Understanding Muscle Growth Mechanisms

Understanding Muscle Growth

• A scoping review by Jorgenson, Phillips, & Hornberger (2020) identified structural adaptations driving skeletal muscle growth from mechanical load.

Learning Objectives

• Explain how mechanical loading affects muscle size.
• Differentiate hypertrophy mechanisms.
• Describe methods to identify sarcoplasmic hypotrophy after mechanical loading.

Skeletal Muscle Structure Overview

• Skeletal muscle is composed of fascicles, bundles of myofibers, which are encased in connective tissue.
• Myofibers contain myofibrils, which appear as repeating units (sarcomeres) aligned along their length.
• Myofibrils are made up of actin and myosin filaments.
• Satellite cells reside within the endomysium and are involved in muscle repair and growth.

Mechanical Load-Induced Growth at the Macroscopic Level

• Longitudinal Growth: Results from an increase in muscle length, primarily during development. In adults, it can occur with chronic stretching or potentially, only slightly, with high-load resistance training.
• Radial Growth: Results from an increase in muscle cross-sectional area (CSA). A 5-30% increase in CSA is typically observed after 8-16 weeks of resistance training, though this varies. Some studies indicate a 14% increase in muscle volume and a 23% increase in CSA with 12 weeks of training

Muscle Fascicles

• A 30% increase in CSA could be caused by 11% increase in fascicle length; this wouldn't change the pennation angle.
• Another possibility is that fascicle diameter increases by 14% and pennation angle increases by 15%.
• Ema et al. (2016) found strong evidence supporting longitudinal and radial growth from mechanical loading.

Mechanical Load-Induced Growth at the Microscopic Level

Longitudinal Growth of Fascicles

• Myofibers can run the entire length of the fascicle or terminate within the fascicle.
• Longitudinal growth of fascicles depends on myofiber growth, especially if fascicles show intrafascicular termination.
• Eccentric contractions might increase sarcomeres but this hasn't been measured in myofibers.

Radial Growth of Fascicles

• Radial growth can occur due to myofiber hypertrophy (increased diameter) or hyperplasia (increased number of myofibers).

Myofiber Hypertrophy

• Resistance training leads to 10-15% increase in myofiber CSA, consistent with known myofiber hypertrophy.

Myofiber Splitting

• Myofiber splitting is normal, but the proportion doesn't increase with mechanical loading. Myofiber splitting isn't believed to meaningfully add to the overall muscle, structurally.

Hyperplasia

• Hyperplasia, the creation of new myofibers, is observed during development, but not consistently in adults under resistance training. Muscle growth, in longitudinal dimensions, can increase the number of fascicles per cross-section and thus increase myofibers per cross-section, but further research is needed.

Mechanical Load-Induced Growth at the Ultrastructural Level

Longitudinal Growth of Myofibers

• Sarcomeres typically maintain a length of ~2.5µm
• Sarcomere addition, likely at the distal end of myofibers, is thought to be a mechanism for longitudinal myofiber growth.
• Myosin splitting, followed by elongation, is a possible mechanism, but not fully determined.

Radial Growth of Myofibers

• Myofibers primarily grow from more contractile elements than other cellular structures, causing an increase in specific tension. Specific tension is the maximal isometric force produced per cross-sectional area (CSA).
• Sarcoplasmic hypertrophy, adding material not directly contributing to contractile force (like accessory proteins), could cause a decrease in specific tension.

Sarcoplasmic Hypertrophy

• Studies by Dankel et al. (2019) show that specific tension following hypertrophy exercise does not change or will only slightly increase.
• The data suggests a lack of support for significant increases in sarcoplasmic hypertrophy in response to resistance training.

Expansion of Force-Generating Elements

• Radial growth seems proportionate to the increases in other cellular structures. This is the most reliable basis for radial growth.
• Myofiber hypertrophy likely results from the addition of contractile proteins.
• Myofibril hypertrophy likely results from contractile protein expansion.

Myofibril Hypertrophy

• Ribosomes in the intermyofibrillar space are necessary for myofilament synthesis.
• The growth mechanisms remain unclear and require further technological advances.

Myofibril Hyperplasia

• Actin does not run perpendicular to the Z-disc but is instead oblique.
• It's believed this oblique angle strains the Z-disc and possibly splits existing sarcomeres.

Radial Growth of Myofibers - Closing Remarks

• It's probable that radial growth is a combination of myofibril hypertrophy and hyperplasia.
• Myofibers are predicted to split into two separate units before growing.

Take Home Messages

• Muscular hypertrophy is complex, encompassing radial and longitudinal growth.
• Interpreting cross-sectional area (CSA) data requires understanding these varied components.
• Hyperplasia of the myofiber likely relates to a consistent process. The concept of sarcoplasmic hypertrophy may be, potentially, outdated.