First of all, to understand how our muscles grow we need to have an understanding of their function and composition.
The main function of the muscular system is movement. Muscles are the only tissue in the body that is able to contract and therefore make the other parts of the body move.
Besides the ability of movement our muscles are also responsible for the maintenance of posture and body position. Muscles often contract to stabilize the body to hold certain position (e.g. sitting, standing etc.) rather than to cause movement. Since these muscles hold up the body throughout the day without becoming tired they have the greatest endurance of all muscles in the body. Each of our skeletal muscles, therefore, has to be able to contract with different levels of tension to adapt to different situations.
Skeletal muscles consist of muscle cells, also known as muscle fibers or myocytes. These muscle cells are composed of myofibrils which consist of repeating sections of sarcomeres. Sarcomeres perform the muscle’s contraction with the help of the contractile proteins myosin and actin, which cause the sarcomere to shorten. I highly encourage you to watch this video to get a good overview of the different components of skeletal muscles.
Your skeletal muscles usually comprise about 40% of your body weight and contain 50-75% of all proteins in the human body. There is a constant turnover of body proteins and researchers estimate that 1-2% of our body’s skeletal muscle mass is synthesized and broken down every day.
Net muscle protein balance
Therefore the maintenance of muscle mass is a balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). The difference between these two equals the net muscle protein balance (NPB) as shown in the equation below:
So depending on the NPB being positive or negative you will either gain or lose muscle. To increase the size of your muscle (whole muscle hypertrophy) will, therefore, require a positive NPB.
Whole muscle hypertrophy can be divided into two different areas:
- Muscle hypertrophy
- Muscle hyperplasia
It is still not 100% clear if hyperplasia occurs in humans, nevertheless to give you the complete picture I included it in this post. The following graphic shows the concept of hypertrophy as well as hyperplasia. Hyperplasia is the increase of cells in number, so in the context of muscle cells, it would mean an increase of muscle fibers. Hypertrophy, on the other hand, refers to the increase of cells in size which leads to an increase in muscle volume.
From the day you were born until adulthood your muscle size increases drastically. During muscle hypertrophy, there is no change in the number of muscle fibers. Instead, the whole muscle fiber gets larger. Muscles are always trying to adapt their structural and functional properties to physical stress. So if your body has to manage a lot of physical stress during the day your muscles will grow, which will lead to muscle hypertrophy. If the muscles are not used often excess muscles will decrease in volume (atrophy).
The concept of muscle fiber hyperplasia is based on either the splitting of muscle fibers or the formation of new fibers from satellite cells. Several studies support this thesis, however it is still controversial since most studies are not able to evaluate whether greater than normal fiber amount was determined genetically or increased by prolonged training. Most studies agree that it might be possible to achieve hyperplasia with the use of steroids or with an extremely severe exercise regimen. But since the effect of muscle hyperplasia on overall muscle size compared to hypertrophy is rather small you shouldn’t worry about it much.
What is more interesting in this context is the concept myonuclei which will be covered later in this post.
Muscle repair mechanism
After physical stress, such as resistance training damaged muscle tissue has to be repaired and additional contractile proteins incorporated to optimize and adapt for the future. The following overcompensation should decrease the risk of damage and make the muscle more resistant to stress. This is achieved through gene transcription which is followed by the processes of translation and assembly of proteins into the contractile architecture. Since muscle cells are three dimensional, they are able to increase their volume either in width or in length. They do this by either adding sarcomeres end-to-end in series which will make the muscle cell longer, or by the adding sarcomeres side-by-side in parallel which will make the cell wider.
Different growth hormones (e.g. IGF-1, testosterone), satellite cells, macrophages and other mechanisms also facilitate the repair processes but are not discussed in further detail here. To read more about these mechanisms you can read this study.
Maximizing muscle growth potential
The following factors play a huge role in maximizing your muscle growth potential:
- First of all, a caloric surplus is important if you want to grow muscle. If you don’t know your daily caloric requirements yet read this post.
- Second, you should know about concentric and eccentric movement. Concentric work is defined as the production of active tension while the muscle is shortening. Eccentric work in skeletal muscle is defined as the production of active tension while the muscle is lengthening. In skeletal muscle, concentric work occurs when a weight is lifted against gravity and eccentric work occurs when a weight is lowered in a controlled fashion. In skeletal muscle, eccentric exercise is the most potent stimulus for functional hypertrophy, leading to bulkier and stronger muscles. That’s why it is really important to focus on the negative movement of every exercise.
- Thirdly studies show that consuming essential amino acids 1-3h after training can enhance muscle growth.
- The fourth factor is fast carbs after training. Since your glycogen has to be restored after a training session it is important to consume carbohydrates with a high-glycemic index to boost glycogen resynthesis.
- Stay hydrated! Your skeletal muscles are 70% water so it is important to keep your body hydrated.
Also intensity, volume, exercise variation and rest play an important role in muscle growth. To learn more about this you can read this article.
So to sum this up keep in mind that you have to serve four ‘‘masters’’: hydration, restoration of metabolized carbohydrate, restoration/repair of damaged proteins, and remodeling proteins.
Myonuclei – A reason for muscle memory?
As mentioned earlier, myonuclei play an important role in the muscle’s growth mechanisms. If you work out on a regular basis you probably already know the phenomenon. Usually, if you are unable to work out, no matter if it is because of injury or you just don’t feel like exercising, the muscles will notice and the volume will decrease. If you now go back to training you will regain muscle force and mass more quickly compared to how long it took initially. Your body has built “muscle memory” to be able to adapt more quickly to the change. Maybe you already know that this is due to the neurological pathways that were strengthened in your brain, however, new studies suggest that myonuclei also play a great role.
Since muscle cells are pretty large they require multiple nuclei. If you work out, more nuclei are needed to support the growing muscle fiber. The cool thing is that if you stop working out the volume of your muscle might decrease with time but the elevated number of nuclei is maintained. Furthermore the ability to create myonuclei is impaired with increasing age, therefore the earlier you start, the better. The following graphic gives you a nice overview of this process:
However, this shouldn’t encourage you to stop working out.
So that’s it for this post. I hope you enjoyed reading and learned something new, if not thanks for reading anyway!
Take care and talk to you soon!
 Rasmussen, B. B., & Phillips, S. M. (2003). Contractile and nutritional regulation of human muscle growth. Exercise and sport sciences reviews, 31(3), 127-131.
 Burd, N. A., Tang, J. E., Moore, D. R., & Phillips, S. M. (2009). Exercise training and protein metabolism: influences of contraction, protein intake, and sex-based differences. Journal of Applied Physiology, 106(5), 1692-1701.
 McCall, G. E., Byrnes, W. C., Dickinson, A., Pattany, P. M., & Fleck, S. J. (1996). Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training. Journal of applied physiology, 81(5), 2004-2012.
 Hyperplasia vs. Hypertrophy in Skeletal Muscle: http://www.bodyrecomposition.com/training/hypetrophy-hyperplasia-skeletal-muscle.html/.
 Larsson, L., & Tesch, P. A. (1986). Motor unit fibre density in extremely hypertrophied skeletal muscles in man. European journal of applied physiology and occupational physiology, 55(2), 130-136.
 Skeletal Muscle Fiber Hyperplasia: http://www.theissnscoop.com/skeletal-muscle-fiber-hyperplasia/.
 How do muscles grow?: http://www.unm.edu/~lkravitz/Article%20folder/musclesgrowLK.html.
 Russell, B., Motlagh, D., & Ashley, W. W. (2000). Form follows function: how muscle shape is regulated by work. Journal of Applied Physiology, 88(3), 1127-1132.
 Bruusgaard, J. C., Johansen, I. B., Egner, I. M., Rana, Z. A., & Gundersen, K. (2010). Myonuclei acquired by overload exercise precede hypertrophy and are not lost on detraining. Proceedings of the National Academy of Sciences, 107(34), 15111-15116.
Photo credits: © lordn / Fotolia