The Science Behind Muscle Growth

What is the science behind muscle growth? Let’s start with the ingredients. These include blood, oxygen, hormones, and proteins. Your muscles consist of muscle fiber bundles, both slow-twitch, and fast-twitch.  If you want to build muscle, you have to work your fast-twitch fibers. For endurance, work slow-twitch.

The Science Behind Muscle Growth – What Happens When You Lift A Weight

The science behind muscle growth begins with the rep. When you perform a rep, you generate a stimulus within your central nervous system. Your muscle fibers then receive nerve impulses and the muscle contracts, or shortens. It is able to do this because it has a supply of glycogen, a form of stored carbohydrate. That’s what it uses for energy. Your muscle fibers convert chemical energy into mechanical energy, causing the muscle to contract.

As you continue doing repetitions of an exercise, you see the muscle growing bigger before your eyes. This is the pumped look that we all lust for in the gym. This is where your muscles seem inflated and your veins bulge as if you had a sack of serpents beneath your skin.

The pumped look occurs because the muscle has become engorged with blood that is rushing to it via capillaries. The blood carries with it the nutrients and oxygen needed for the growth and repair of muscle fibers. The pumped look you have after doing two or three sets of an exercise soon dissipates because the initial rush of blood empties out into your system.

Muscles, like siblings in some families, always act in opposite ways. If you do a curl, your biceps contracts; but it’s opposite muscle, the triceps, relaxes. If you straighten your elbow, you’ll extend or flex your triceps, but the biceps will relax.

Genetics overpowers desire when it comes to how much muscle you can pack on during a weight training program. We’ve all seen the guy who does minimal workouts and looks like he could compete on one of those gladiator shows, while another man trains harder and still looks like an accountant. If you’re one of the latter, don’t despair.

Yes, there are natural limits, but everybody can build muscle. Regardless of your body type, you can get bigger and stronger. That applies to males and females and individuals of any age.

The Science Behind Muscle Growth – Mighty Morphs

William Herbert Sheldon photographed 46,000 men and women to come up with his widely cited system for classifying body types. He identified 88 categories. To keep things simple, he grouped them into three main categories:

  • Endomorph. Generally round and short in stature, guys in this group tend to have more fat cells than those in the other two groups.
  • Mesomorph. These muscular men tend to add muscle easily and have wide shoulders, small waists, and low levels of body fat. Think of a running back in football or a competition bodybuilder as being this type.
  • Ectomorph. These men are tall and lanky, like NBA players. They have a tougher time adding muscle bulk. One way for them to amass muscle faster while training is to consume 600 more calories.


Most of us have a combination of characteristics. We’re part Endo, part Meso, and part Ecto but a bit more of one than the others. The dominating characteristics determine just how massive we will look, no matter how hard we work out.

Fast-Twitch, Slow Twitch, We All Twitch

The science behind muscle growth is conducted by people who study the muscular system. They are more likely to talk about the body’s slow-twitch and fast-twitch muscle fibers than they are to talk about body types. The muscle fibers also are called Type I and Type II fibers, respectively. As many as three-quarters of us have a ratio of these fibers that falls between 60:40 and 40:60. Elite athletes, however, are typically outside these parameters. Some sprinters, for example, have been measured with 85 percent of their leg muscle fibers being fast-twitch.

You should think of it as a continuum. Fast-twitch at one end being the very powerful, rapidly contracting muscle fibers that fatigue very quickly. At the other end are the muscle fibers that don’t generate nearly as much force but have much more endurance.

If you lift weights so heavy that you can only manage a few repetitions, you recruit the fast-twitch fibers. That’s because they are brought into play when the slow-twitch fibers lack the force or power to finish the job. If you use lighter weights and perform a higher number of repetitions, you tap into the less powerful slow-twitch fibers.

If you typically lift nothing heavier than the TV remote in your daily routine, your muscles will break down old cells every 7 to 15 days. But if you lift weights or do other forms of resistance exercise, that process is accelerated since the exercise is causing microscopic tears in these fibers.

Given proper rest before the next workout, the muscle fibers repair themselves and come back bigger and stronger than before. That’s why you need to gradually increase the load you lift in order to continue to see muscle growth.

How does the science behind muscle growth apply to women and older people? Women usually don’t bulk up the way men do when they lift weights. That’s because they have fewer fast-twitch and slow-twitch fibers. They also have less testosterone. As you’d expect, then, men with high levels of testosterone see faster and bigger muscle gains than guys with less of the hormone.

As people age, they often exercise fewer of their fast-twitch fibers and develop a predominance of slow-twitch fibers. They probably could ward off much of this muscle fiber loss if they lifted weights during their forties and beyond. It’s not known, however, whether fast-twitch fibers can be regained through weight training once they have been lost.

The Science Behind Muscle Growth – The Pump

Researchers such as Dr. Lemon once thought that your potential for muscle development was pretty much predetermined by the number of fast-twitch and slow-twitch muscle fibers you were born with.

That thinking has changed.There is more opportunity to manipulate muscle development than was once thought. We all have the potential to develop more of either type of fiber through specific exercises.

What if, however, you have lots of endurance-enhancing slow-twitch fibers that are ideal for running a marathon? You probably won’t be able to make yourself into a world-class sprinter by concentrating on weight training exercises that tax fast-twitch fibers. It won’t happen because you can’t overcome the fact that the best sprinters got a head start genetically by having an unusually high number of fast-twitch muscle fibers. You could make yourself into a much more respectable sprinter, though.

Similarly, what if you were born with a high percentage of fast-twitch fibers? You aren’t likely to convert them all to slow-twitch via weight training or any other type of training.

The Science Behind Muscle Growth – The Fiber Question

Okay, back to the basics. Exercising a muscle causes tiny tears in the muscle fibers. As the muscle fibers heal, they come back bigger than before. That’s how you accomplish muscle growth. But some scientists think that once a muscle fiber reaches a certain size, it splits. This increases the number of fibers that you have available and makes room for even more muscle growth. This splitting process is known as hyperplasia.

We are all born with a different number of muscle fibers. A guy with, say, 25 percent more fibers than you has the potential to develop bigger muscles because he has more of these growth-inducing cells to recruit during his weight training. So if you could train some of your fibers to the point where they split, you in theory would increase your muscle-building potential.

Trouble is, it’s not clear yet whether this really happens in humans. Studies on cats and rodents seem to show that they can increase their number of muscle fibers through weight training. You want a strong cat, buy it a miniature barbell set. You want a strong you, keep up with your lifting program and keep the faith. This fiber-splitting theory may actually pan out. It’s a research question that is difficult to answer in humans because you have to remove and destroy the muscle, and you don’t get many volunteers to do that.

The Muscle Is The Message

The science behind muscle growth also has to do with neuromuscular response. This means that as you start to pump iron. It is a new activity that is telegraphed to your spinal cord and brain via nerves. The nerves tell the muscle being exercised that it must contract, and the muscle follows these orders like a good soldier. Only then can the muscle relax. And only then will your muscle fibers respond to the stimulus of lifting weights.

Once it does, within just a couple of weight training sessions your nerves begin to communicate more efficiently with your muscle fibers, bringing more of them into play. This is half the formula for getting stronger.

The other half is creating more muscle mass, and this takes longer, typically a few weeks. That’s because your body needs to synthesize the proteins that are used in muscle contractions. And that takes time.

So let’s say that you’ve just started your workout plan and you can do only six repetitions of an exercise with a particular weight. A couple of days later, you can do eight repetitions, and by the end of the week, you can do nine. Yet you look no different. Why? Your nervous system is already communicating better with your muscles—hence the increased number of repetitions—but you can’t see the results of this increased strength because your body hasn’t yet synthesized the proteins that are needed for those muscle contractions.

Within 3 weeks of starting your training program, you will likely feel significant strength gains.

Yes, it happens that quickly. You’ll find that dragging loaded trash cans to the street has suddenly gotten easier. You’ll find yourself sprinting up two flights of stairs where before one might have been your limit. You’ll notice that you’re stronger. But it may be a few more weeks before you actually see the changes in your body

Of course, you need to train properly to get results. What if you are lifting weights at 40 to 50 percent of your maximum capability? You will trigger the neuromuscular response but you may still look like Pee-Wee Herman. This is because you’re not asking that muscle to do anything above and beyond what it could do on a normal basis.

So you might try adding more weight to some of your exercise routines. The danger, of course, is in overdoing it. In the early going, you’ll probably be sore because sometimes it’s a trial-and-error method. You don’t know what you can do and can’t do.

It’s a balancing act. If you feel no soreness and see no muscle growth over a period of a few weeks, chances are that you need to add weight. But if you’re feeling pain, not soreness, after workouts, you need to go lighter.


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