This won’t be your typical bodybuilding article on muscle growth. I’m not trying to sell you the latest and greatest workout shake or whatever (because I don’t use them myself). Instead it’s going to be a proper, in depth look at the main factors behind muscle growth, an explanation of the growth process itself, and some recommendations on what to do next to increase your own muscle growth.
If you’re the lazy type I can save you the effort of reading – the same rules will apply at the end of this article that have always applied. You need to lift heavy objects, follow progressive overload (to get stronger) and adequately feed your muscles for growth. You’ll also need to have enough rest to allow the hormones mentioned below a chance to be released and to have some impact within your body.
So, with that preamble out of the way let’s get on with finding out how muscles grow, and what we can do to help them. As usual, I’ll start basic and work up to the detail at the end. Stick with it.
What determines muscular strength?
And why is it important anyway? Let’s put the question another way. Why are you obsessed with protein if you’re a bodybuilder?
You’re obsessed with protein because protein is the important functional material within muscle. Protein filaments line up inside your muscle cells along the muscle cell length, end-to-end. When you activate that muscle (try to move it), the protein filaments “crawl” over the top of one another, shortening the muscle. It’s a bit like a telescope – the filaments slot together to reduce the length of the muscle. Because the muscle is attached at both ends, this contraction causes a “pull” against the attachment points. In the real world this pulls one part of our body closer to another – like your forearm being pulled toward your shoulder when doing a bicep curl.
The filaments are like a tug of war where everyone is facing the wrong way. Rather than pulling away from the middle of the rope, everyone pulls toward the centre and it collapses inwards. The more filaments you have the more strength you can exert. So the main determinant of muscular strength is the cross sectional area of a muscle. Imagine a chicken breast sliced through the middle – like the photo opposite – this area determines how strong your muscle is. You can see the filaments (strands) of muscle in the chicken breast, as you get more of these, the muscle gets stronger. That’s why getting stronger also ensures you get bigger. Nice.
What is muscle growth?
What do we actually mean by muscle growth anyway? There are two main types of growth that are commonly talked about: myofibrillar and sarcoplasmic hypertrophy. Don’t run a mile. Whilst these things sound like something from Star Trek, they’re just describing the swelling/growth (hypertrophy) or different parts of the muscle. The first (myofibrillar) looks at the amount of muscle protein within each muscle cell. The second (sarcoplasmic) looks at the volume of the fluid within the muscle cell.
Fairly obviously myofibrillar growth is associated with an increase in muscular strength (because you get more protein in the muscle, and the protein is what does the work). So in our above example the myofibrillar growth occurs when we get more muscle filaments, and when each filament has more protein inside it. This is a good thing.
Muscles will also swell up as a result of high rep training. This is sarcoplasmic hypertrophy, and increases the volume of fluid in the muscle as well as the non-contractile proteins. Non-contractile means they aren’t involved in creating the muscular force mentioned above. We won’t be looking into this form of training in this article, because the aim is to grow the muscle in order to get stronger. “Pumping” the muscles full of fluid isn’t going to get you bigger over the long run – filling them full of protein will.
The final mechanism of muscular growth is called hyperplasia – this means growing more muscle cells. It’s a bit of a controversial one, so we’ll skirt around the issue a little bit here. If it does happen to any great degree then it’s in response to the same stimulus that provides myofibrillar hypertrophy anyway. It will also be delivered by the same response mechanism, so functionally the recommendations for training, diet and rest won’t change. I’m only including the discussion here for completeness.
What determines whether a muscle will grow or not?
Here’s where things get interesting. In order for a muscle to grow bigger, it needs to receive a stimulus. This is fairly obvious – no change happens inside your body without a signal to initiate the change. In this case the signal we will apply comes from resistance weight training. The resistance weight training forces your muscles to do work. The protein filaments within the muscle cells must crawl over one another to shorten the length of your muscle cells, as mentioned above. But like any other form of machinery, as you use the machine it starts to break down and needs to be repaired. The human body is also quite unlike the machines you use at work – when it receives stimulus to work, it further ramps up the repair mechanisms to ensure a similar amount of work will cause less damage in the future. This is achieved by causing muscle growth – a bigger muscle with a broader cross sectional area will be stronger, so it will be more capable to do the work required in the future.
As I mentioned in the introduction, this is why anyone serious about getting bigger should also be serious about getting stronger. Progressive overload is the tool we use in the gym to exploit our bodies’ natural desire to improve.
Muscle Cell Biology
Hmm, sorry about this. There’s no way to explain how the muscle grows without a little bit of science.
Human cells are a lot like a boiled egg in appearance. The egg white is called the “cytoplasm”, and that’s where a lot of the normal day to day chemical processes go on inside the cell. The shell is the “membrane” which acts like a semi-porous barrier allowing some things into the cell whilst blocking others. There are also gateways on this membrane that act both actively and passively to encourage the flow of chemicals across the membrane. The yolk of the egg is like your cell nucleus. This is the cell nerve-centre, and it’s where your DNA is housed. It is also wrapped in a membrane of its own, and inside this membrane is where all the DNA activity goes on.
The nucleus is the important part for us to focus on here. The function of your DNA is to tell the cell what to do. It tells the cell what type of cell it is (ever wondered why your muscle cells turned out that way, when other cells turned into eyeballs or liver or immune cells?). The instructions written on the DNA tell the cell how to behave, what shape to be, where to sit and so on. But these instructions aren’t continuous – the DNA is constantly being informed of external activities so it can decide how to respond. And in the case of a muscle cell, those external activities are looking at the amount of tension that has been placed across the length of the cell.
When a signal is sent to the DNA inside the nucleus, the response is delivered by interpreting – or reading – certain parts of the DNA. In reality, when your body “reads” the code on your DNA it is just taking instructions on how to build proteins. Each different part of the DNA is a “recipe” for a different protein. When your cell “reads” the DNA in the nucleus it simultaneously makes the new protein specified by that bit of the DNA “recipe”. The proteins then head off into the cell (and beyond) to pass the message on. If they’re muscle filament proteins they join the muscular filaments, making them thicker and better able to handle the next exercise session.
How does this help us understand muscle growth?
Good point – I’ll get there. Normally your cells have one nucleus each. One single nucleus to tell them how to behave. But that isn’t the case in muscle cells – they’re multinucleate – or have more than one nucleus per cell. Why? Because the role of the nucleus is to interpret DNA into protein – the more nuclei you have, the more DNA each muscle cell has, and the more opportunity each muscle cell has to create protein.
So when you want to increase the size of a muscle cell, you need to give it the machinery and instructions to add more protein – by adding in more nuclei. Our muscles do that through the use of a specialised type of stem cell called Satellite Cells.
Bring on the Satellites
These specialised cells normally rest on the membranes that wrap our muscle fibers. On a day to day basis they’re dormant, not providing any functional or mechanical assistance to the movement of our muscles. Under the stress of resistance exercise (and the associated release of a variety of hormones) the satellite cells “wake up” and start dividing (increasing in number). Some remain where they are, awaiting the next bout of training. Others migrate to the site of the damage to your muscles and fuse with existing muscle cells. This is orchestrated by your immune cells – they detect the trauma caused by the workout and come to the scene to investigate. Once there the immune cells start releasing cytokine chemicals that recruit and direct the satellite cells into action.
The satellite cells bind to the muscle fiber and differentiate (change) into muscle cell tissue. Functionally the nucleus of the satellite cell becomes available as a “new” nucleus for the muscle cell. This means the muscle cell has been repaired and has grown in size. It also has a “new nucleus” that is able to supply the cell with additional contractile proteins and direct further growth. This is the foundation of myofibrillar hypertrophy and it is extremely important – the function of these extra nuclei is to increase the muscle’s ability for peak force generation (strength).
Implications for strength training routines
This ought to be pretty obvious, but there are still people out there who dispute the basic facts of muscle physiology. By putting the muscle under sufficient stimulus to cause micro trauma (through short bursts of high intensity resistance exercise) you directly cause an increase in both the number of nuclei within your individual muscle cells and also the amount of protein contained within each muscle cell. This increase provides you with more strength. In order to keep getting stronger, you need to keep adding stress to the muscle cells. Given your muscles are now stronger; you need to add more stress each and every workout to get the same response. This is the basis of progressive overload in weight lifting, and hopefully you are now all completely convinced that barbell training is the best mechanism to deliver this. If not, go and read the article linked above.
What else do we need to know about muscle growth?
Well, fairly obviously this doesn’t occur in a vacuum. The muscles cells are bundled together in spindles, which are held together into actual muscles. They’re fed and watered by capillaries bringing fresh blood and nutrients around your body. And they’re directed by your nervous system to fire at will – via something called the “neuromuscular junction”.
One mechanism that delivers an increase in strength is a direct result of increased neural links to your muscles. When you start to work out, your muscles have limited connections to your brain. After training these connections begin to increase. This means the end point junctions where the nerve passes the signal to the muscle increase in number, the rate at which signals can be sent increases in frequency and the amplitude (strength) of the signal increases too. In fact the first increases in strength seen in a new trainee almost all come from these neural adaptations to strength training. Luckily, these junctions are also a rich site for Satellite cells. Downstream impacts on the recruitment of these junctions include an increased hormonal output. By recruiting more neuromuscular junctions you are able to increase the feedback to the brain, where the growth hormone is released.
This is important for the type of exercises you choose. Isolation exercises may have their benefits for specific improvement of symmetry and as assistance work, but they do not cause the same level of input into our central nervous system as the large, multi-joint compound weight movements. Squats, Deadlifts, Bench, Overhead Press and Chins are all excellent ways to tax multiple neuromuscular junctions simultaneously, delivering a greater hormonal response. Read on to find out more.
You can’t talk about muscle growth without discussing hormones. All well and good to know how the muscle grows, but the orchestration of muscle growth comes from hormonal responses. Let’s look at the big one I just mentioned, Growth Hormone, as well as a few of the other players in this space.
This is responsible for a wide range of beneficial changes in the body, starting with the basics (your growth spurt as a teenager) through increased turnover of muscle, collagen and bone, boosting the muscles use of fat as a fuel, and improving body composition. All good stuff. The two most effective methods to boost growth hormone circulation are sleep (you normally release a pulse of the stuff every night about 2 hours after falling asleep) and exercise. Anyone who is a regular reader on this site will also know that fasting causes an increase in circulating Growth Hormone levels, which is the reason why you don’t lose muscle mass whilst on an intermittent fasting protocol (because the Growth Hormone boost selectively spares your muscle tissue).
However, whilst Growth Hormone is useful for preventing muscle breakdown, it actually has limited direct effects on the synthesis of new protein within muscle cells, mostly delivered through interactions with the cell membrane. The big player in protein synthesis is IGF-1 – Insulin-Like Growth Factor 1.
As you might guess from the name, it’s similar in structure to insulin, although the target and role of the hormone is remarkably different. IGF-1 is produced in the liver in response to Growth Hormone signalling. So whilst the growth hormone has limited direct input into building new protein in muscle cells, it has considerable global impact on the overall growth signal.
Most of the bodies’ cells have IGF-1 receptors. They are especially prominent in the target tissues we’re interested in – the skeletal muscle, tendons, ligaments and bones. Skeletal muscle is also able to secrete this hormone itself in response to exercise. IGF-1 is able to play a small role in insulin sensitivity, but the major response here is to increase the number of satellite cells and then encourage them to begin the differentiation process (turn from satellite cell into muscle cell nucleus).
Other growth factors
IGF-1 isn’t the only growth factor – the immune system cells that immediately head to the site of muscle trauma are directing events through “cytokines”. These cause further growth factors – FGF (Fibroblast Growth Factor) and HGF (Hepatocyte Growth Factor) to be released from the muscle cells and stem cells to boost the response.
FGF release is proportional to the amount of stress and damage caused to the muscle. This means the bigger the training growth signal, the greater the FGF release level is. It is also responsible for boosting the satellite cell response, and for increasing the growth of new capillaries to the muscles and thus ensuring an adequate supply of nutrients.
HGF on the other hand is released by the stem cell lines and has a huge range of downstream impacts on all manner of cell types. Of interest here, HGF seems to enable the satellite cells to move to the site of muscle trauma, where they carry out the repair and rebuilding work. The level of HGF release is also proportional to the amount of trauma delivered to the muscle through training.
Given that we know intense exercise causes more trauma to the muscle than easy exercise, this is a good reason to focus on heavier strength building routines. They typically use weights that are upward of 85% of the 1 rep max, for sets of 1-5 repetitions. It’s important to get enough volume in too, so once the intensity training is complete it’s always worth putting some volume in too.
Well, it wouldn’t be a muscle building article without mention of the big T. Given the length of the article already I won’t dwell on this one. We know that resistance weight training has a large positive impact on testosterone production in both men and women. It increases the free testosterone in the bloodstream and causes a higher daily pulse-release of the hormone into your bloodstream. And testosterone does everything from improving body composition to increasing resting metabolic rate, as well as increasing muscle size.
How do we boost Testosterone levels? Well, through the same mechanisms we already discussed – heavy resistance weights exercise that recruits the largest number of muscle fibers and is at a significant percentage of your maximum weight lifting ability – the squat and the Deadlift are your two best friends here.
What about the nutrients?
Aha, obviously we have to come to that eventually too. You can stimulate these satellite cells all day long but there won’t be any growth without building blocks and fuel. Ultimately the satellite cells need glucose from the bloodstream to differentiate and move to the site where they’re needed. Once they have fused with the muscle cell, they need fatty acids, glucose and amino acids in order to start the process of growing the muscle fibers by laying down more protein inside the muscle cells.
The actions of Growth Hormone are important here (as discussed above) and also of Insulin. Resistance weight training makes the muscle cells more insulin sensitive, meaning they more easily succumb to the action of insulin. In practice this means that your muscle cells are more easily able to take up glucose and amino acids from the bloodstream immediately post workout – when these nutrients are needed to repair the proteins that were damaged during the exercise bout.
Sufficient post workout nutrition is very important – supplying the body with an adequate dose of protein (especially leucine) and fuel (especially glucose) in the 2-4 hours immediately post workout is the time when you will get the most response from your muscle cells. The satellite cells have been shown to continue to migrate to damaged parts of the muscle for up to 48 hours post workout, which is why I recommend consuming the bulk of your daily calories in the post workout window (>60% if possible), training later in the day, eating a meal full of complete animal proteins post workout, and ensuring the first meal on your recovery day also contains a significant amount of that day’s protein intake.
For more information on how much you need to eat to grow muscle you should check out my post “The 7 Fundamental Questions for a One Pound Muscle Mass Gain”.
Bringing it together
As I mentioned at the outset, the knowledge on how muscle grows won’t change the facts about optimal training. These facts state that you need to get stronger if you want to gain muscle. You need to add weight to the barbell on a consistent and continuous basis if you want to get stronger. And you need to eat right and rest properly (7-9 hours a night is ideal) in order to allow the signals from this muscular exertion enough time to be released into your bloodstream and work their magic.
If you’re new to building muscle – choose a weight training programme using free weights (more muscle recruitment for stabilisation), and implement progressive overload. Click here for an article to help you pick the right programme for you.
If you’re an old hand at the iron game you already know how powerful progressive overload is and how to continually build more muscle. Find a programme that lets you continue to challenge your muscles with ever heavier weights, and ensure you recruit as many muscle fibers as you can for each exercise by mixing up your slow lifts with more explosive efforts.
What have you found to be most important when you build muscle? Why not share your thoughts in the comment section below: