0:00 Intro
1:14 Structure of a Muscle
2:00 Myofibrillar Hypertrophy
4:27 Sarcoplasmic Hypertrophy
5:31 Myofibrillar Packing
7:13 MiniReview
7:45 Muscle Hyperplasia
8:48 Increase in Muscle Length
11:55 Summary

Resistance training, be it with barbells, dumbbells, machines, or even bodyweight, often produces an increase in muscle size. But, have you ever wondered what, within the muscle, drives an increase in whole muscle size?

In this video, we explore this. In the human body, muscles increase in size via increasing in crosssectional area. By my count, there appear to be five ways in which this can occur: myofibrillar hypertrophy, sarcoplasmic hypertrophy, myofibrillar packing, muscle hyperplasia, and an increase in muscle length.

Firstly, before detailing what these are, we need to understand muscles are organized in hierarchical layers. Namely, within whole muscles are fascicles, within fascicles are muscle fibers, and within muscle fibers are myofibrils. Myofibrils consist of an array of sarcomeres, and sarcomeres are what generate muscle force.

Myofibrillar hypertrophy is where at the muscle fiber level, the myofibrils and sarcoplasm grow at the same pace, meaning the relative space taken up by both remains the same.

Myofibrils may grow by increasing in number, or by increasing the crosssectional area of preexisting myofibrils. In the latter case, it's believed myofibrils can increase in crosssectional area through adding sarcomeres below or above preexisting sarcomeres. This phenomenon has been called an increase in sarcomeres in parallel).

Sarcoplasmic hypertrophy is where at the muscle fiber level, the sarcoplasm grows at a greater pace than the myofibrils, meaning the relative space taken up by the sarcoplasm becomes greater (and concurrently the relative space taken up by the myofibrils would decrease).

Myofibrillar packing is where at the muscle fiber level, the myofibrils grow at a greater pace than the sarcoplasm, meaning the relative space taken up by the myofibrils becomes greater (and concurrently the relative space taken up by the sarcoplasm decreases).

All three of these (myofibrillar hypertrophy, sarcoplasmic hypertrophy, and myofibrillar packing) are related, as all of them have the capacity to increase muscle fiber crosssectional area (and subsequently the whole muscle's crosssectional area).

Muscle hyperplasia is a little more different. This is where the number of muscle fibers is increased within a muscle (either via the creation of completely new muscle fibers or the splitting of preexisting muscle fibers). Now, it's important to mention that currently, the research on whether muscle hyperplasia occurs in humans is far from clear.

An increase in muscle length literally refers to an increase in the length of the whole muscle. Now, in the human body, muscles are attached to bones via tendons. These attachment locations do not change. Thus, any increase in muscle length is not visually apparent. Rather, due to these fixed attachment locations, a longer muscle would bulge outwards, and we can consider this an increase in crosssectional area. As some extra detail, it seems that an increase in whole muscle length can be explained by events at the muscle fiber level. Specifically, myofibrils (which, as we've established, are within muscle fibers) can increase in length via adding sarcomeres in a serial line. This phenomenon has been called an increase in sarcomeres in series. Now, although there is indirect evidence to support the notion that an increase in whole muscle length can occur with resistance training (and there may be specific ways to train for it, as we see in another video), there is evidence that it may be limited.

References:
Haun et al. https://www.frontiersin.org/articles/...
Muscle contraction details https://www.ncbi.nlm.nih.gov/books/NB...
Jorgenson et al. https://www.mdpi.com/20734409/9/7/16...
Haun et al. (2) https://www.frontiersin.org/articles/...
Phillips et al. https://pubmed.ncbi.nlm.nih.gov/10932...
Kubo et al. https://www.jstage.jst.go.jp/article/...
INFANTOLINO et al. https://anatomypubs.onlinelibrary.wil...

Music:
Song 1) L'Indécis Soulful https://chll.to/64a098ba
Song 2) L'Indécis Departure https://chll.to/e8099930
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