Calisthenics 2: Physiology of Strength and Hypertrophy

Strength = Neural Adaptations * Muscle Cross-Sectional Area

The output force of a muscle is determined by four factors: angle of attack on the joint, limb length, muscle cross-sectional area, and most importantly, neural adaptations. Unfortunately, we can’t control the angle of attack on the joint and limb length so they are excluded from the aforementioned equation.

The most surprising fact about strength training is that it’s mostly about neural adaptations, not on hypertrophy, which is just a fancy term for muscle cross-sectional area.

The Central Nervous System, Motor Units, and Muscle Fiber Types

A motor unit is composed of a motor neuron and all the muscle fibers it innervates. Innervation is the pathway from a motor neuron to muscle fibers. A single motor neuron could innervate lots of fibers in a muscle but it will only innervate muscle fibers of one of the three types.

Type I: Slow-twitch fibers
Type IIa: Adaptable fibers
Type IIx: Fast-twitch fibers

Motor units are classified in a similar continuum when compared to muscle fibers. Low Threshold Motor Units (LTMU) correspond to Type I slow-twitch fibers and High Threshold Motor Units correspond to Type IIx fast-twitch fibers. The reason behind their names (low or high) is the activation potential (electrochemical signal) needed to fire them. When we look at motor units in a strength continuum as in the previous post, LTMUs are on the endurance side and HTMUs are on the strength side.

Low Threshold Motor Units (LTMU) -> Type I: slow-twitch fibers
LTMUs innervate Type I fibers, slow-twitch fibers, which are red in color because of the immense number of mitochondria they contain. They do not get fatigued very easily due to the number of mitochondria and aerobic (with oxygen) respiration that takes place inside them as a means of producing energy. These fibers have the least potential for hypertrophy.
Medium Threshold Motor Units (MTMU) -> Type IIa: adaptable fibers
MTMUs innervate Type IIa fibers, adaptable fibers, which are pink in color. They have characteristics from both Type I and Type IIx fibers. Sport-specific training becomes crucial in the development of these fibers because they can be trained to bias towards endurance or strength. Doing high-repetition endurance work when your sport is sprinting will adapt your muscles toward endurance, which is not a good way to train. Specificity in sport is king.
High Threshold Motor Units (HTMU) -> Type IIx: fast-twitch fibers
HTMUs innervate Type IIx fibers, fast-twitch fibers, which are white in color. They produce energy by anaerobic (without oxygen) respiration which is very suitable for short bursts of energy but not good for sustained energy production. Since the byproducts of anaerobic respiration include lactic acid, fast-twitch fibers get fatigued very fast. However, these muscles also contract the fastest and have the most potential for hypertrophy.

Neural Adaptations For Strength

Recruitment is an increase in the number of motor units being activated for a specific movement.
Rate Coding or Firing Rate is a decrease in time between each electrochemical signal sent to the muscle, which increases the rate of contraction.
Synchronization or Intra-muscular Coordination is a decrease in time between motor units firing and working together.
Contribution or Inter-muscular Coordination is how effectively timed the different contributing muscles to a movement are fired.
Antagonist Inhibition or Reciprocal Inhibition is the decrease in the resistance from antagonist muscles for a movement. For example, a decrease in the resistance from biceps while lifting weights in a bench press.
Motor Learning refers to the neural connections that are established in your brain as you repeat certain movements. Strong neural connections increase your capacity to learn more complex movements and eases the execution of those you've already learned.

Recruitment increases as the force requirements increase. Our nervous system has special limiters on the amount of force we can produce. Structures like Golgi tendon organs in our musculotendinous junctions (where the muscle starts to become a tendon) provide feedback to the brain, which decreases the maximum force output of muscles to prevent injury in untrained people. Fortunately, this inhibitory effect can be minimized surprisingly well with 85-90% 1 RM (Repetitions Maximum), i.e. 3 RM, training. Thus, if your primary goal is strength, you will be training in 1-3 rep range.

Rate Coding or Firing Rate increases begin after all motor units are recruited due to fatigue. If the nervous system senses the need for even more strength, it sends more rapid signals to the motor units and thus telling them to contract faster. For most large muscles, such as those for locomotion, this occurs at 90-92% of 1 RM (3RM). For smaller muscles like the forearms, rate coding can even occur at 50% of 1 RM.

Rate coding does not have a huge impact on strength training. But for those who seek hypertrophy, knowing that muscles with a greater percentage of slow-twitch fibers like the abs, calves and forearms respond better to higher repetition training might be useful. However, two-joint muscles e.g. hamstrings, biceps and many of the larger muscles, such as the glutes, tend to respond to more difficult exercises with fewer repetitions because of the preponderance of fast-twitch fibers.

Synchronization or Intra-muscular Coordination is the term for your nervous system's ability to get more efficient at recruiting different motor units to complement each other. It starts to occur as you do the exercise more often. This is especially important in skill and technique based sports more than it is on strength based ones. For instance swimmers, runners, gymnasts and the like, benefit from longer and more repetitive training sessions even though their competition might require shorter maximal effort.

Contribution or Inter-muscular Coordination is essentially the correctness of your technique. Contracting the scapula at the end of a push-up is one example. Activating the scapular muscles at the start of pull-up is another. In order to prevent over-training or under-training some muscles, you have to take the time to learn which muscles are recruited in the movement. It is only safe for you to move on to the next progression if and only if you've mastered the previous one.

Antagonist Inhibition or Reciprocal Inhibition can be improved by stretching the antagonist muscles beforehand.

Motor Learning occurs automatically in the brain as you repeat the same exercises in good form. It is sometimes better for you to not think about how to perform the movement if you have solid form. (Conscious awareness of muscle movements sometimes lead to a phenomenon called choking) Be careful though, if you're just starting out, you have to concentrate 100% on the movement. If you don't have the perfect form yet, it is better that you focus on recruited muscles.

It has been said: "Practice makes perfect." However, it is more accurately stated: "Perfect practice makes perfect."
- Steven Low

Mechanisms of Hypertrophy

There are 3 main pathways that lead to hypertrophy.

Mechanical Tension
Eccentric Damage/Microtrauma
Metabolic Accumulation, Local Growth Factors, Hypoxia

Mechanical Tension-based hypertrophy is activated with high-intensity exercises like heavyweights and fast movements. This is also referred to as HTMU or fast-twitch fatigue hypertrophy.

Eccentric Damage/Microtrauma is all about the volume of an exercise. An individual exercise is intense enough to create damage on the tissue but also light enough to perform for enough repetitions to create the damage. The time it takes for the repetitions to be completed is also referred to as time under tension by many athletes.

Metabolic Accumulation, Local Growth Factors, Hypoxia, can be thought of as the low-intensity exercises performed for high repetitions. Some examples are cyclists having large quadriceps and rowers having large back muscles.

Eccentric and isometric exercises are crucial for achieving hypertrophy with bodyweight training. Examples of isometric exercises are the back lever, front lever, and planches. An example of an eccentric exercise could be lowering from the top of a pull-up or starting in the top position of a push-up/dip and slowly going down. Isometrics are biased toward mechanical tension and metabolic accumulation, while eccentrics are biased toward mechanical tension and eccentric damage/microtrauma.

A surprising fact about isometric exercises is that shorter hold periods tend to stimulate hypertrophy more. This is due to the adaptation of muscle fibers towards endurance-based slow-twitch fibers for longer holds. One second holds for these difficult moments stimulate strength adaptations and better recruitment of HTMUs, rather than longer holds, like 5 seconds.