The physiological process of increasing the size of an entire muscle is known as hypertrophy. Human muscle has a huge capacity for hypertrophy and is able to increase in size by 200% or more. A person maintains this ability throughout his or her lifespan.
Hypertrophy is considered a fairly straightforward process that adds thickness to muscle. Adding girth to muscle is also known as increasing cross-sectional area. Muscles get bigger in circumference; despite the claims from some exercise systems, muscles do not get bigger by becoming longer (other than in pediatric growth) since the distance between skeletal attachments cannot be extended by exercise.
Figure 1: Muscle hypertrophy
Although conceptually simple, the precise mechanism behind muscle growth largely is not understood. We know a good amount about the individual cellular players. However, the ways in which those players interact to contribute to cell and muscle growth are often debated to no resolution because there is very little quality research that provides definitive answers. In fact, there are numerous and differing methods for measuring hypertrophy and similarly many different laboratory models used to mimic real-world hypertrophy. These various methods and models create discord within the data.
A basic concept with robust support is that in order to increase muscle cross-sectional area, individual cells within the muscle enlarge and cumulatively increase the size of the complete muscle (1). Cellular hypertrophy results in whole-muscle hypertrophy.
A secondary route to muscle hypertrophy is through hyperplasia, where one muscle cells divides into two (basically mitosis) or a new cell is created from a progenitor cell. Hypertrophy through this mechanism occurs via increased cell number. Research on hyperplasia is not strong — if it does occur, it may be only a transient step in satellite cell fusion into an existing cell, or at best, a contributor of only about 3% toward overall muscle hypertrophy.
What Gets Added?
When we look at the chemical composition of whole muscle, we find there are four basic groups of components: (1) water, (2) cellular proteins, (3) carbohydrate and lipids, and (4) connective tissue proteins. Adding to any of these components can positively affect muscle hypertrophy.
Figure 2: The four components of a muscle
We see a similar distribution of components at the level of the individual muscle cell. If we look at which proteins are present inside individual muscle cells, we see a separation into three categories: (1) myofibrillar proteins, such as myosin, actin, titin, and more than a dozen more; (2) sarcoplasmic proteins, such as creatine kinase, lactate dehydrogenase, pyruvate kinase, myoglobin, and about 80 more; and (3) mitochondrial proteins, such as citrate synthase, ATP-synthase, cytochrome-C, and many more.
The composition of the increase in cellular mass is dependent on the type of training performed. Higher-intensity training is most likely to result in myofibrillar hypertrophy, whereas architectural and contractile protein additions provide the lion’s share of increased muscle mass. Lower-intensity and exhaustive exercise will most likely result in sarcoplasmic hypertrophy, where the added mass results from increased intracellular energy stores, increased presence of metabolic chemicals, increased mitochondrial content, and augmentation of other metabolic elements. It is important to note that this is not a black-and-white, either-or circumstance — myofibrillar and sarcoplasmic component additions occur in both types of hypertrophy, though there tends to be a bias toward one or the other end of the continuum. It is also important to understand the concepts of myofibrillar and sarcoplasmic hypertrophy are based upon conjecture. The weakness of the body of evidence related to hypertrophic mechanisms allows no more.
Reference
- Gollnick, P.D., B.F. Timson, R.L. Moore, and M. Riedy. Muscular enlargement and number of fibers in skeletal muscles of rats. Journal of Applied Physiology. 50(1981): 936–943.
Additional Reading
- Muscle Basics, Part 1: Cells, Proteins, and Sarcomeres
- Muscle Basics, Part 2: Anatomy of Muscle Contraction
To learn more about human movement and the CrossFit methodology, visit CrossFit Training.
Comments on Muscle Basics, Part 3: Hypertrophy
It would seem to be important to have an optimal balance of myofibrillar and sarcoplasmic components for GPP. Since we don't really know what that balance is, we should not bias our training. This really brings us, as CrossFitters, full circle back to Coach Glassman's idea of constantly varied functional movements across broad time and modal domains. This ostensibly fosters growth in both domains in the appropiate ratio, and as we can and do measure, allows for unrivaled GPP.
This part requires further elucidation:
"Higher-intensity training is most likely to result in myofibrillar hypertrophy, whereas architectural and contractile protein additions provide the lion’s share of increased muscle mass. Lower-intensity and exhaustive exercise will most likely result in sarcoplasmic hypertrophy, where the added mass results from increased intracellular energy stores, increased presence of metabolic chemicals, increased mitochondrial content, and augmentation of other metabolic elements."
Intensity in exercise science can refer to several different things.
I presume that "higher-intensity" here indicates intensity relative to an athlete's one rep max lift in strength training, and not higher intensity in the standard CrossFit sense of intensity relative to maximal power output in a particular context. CrossFit's benchmark workouts such as "Fran" and "Elizabeth" are low to medium intensity for experienced athletes in the one-rep max sense of intensity. That is, 95 pounds on thrusters and 135 pounds on cleans will tend to be relatively low percentages of the one-rep-max thrusters and cleans for this population.
Nonetheless, "Fran" and "Elizabeth" are usually performed at a high intensity relative to the athletes' maximal power output for their particular time and modal domains. And this for obvious reasons: no one wants to post a slow time on the board, and everyone wants to PR.
A third possible definition of intensity would be intensity relative to VO2Max, though you could argue that this is functionally equivalent to CrossFit's definition of intensity when applied to time domains of around 3-4 minutes, and longer.
This confusion surrounding the definition intensity is important if we are to know what muscular adaptations we can expect from "Fran" and "Elizabeth," whether myofibrillar or sarcoplasmic. Though indeed, if it is all based upon "conjecture," as the author states, the distinction may in the end be moot.
Lastly, I appreciate the author's frankness and epistemic humility here: "the precise mechanism behind muscle growth largely is not understood."
In our current absence of well-supported theories and laws governing the physiological response to exercise, we should base training and programming off of documented physical, not notional physiological, responses. That is, we should measure (F*D)/T at a variety of time and modal domains prior to an intervention, perform the intervention, and then measure it again. Efficacy depends on reliably increasing (F*D)/T, not on conforming with a particular model of expected physiological adaptation (which is likely quite distinct from reality besides). As Greg has said, exercise science is not a mature enough field to derive conclusions from first principles and models.
not as eloquently written, but I had the same question as to what the definition of intensity is in terms of hypertrophy training.
"Intensity" is one of those words, like "fitness", that used to be very poorly defined. People on all sides of the debate wanted to own it exclusively for their domain-specific use. Exercise science tends to define it as %1RM and powerlifters were happy to go along with this definition. For runners and other athletes, this definition is pretty useless, so they go by pace, HR or VO2max. For others, intensity seems to correlate to the amount of Mountain Dew consumed...
I agree with you, that intensity should be defined as percentage of power output under given conditions. This definition is inclusive of powerlifting, CrossFit, marathon running, etc.
Astute as always Russ! I am specifically talking force of contraction required, i.e., in the weight room how much weight is on the bar. But I did not specify. The discussion of intensity as you note is deep, and beyond the intent of the primer presented ... although at some point on this journey I'd really like to take the time to simplify and unify the concepts across modes of exercise.
Muscle Basics, Part 3: Hypertrophy
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