strength of your contraction is completely determined by how many total muscle fibers your brain ends up recruiting.
So play this out comparing concentric to eccentric considering what we just talked about is the only way your muscle can be activated. The shortening (concentric obviously) of your muscle requires much more strength. Looking at curls again, it's vastly easier to ease the weight down gently than it is to curl it up. Being as these two motions require different levels of strength, your body must change the contraction force it's producing. The only way your body can do this, is to recruit different amounts of muscle fibers. During the concentric part of the movement, your brain recruits way more muscle fibers than during the eccentric phase. If you recruited just as many eccentrically, realize that the weight would not go down at all. It would just stay up. This should be obvious. So effectively, the muscle contracts identically concentrically and eccentrically, but eccentrically, less total muscle fibers are used.
Now, because the muscle contracts the exact same, this means that, eccentrically, while the muscle is lengthening, the contractile proteins are linking up, holding hands, trying to pull themselves closer to each other, and are then ripped apart due to the lengthening muscle. Concentrically, the muscle is shortening, so they just pull happily and create the movement. But eccentrically, the muscle moves in the opposite direction they're trying to pull, and they're ripped apart. Possibly you've heard the term "micro tears" regarding muscle damage. This happens eccentrically. It happens during the easier part of the movement. Virtually all muscular damage, particularly to the muscle-tendon junction, happens as a result of eccentric muscle activation. Muscle damage causes fibroblasts to come into the area. Fibroblasts create increases in the size and number of your contractile proteins in each muscle fiber. This causes the muscle fibers to grow. This is hypertrophy- muscle growth and development.
So here's what you need to know. Eccentric muscle activation causes the majority of all the muscular damage. Muscular damage causes the physiological mechanisms that account for hypertrophy.
Strength, however, is different than hypertrophy. To some degree it is a product of hypertrophy, as more contractile proteins in a muscle fiber you're recruiting creates a larger force output. But it's also, and primarily, the result of neurological components. The neuromuscular connection of your brain is how many muscle fibers you're actually recruiting, As I mentioned earlier, you cannot activate them all at once. You kind of can, but you don't want to. It's a safety mechanism monitored by this thing called the golgi tendon organ. If you activate all your muscle fibers at once, you're a mother lifting a car off a child. It's a chemical and neuromuscular override of the golgi tendon organ that allows you to do it. And then allows you to be hospitalized because you ripped your tendons from your bones due to the force production they're capable of, performing but not defending. But this gives you some idea of the strength potential as a result of neuromuscular development. And like I said, you will never get to the point of activating them all, but you can
vastly increase the number you are
recruiting during an exercise. And training mechanisms, amount of resistance and force demanded, time under tension, etc. All these things will account for that. But that's leaving the physiology section, and moving into specificity of training. So for this section, you now know everything you need to.