Hypertrophy Mechanisms: Part 2

Written by Stefan Ianev (Clean Health Research & Development Specialist)

In part 1 of this article we made an argument for why muscle damage and metabolic stress don’t contribute to hypertrophy, and why mechanical tension is the only direct driver of muscle hypertrophy. 

The mechanically induced conformational change of individual muscle fibers may directly activate downstream signalling and may trigger messenger systems to activate signalling indirectly (1).

This is an important factor, because it means that we need to define the mechanical tension stimulus in relation to the forces experienced by each individual muscle fiber, and not by the whole muscle.

There are several factors that affect the level of mechanical tension that is experienced by a muscle fiber during a muscular contraction, including:

  • Motor Unit Recruitment
  • Force-Velocity Relationship
  • Length-Tension Relationship

Let’s take a look how at each of these factors in a little bit more detail. 

Motor Unit Recruitment

A motor unit is made up of a single nerve cell or neuron and all the muscle fibers that it innervates. When a neuron is stimulated by a level of force beyond a certain threshold, all the muscle fibers it enervates will contract. This is called the all-or-none principle. 

The recruitment threshold of a motor unit is the level of force that a muscle must produce before that motor unit is recruited. Motor units are always recruited in order of low-threshold to high-threshold. Low-threshold motor units, which govern a small number of muscle fibers, are recruited in muscular contractions involving low forces, while high-threshold motor units, which govern a large number of muscle fibers, are recruited in addition to low-threshold motor units in muscular contractions involving high forces, or when fatigue is present and the low-threshold motor units are no longer able to produce force .   

Typically, low-threshold motor units govern only a dozen or so muscle fibers which are all slow-twitch fibers. High-threshold motor units govern thousands of muscle fibers including slow twitch and fast twitch fibers. For this reason, the high-threshold motor units are far more responsive to mechanical loading because they govern a much larger pool of muscle fibres, including the fast twitch fibers which have a much greater capacity to hypertrophy (2,3). 

Overall, the literature indicates that the growth capacity of fast twitch fibers is approximately 50% greater than that of slow twitch fibers (3). That is why, in any set that the high threshold motor units are not recruited, the hypertrophy stimulus will be minimal. 

Force-Velocity Relationship

Simply recruiting the high-threshold motor units does not guarantee that each muscle fiber will be exposed to sufficient mechanical tension to trigger hypertrophy. For example, ballistic contractions can achieve a high level of motor unit recruitment, even with submaximal loads (4). However, low-load high-velocity ballistic contractions have not been associated with hypertrophy (5).

That is because when a single fiber contracts rapidly, it can only exert a low level of force due to the force-velocity relationship (6). The force-velocity relationship of a muscle fiber is determined by the number of actin-myosin cross bridges that are attached at any one time. When muscle fibers shorten slowly, they can form more simultaneous cross bridges, but when they shorten rapidly, they form fewer cross bridges at the same time. 

For this reason, only those contractions in which the high-threshold motor units are being recruited, and the bar is moving slowly, will provide sufficient mechanical loading for the high-threshold motor units. That means that either the load needs to be heavy enough or sufficient fatigue needs to be present so that the high-threshold fibers are recruited, and the bar is unintentionally moving slow. 

Length-Tension Relationship

The length-tension relationship refers to the level of force or tension that a muscle fiber produces at a given length. The length-tension relationship is determined both by the active and passive elements of a muscle fiber (7). 

This is demonstrated in the following graph adapted from Brughelli and Cronin (7).

The active tension a muscle fiber produces is determined by the degree of overlap between the actin and myosin filaments, which affects how many crossbridges can form. Peak active tension occurs near the resting length of a muscle fiber where the actin and myosin filaments are neither too bundled up or spaced too far apart (7). 

This is shown in the following graph adapted from Power (8). 

The passive tension a muscle fiber produces is determined by the stretch of the elastic structural elements inside the fiber, such as the cell cytoskeleton, titin, and the surrounding collagen layer of the fiber, called the endomysium. Passive tension increases exponentially as a muscle fiber is stretched to longer lengths. 

That is why exercises with an ascending or a bell strength curve where most of the tension is at the beginning or at the middle of the movement are generally more effective for increasing hypertrophy than exercises with a descending strength curve. For example, squats are more effective for increasing hypertrophy than leg extensions.  

That is not to say exercises with a descending strength curve should not be used at all. They are effective for increasing the mind-to-muscle connection for muscle groups that are less neurologically efficient. They also usually cause less joint stress, so they can be useful for unloading the joints. However, they are generally not going to create as much mechanical loading as exercises with an ascending or a bell strength curve. 

The main difference between exercises with an ascending verses a bell strength curve is that exercises with an ascending strength curve increase passive tension, while exercises with a bell strength curve increase active tension. 

When a muscle fiber produces a large contractile force with its passive elements, it deforms longitudinally, which stimulates the fiber to increase in length. When a muscle fiber produces a large contractile force with its active elements, it bulges outwards, which stimulates the fiber to increase in diameter (9). 

That is why for example, both full squats and half squats can be effective for increasing hypertrophy by stimulating the muscle fibers in different ways. Half squats allow you to overload the active elements of a muscle fiber with a greater load, while full squats allow you to overload the passive elements by stretching the muscles under load. Both produce high tension on the muscle fibers but one is more active tension while the other is more passive tension. 

Overall, stretching the muscles under load produces the highest combined tension as we showed previously in the graph by Brughelli and Cronin (7). Therefore, full squats would be the preferred option if you could only do one squat variation but, adding in half squats can be effective for increasing active tension and stimulating the belly of the muscle to a greater degree.  

Much like maximal eccentrics, we previously believed that movements which stretch the muscles under load were more effective for increasing hypertrophy because they cause more muscle damage. This is because when you stretch the muscles under load, the actin and myosin filaments have less overlap, and more microtears to the muscle fibers occur.

However, it now appears that the superior hypertrophic effect of such movements is more related to the high level of tensile forces achieved by stretching the passive elements of the muscle fibers. Muscle damage just happens to occur in conjunction with these higher tensile forces, but it’s not the primary trigger of hypertrophy. For this reason, doing too much volume of maximal eccentrics, or loaded stretching can become detrimental because of the excess muscle damage that occurs with these methods.  

The practical application of these methods and many others are discussed in more detail in our Performance PT Certification! Click here to enrol.

References 

  1. Burkholder TJ. Mechanotransduction in skeletal muscle. Front Biosci. 2007;12:174-191. Published 2007 Jan 1. doi:10.2741/2057
  1. Pope ZK, Hester GM, Benik FM, DeFreitas JM. Action potential amplitude as a noninvasive indicator of motor unit-specific hypertrophy. J Neurophysiol. 2016 May 1;115(5):2608-14. doi: 10.1152/jn.00039.2016. Epub 2016 Mar 2. PMID: 26936975; PMCID: PMC4922476.
  1. Adams GR, Bamman MM. Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy. Compr Physiol. 2012 Oct;2(4):2829-70. doi: 10.1002/cphy.c110066. PMID: 23720267.
  1. Harwood B, Rice CL. Changes in motor unit recruitment thresholds of the human anconeus muscle during torque development preceding shortening elbow extensions. J Neurophysiol. 2012;107(10):2876-2884. doi:10.1152/jn.00902.2011
  1. Cormie P, McGuigan MR, Newton RU. Adaptations in athletic performance after ballistic power versus strength training. Med Sci Sports Exerc. 2010 Aug;42(8):1582-98. doi: 10.1249/MSS.0b013e3181d2013a. PMID: 20139780.
  1. Piazzesi G, Reconditi M, Linari M, et al. Skeletal muscle performance determined by modulation of number of myosin motors rather than motor force or stroke size. Cell. 2007;131(4):784-795. doi:10.1016/j.cell.2007.09.045
  1. Brughelli M, Cronin J. Altering the length-tension relationship with eccentric exercise : implications for performance and injury. Sports Med. 2007;37(9):807-26. doi: 10.2165/00007256-200737090-00004. PMID: 17722950.
  1. Power GA. “Neuromuscular Function Following Lengthening Contractions.” (2012).
  1. Valamatos MJ, Tavares F, Santos RM, Veloso AP, Mil-Homens P. Influence of full range of motion vs. equalized partial range of motion training on muscle architecture and mechanical properties. Eur J Appl Physiol. 2018;118(9):1969-1983. doi:10.1007/s00421-018-3932-x

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